Network Working Group P. Sangster
Request for Comments: 5209 Symantec
Category: Informational H. Khosravi
Intel
M. Mani
Avaya
K. Narayan
Cisco Systems
J. Tardo
Nevis Networks
June 2008
Network Endpoint Assessment (NEA): Overview and Requirements
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This document defines the problem statement, scope, and protocol
requirements between the components of the NEA (Network Endpoint
Assessment) reference model. NEA provides owners of networks (e.g.,
an enterprise offering remote access) a mechanism to evaluate the
posture of a system. This may take place during the request for
network access and/or subsequently at any time while connected to the
network. The learned posture information can then be applied to a
variety of compliance-oriented decisions. The posture information is
frequently useful for detecting systems that are lacking or have
out-of-date security protection mechanisms such as: anti-virus and
host-based firewall software. In order to provide context for the
requirements, a reference model and terminology are introduced.
Sangster, et al. Informational [Page 1]RFC 5209 NEA Requirements June 2008
Table of Contents
1. Introduction ....................................................3
1.1. Requirements Language ......................................4
2. Terminology .....................................................5
3. Applicability ...................................................7
3.1. Scope ......................................................7
3.2. Applicability of Environments ..............................8
4. Problem Statement ...............................................9
5. Reference Model ................................................10
5.1. NEA Client and Server .....................................12
5.1.1. NEA Client .........................................12
5.1.1.1. Posture Collector .........................12
5.1.1.2. Posture Broker Client .....................14
5.1.1.3. Posture Transport Client ..................15
5.1.2. NEA Server .........................................15
5.1.2.1. Posture Validator .........................15
5.1.2.2. Posture Broker Server .....................17
5.1.2.3. Posture Transport Server ..................18
5.2. Protocols .................................................18
5.2.1. Posture Attribute Protocol (PA) ....................18
5.2.2. Posture Broker Protocol (PB) .......................19
5.2.3. Posture Transport Protocol (PT) ....................19
5.3. Attributes ................................................20
5.3.1. Attributes Normally Sent by NEA Client: ............21
5.3.2. Attributes Normally Sent by NEA Server: ............21
6. Use Cases ......................................................22
6.1. Initial Assessment ........................................22
6.1.1. Triggered by Network Connection or Service
Request ............................................22
6.1.1.1. Example ...................................23
6.1.1.2. Possible Flows and Protocol Usage .........23
6.1.1.3. Impact on Requirements ....................25
6.1.2. Triggered by Endpoint ..............................25
6.1.2.1. Example ...................................25
6.1.2.2. Possible Flows and Protocol Usage .........26
6.1.2.3. Impact on Requirements ....................28
6.2. Posture Reassessment ......................................28
6.2.1. Triggered by NEA Client ............................28
6.2.1.1. Example ...................................28
6.2.1.2. Possible Flows & Protocol Usage ...........29
6.2.1.3. Impact on Requirements ....................30
6.2.2. Triggered by NEA Server ............................30
6.2.2.1. Example ...................................30
6.2.2.2. Possible Flows and Protocol Usage .........31
6.2.2.3. Impact on Requirements ....................33
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7. Requirements ...................................................34
7.1. Common Protocol Requirements ..............................34
7.2. Posture Attribute (PA) Protocol Requirements ..............35
7.3. Posture Broker (PB) Protocol Requirements .................36
7.4. Posture Transport (PT) Protocol Requirements ..............38
8. Security Considerations ........................................38
8.1. Trust .....................................................39
8.1.1. Endpoint ...........................................40
8.1.2. Network Communications .............................41
8.1.3. NEA Server .........................................42
8.2. Protection Mechanisms at Multiple Layers ..................43
8.3. Relevant Classes of Attack ................................43
8.3.1. Man-in-the-Middle (MITM) ...........................44
8.3.2. Message Modification ...............................45
8.3.3. Message Replay or Attribute Theft ..................45
8.3.4. Other Types of Attack ..............................46
9. Privacy Considerations .........................................46
9.1. Implementer Considerations ................................47
9.2. Minimizing Attribute Disclosure ...........................49
10. References ....................................................50
10.1. Normative References .....................................50
10.2. Informative References ...................................50
11. Acknowledgments ...............................................51
1. Introduction
Endpoints connected to a network may be exposed to a wide variety of
threats. Some protection against these threats can be provided by
ensuring that endpoints conform to security policies. Therefore, the
intent of NEA is to assess these endpoints to determine their
compliance with security policies so that corrective measures can be
provided before they are exposed to those threats. For example, if a
system is determined to be out of compliance because it is lacking
proper defensive mechanisms such as host-based firewalls, anti-virus
software, or the absence of critical security patches, the NEA
protocols provide a mechanism to detect this fact and indicate
appropriate remediation actions to be taken. Note that an endpoint
that is deemed compliant may still be vulnerable to threats that may
exist on the network.
NEA typically involves the use of special client software running on
the requesting endpoint that observes and reports on the
configuration of the system to the network infrastructure. The
infrastructure has corresponding validation software that is capable
of comparing the endpoint's configuration information with network
compliance policies and providing the result to appropriate
authorization entities that make decisions about network and
application access. Some endpoints may be incapable of running the
Sangster, et al. Informational [Page 3]RFC 5209 NEA Requirements June 2008
NEA Client software (e.g., printer) or be unwilling to share
information about their configuration. This situation is outside the
scope of NEA and is subject to local policies.
The result of an endpoint assessment may influence an access decision
that is provisioned to the enforcement mechanisms on the network
and/or endpoint requesting access. While the NEA Working Group
recognizes there may be a link between an assessment and the
enforcement of a resulting access decision, the mechanisms and
protocols for enforcement are not in scope for this specification.
Architectures, similar to NEA, have existed in the industry for some
time and are present in shipping products, but do not offer adequate
interoperability. Some examples of such architectures include:
Trusted Computing Group's Trusted Network Connect [TNC], Microsoft's
Network Access Protection [NAP], and Cisco's Cisco Network Admission
Control [CNAC]. These technologies assess the software and/or
hardware configuration of endpoint devices for the purposes of
monitoring or enforcing compliance to an organization's policy.
The NEA Working Group is developing standard protocols that can be
used to communicate compliance information between a NEA Client and a
NEA Server. This document provides the context for NEA including:
terminology, applicability, problem statement, reference model, and
use cases. It then identifies requirements for the protocols used to
communicate between a NEA Client and NEA server. Finally, this
document discusses some potential security and privacy considerations
with the use of NEA. The majority of this specification provides
informative text describing the context of NEA.
1.1. Requirements Language
Use of each capitalized word within a sentence or phrase carries the
following meaning during the NEA WG's protocol selection process:
MUST - indicates an absolute requirement
MUST NOT - indicates something absolutely prohibited
SHOULD - indicates a strong recommendation of a desired result
SHOULD NOT - indicates a strong recommendation against a result
MAY - indicates a willingness to allow an optional outcome
Lower case use of "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and
"MAY" carry their normal meaning and are not subject to these
definitions.
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2. Terminology
This section defines a set of terms used throughout this document.
In some cases these terms have been used in other contexts with
different meanings so this section attempts to describe each term's
meaning with respect to the NEA WG activities.
Assessment - The process of collecting posture for a set of
capabilities on the endpoint (e.g., host-based firewall) such that
the appropriate validators may evaluate the posture against
compliance policy.
Assertion Attributes - Attributes that include reusable information
about the success of a prior assessment of the endpoint. This
could be used to optimize subsequent assessments by avoiding a
full posture reassessment. For example, this classification of
attribute might be issued specifically to a particular endpoint,
dated and signed by the NEA Server allowing that endpoint to reuse
it for a time period to assert compliance to a set of policies.
The NEA Server might accept this in lieu of obtaining posture
information.
Attribute - Data element including any requisite meta-data describing
an observed, expected, or the operational status of an endpoint
feature (e.g., anti-virus software is currently in use).
Attributes are exchanged as part of the NEA protocols (see section
5.2). NEA recognizes a variety of usage scenarios where the use
of an attribute in a particular type of message could indicate:
o previously assessed status (Assertion Attributes),
o observed configuration or property (Posture Attributes),
o request for configuration or property information (Request
Attributes),
o assessment decision (Result Attributes), or
o repair instructions (Remediation Attributes).
The NEA WG will standardize a subset of the attribute namespace
known as standard attributes. Those attributes not standardized
are referred to in this specification as vendor-specific.
Dialog - Sequence of request/response messages exchanged.
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Endpoint - Any computing device that can be connected to a network.
Such devices normally are associated with a particular link layer
address before joining the network and potentially an IP address
once on the network. This includes: laptops, desktops, servers,
cell phones, or any device that may have an IP address.
Message - Self contained unit of communication between the NEA Client
and Server. For example, a posture attribute message might carry
a set of attributes describing the configuration of the anti-virus
software on an endpoint.
Owner - the role of an entity who is the legal, rightful possessor of
an asset (e.g., endpoint). The owner is entitled to maintain
control over the policies enforced on the device even if the asset
is not within the owner's possession. The owner may permit user
override or augmentation of control policies or may choose to not
assert any policies limiting use of asset.
Posture - Configuration and/or status of hardware or software on an
endpoint as it pertains to an organization's security policy.
Posture Attributes - Attributes describing the configuration or
status (posture) of a feature of the endpoint. For example, a
Posture Attribute might describe the version of the operating
system installed on the system.
Request Attributes - Attributes sent by a NEA Server identifying the
posture information requested from the NEA Client. For example, a
Request Attribute might be an attribute included in a request
message from the NEA Server that is asking for the version
information for the operating system on the endpoint.
Remediation Attributes - Attributes containing the remediation
instructions for how to bring an endpoint into compliance with one
or more policies. The NEA WG will not define standard remediation
attributes, but this specification does describe where they are
used within the reference model and protocols.
Result Attributes - Attributes describing whether the endpoint is in
compliance with NEA policy. The Result Attribute is created by
the NEA Server normally at the conclusion of the assessment to
indicate whether or not the endpoint was considered compliant.
More than one of these attributes may be used allowing for more
granular feature level decisions to be communicated in addition to
an overall, global assessment decision.
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Session - Stateful connection capable of carrying multiple message
exchanges associated with (an) assessment(s) of a particular
endpoint. This document defines the term session at a conceptual
level and does not describe the properties of the session or
specify requirements for the NEA protocols to manage these
sessions.
User - Role of a person that is making use of the services of an
endpoint. The user may not own the endpoint so he or she might
need to operate within the acceptable use constraints defined by
the endpoint's owner. For example, an enterprise employee might
be a user of a computer provided by the enterprise (owner) for
business purposes.
3. Applicability
This section discusses the scope of the technologies being
standardized and the network environments where it is envisioned that
the NEA technologies might be applicable.
3.1. Scope
The priority of the NEA Working Group is to develop standard
protocols at the higher layers in the reference model (see section
5): the Posture Attribute protocol (PA) and the Posture Broker
protocol (PB). PA and PB will be designed to be carried over a
variety of lower layer transport (PT) protocols. The NEA WG will
identify standard PT protocol(s) that are mandatory to implement. PT
protocols may be defined in other WGs because the requirements may
not be specific to NEA. When used with a standard PT protocol (e.g.,
Extensible Authentication Protocol (EAP), Transport Layer Security
(TLS) [TLS]), the PA and PB protocols will allow interoperability
between a NEA Client from one vendor and a NEA Server from another.
This specification will not focus on the other interfaces between the
functional components of the NEA reference model nor requirements on
their internals. Any discussion of these aspects is included to
provide context for understanding the model and resulting
requirements.
Some tangent areas not shown in the reference model that are also out
of scope for the NEA working group, and thus this specification,
include:
o Standardizing the protocols and mechanisms for enforcing
restricted network access,
o Developing standard protocols for remediation of non-compliant
endpoints,
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o Specifying protocols used to communicate with remote portions of
the NEA Client or Server (e.g., remote collectors or validators
of posture),
o Supporting a NEA Client providing posture for other endpoints
(e.g., a NEA Client on an Intrusion Detection System (IDS)
providing posture for an endpoint without a NEA Client),
o Defining the set of events or situations that might trigger a
NEA Client or NEA Server to request an assessment,
o Detecting or handling lying endpoints (see section 8.1.1 for
more information).
3.2. Applicability of Environments
Because the NEA model is based on NEA-oriented software being present
on the endpoint and in the network infrastructure, and due to the
nature of the information being exposed, the use of NEA technologies
may not apply in a variety of situations possible on the Internet.
Therefore, this section discusses some of the scenarios where NEA is
most likely to be applicable and some where it may not be.
Ultimately, the use of NEA within a deployment is not restricted to
just these scenarios. The decision of whether to use NEA
technologies lies in the hands of the deployer (e.g., network
provider) based upon the expected relationship they have with the
owners and users of potential endpoints.
NEA technologies are largely focused on scenarios where the owner of
the endpoint is the same as the owner of the network. This is a very
common model for enterprises that provide equipment to employees to
perform their duties. These employees are likely bound under an
employment contract that outlines what level of visibility the
employer expects to have into the employee's use of company assets
and possibly activities during work hours. This contract may
establish the expectation that the endpoint needs to conform to
policies set forth by the enterprise.
Some other environments may be in a similar situation and thus find
NEA technologies to be beneficial. For example, environments where
the endpoint is owned by a party (possibly even the user) that has
explicitly expressed a desire to conform to the policies established
by a network or service provider in exchange for being able to access
its resources. An example of this might be an independent contractor
with a personal laptop, working for a company imposing NEA assessment
policies on its employees, who may wish a similar level of access and
is willing to conform to the company's policies. NEA technologies
may be applicable to this situation.
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Conversely, some environments where NEA is not expected to be
applicable would be environments where the endpoint is owned by a
user that has not agreed to conform to a network provider's policies.
An example might include when the above contractor visits any public
area like the local coffee shop that offers Internet access. This
coffee shop would not be expected to be able to use NEA technologies
to assess the posture of the contractor's laptop. Because of the
potentially invasive nature of NEA technology, such an assessment
could amount to an invasion of privacy of the contractor.
It is more difficult to determine whether NEA is applicable in other
environments, so the NEA WG will consider them to be out of scope for
consideration and specification. In order for an environment to be
considered applicable for NEA, the owner or user of an endpoint must
have established a clear expectation that it will comply with the
policies of the owner and operator of the network. Such an
expectation likely includes a willingness to disclose appropriate
information necessary for the network to perform compliance checks.
4. Problem Statement
NEA technology may be used for a variety of purposes. This section
highlights some of the major situations where NEA technologies may be
beneficial.
One use is to facilitate endpoint compliance checking against an
organization's security policy when an endpoint connects to the
network. Organizations often require endpoints to run an
IT-specified Operating System (OS) configuration and have certain
security applications enabled, e.g., anti-virus software, host
intrusion detection/prevention systems, personal firewalls, and patch
management software. An endpoint that is not compliant with IT
policy may be vulnerable to a number of known threats that might
exist on the network.
Without NEA technology, ensuring compliance of endpoints to corporate
policy is a time-consuming and difficult task. Not all endpoints are
managed by a corporation's IT organization, e.g., lab assets and
contractor machines. Even for assets that are managed, they may not
receive updates in a timely fashion because they are not permanently
attached to the corporate network, e.g., laptops. With NEA
technology, the network is able to assess an endpoint as soon as it
requests access to the network or at any time after joining the
network. This provides the corporation an opportunity to check
compliance of all NEA-capable endpoints in a timely fashion and
facilitate endpoint remediation potentially while quarantined when
needed.
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NEA technology can be used to provide posture assessment for a range
of ways of connecting to the network including (but not limited to)
wired and wireless LAN access such as using 802.1X [802.1X], remote
access via IPsec [IPSEC], or Secure Socket Layer (SSL) VPN, or
gateway access.
Endpoints that are not NEA-capable or choose not to share sufficient
posture to evaluate compliance may be subject to different access
policies. The decision of how to handle non-compliant or
non-participating endpoints can be made by the network administrator
possibly based on information from other security mechanisms on the
network (e.g., authentication). For example, remediation
instructions or warnings may be sent to a non-compliant endpoint with
a properly authorized user while allowing limited access to the
network. Also, network access technologies can use the NEA results
to restrict or deny access to an endpoint, while allowing
vulnerabilities to be addressed before an endpoint is exposed to
attack. The communication and representation of NEA assessment
results to network access technologies on the network is out of scope
for this document.
Reassessment is a second important use of NEA technology as it allows
for additional assessments of previously considered compliant
endpoints to be performed. This might become necessary because
network compliance policies and/or endpoint posture can change over
time. A system initially assessed as being compliant when it joined
the network may no longer be in compliance after changes occur. For
example, reassessment might be necessary if a user disables a
security protection (e.g., host-based firewall) required by policy or
when the firewall becomes non-compliant after a firewall patch is
issued and network policy is changed to require the patch.
A third use of NEA technology may be to verify or supplement
organization asset information stored in inventory databases.
NEA technology can also be used to check and report compliance for
endpoints when they try to access certain mission critical
applications within an enterprise, employing service (application)
triggered assessment.
5. Reference Model
This section describes the reference model for Network Endpoint
Assessment. This model is provided to establish a context for the
discussion of requirements and may not directly map to any particular
product or deployment architecture. The model identifies the major
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functionality of the NEA Client and Server and their relationships,
as well as the protocols they use to communicate at various levels
(e.g., PA is carried by the PB protocol).
While the diagram shows 3 layered protocols, it is envisioned that PA
is likely a thin message wrapper around a set of attributes and that
it is batched and encapsulated in PB. PB is primarily a lightweight
message batching protocol, so the protocol stack is mostly the
transport (PT). The vertical lines in the model represent APIs
and/or protocols between components within the NEA Client or Server.
These interfaces are out of scope for standardization in the NEA WG.
+-------------+ +--------------+
| Posture | <--------PA--------> | Posture |
| Collectors | | Validators |
| (1 .. N) | | (1 .. N) |
+-------------+ +--------------+
| |
| |
| |
+-------------+ +--------------+
| Posture | | Posture |
| Broker | <--------PB--------> | Broker |
| Client | | Server |
+-------------+ +--------------+
| |
| |
+-------------+ +--------------+
| Posture | | Posture |
| Transport | <--------PT--------> | Transport |
| Client | | Server |
| (1 .. N) | | (1 .. N) |
+-------------+ +--------------+
NEA CLIENT NEA SERVER
Figure 1: NEA Reference Model
The NEA reference model does not include mechanisms for discovery of
NEA Clients and NEA Servers. It is expected that NEA Clients and NEA
Servers are configured with information that allows them to reach
each other. The specific methods of referencing the configuration
and establishing the communication channel are out of scope for the
NEA reference model and should be covered in the specifications of
candidate protocols such as the Posture Transport (PT) protocol.
Sangster, et al. Informational [Page 11]RFC 5209 NEA Requirements June 2008
5.1. NEA Client and Server
5.1.1. NEA Client
The NEA Client is resident on an endpoint device and comprised of the
following functionality:
o Posture Collector(s)
o Posture Broker Client
o Posture Transport Client(s)
The NEA Client is responsible for responding to requests for
attributes describing the configuration of the local operating domain
of the client and handling the assessment results including potential
remediation instructions for how to conform to policy. A NEA Client
is not responsible for reporting on the posture of entities that
might exist on the endpoint or over the network that are outside the
domain of execution (e.g., in other virtual machine domains) of the
NEA Client.
For example, a network address translation (NAT) device might route
communications for many systems behind it, but when the NAT device
joins the network, its NEA Client would only report its own (local)
posture. Similarly, endpoints with virtualization capabilities might
have multiple independent domains of execution (e.g., OS instances).
Each NEA Client is only responsible for reporting posture for its
domain of execution, but this information might be aggregated by
other local mechanisms to represent the posture for multiple domains
on the endpoint. Such posture aggregation mechanisms are outside the
focus of this specification.
Endpoints lacking NEA Client software (which is out of NEA scope) or
choosing not to provide the attributes required by the NEA Server
could be considered non-compliant. The NEA model includes
capabilities to enable the endpoint to update its contents in order
to become compliant.
5.1.1.1. Posture Collector
The Posture Collector is responsible for responding to requests for
posture information in Request Attributes from the NEA Server. The
Posture Collector is also responsible for handling assessment
decisions in Result Attributes and remediation instructions in
Remediation Attributes. A single NEA Client can have several Posture
Collectors capable of collecting standard and/or vendor-specific
Posture Attributes for particular features of the endpoint. Typical
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examples include Posture Collectors that provide information about
Operating System (OS) version and patch levels, anti-virus software,
and security mechanisms on the endpoint such as host-based Intrusion
Detection System (IDS) or firewall.
Each Posture Collector will be associated with one or more
identifiers that enable it to be specified as the destination in a PA
message. The Posture Broker Client uses these identifiers to route
messages to this Collector. An identifier might be dynamic (e.g.,
generated by the Posture Broker Client at run-time during
registration) or more static (e.g., pre-assigned to the Posture
Collector at install-time and passed to the Posture Broker Client
during registration) or a function of the attribute messages the
Collector desires to receive (e.g., message type for subscription).
The NEA model allocates the following responsibilities to the Posture
Collector:
o Consulting with local privacy and security policies that may
restrict what information is allowed to be disclosed to a given
NEA Server.
o Receiving Request Attributes from a Posture Validator and
performing the local processing required to respond
appropriately. This may include:
- Collecting associated posture information for particular
features of the endpoint and returning this information in
Posture Attributes.
- Caching and recognizing the applicability of recently issued
attributes containing reusable assertions that might serve to
prove compliance and returning this attribute instead of
posture information.
o Receiving attributes containing remediation instructions on how
to update functionality on the endpoint. This could require the
Collector to interact with the user, owner, and/or a remediation
server.
o Monitoring the posture of (a) particular features(s) on the
endpoint for posture changes that require notification to the
Posture Broker Client.
o Providing cryptographic verification of the attributes received
from the Validator and offering cryptographic protection to the
attributes returned.
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The above list describes the model's view of the possible
responsibilities of the Posture Collector. Note that this is not a
set of requirements for what each Posture Collector implementation
must support, nor is it an exhaustive list of all the things a
Posture Collector may do.
5.1.1.2. Posture Broker Client
The Posture Broker Client is both a PA message multiplexer and a
de-multiplexer. The Posture Broker Client is responsible for
de-multiplexing the PB message received from the NEA Server and
distributing each encapsulated PA message to the corresponding
Posture Collector(s). The model also allows for the posture
information request to be pre-provisioned on the NEA Client to
improve performance by allowing the NEA Client to report posture
without receiving a request for particular attributes from the NEA
Server.
The Posture Broker Client also multiplexes the responses from the
Posture Collector(s) and returns them to the NEA Server. The Posture
Broker Client constructs one or more PB messages using the PA
message(s) it obtains from the Posture Collector(s) involved in the
assessment. The quantity and ordering of Posture Collector responses
(PA message(s)) multiplexed into the PB response message(s) can be
determined by the Posture Broker Client based on many factors
including policy or characteristics of the underlying network
transport (e.g., MTU). A particular NEA Client will have one Posture
Broker Client.
The Posture Broker Client also handles the global assessment decision
from the Posture Broker Server and may interact with the user to
communicate the global assessment decision and aid in any necessary
remediation steps.
The NEA model allocates the following responsibilities to the Posture
Broker Client:
o Maintaining a registry of known Posture Collectors and allowing
for Posture Collectors to dynamically register and deregister.
o Multiplexing and de-multiplexing attribute messages between the
NEA Server and the relevant Posture Collectors.
o Handling posture change notifications from Posture Collectors
and triggering reassessment.
o Providing user notification about the global assessment decision
and other user messages sent by the NEA Server.
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5.1.1.3. Posture Transport Client
The Posture Transport Client is responsible for establishing a
reliable communication channel with the NEA Server for the message
dialog between the NEA Client and NEA Server. There might be more
than one Posture Transport Client on a particular NEA Client
supporting different transport protocols (e.g., 802.1X, VPN).
Certain Posture Transport Clients may be configured with the address
of the appropriate Posture Transport Server to use for a particular
network.
The NEA model allocates the following responsibilities to the Posture
Transport Client:
o Initiating and maintaining the communication channel to the NEA
Server. The Posture Transport Client hides the details of the
underlying carrier that could be a Layer 2 or Layer 3 protocol.
o Providing cryptographic protection for the message dialog
between the NEA Client and NEA Server.
5.1.2. NEA Server
The NEA Server is typically comprised of the following NEA
functionality:
o Posture Validator(s)
o Posture Broker Server
o Posture Transport Server(s)
The Posture Validators might be located on a separate server from the
Posture Broker Server, requiring the Posture Broker Server to deal
with both local and remote Posture Validators.
5.1.2.1. Posture Validator
A Posture Validator is responsible for handling Posture Attributes
from corresponding Posture Collector(s). A Posture Validator can
handle Posture Attributes from one or more Posture Collectors and
vice-versa. The Posture Validator performs the posture assessment
for one or more features of the endpoint (e.g., anti-virus software)
and creates the result and, if necessary, the remediation
instructions, or it may choose to request additional attributes from
one or more Collectors.
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Each Posture Validator will be associated with one or more
identifiers that enable it to be specified as the destination in a PA
message. The Posture Broker Server uses this identifier to route
messages to this Validator. This identifier might be dynamic (e.g.,
generated by the Posture Broker Server at run-time during
registration) or more static (e.g., pre-assigned to a Posture
Validator at install-time and passed to the Posture Broker Server
during registration) or a function of the attribute messages the
Validator desires to receive (e.g., message type for subscription).
Posture Validators can be co-located on the NEA Server or can be
hosted on separate servers. A particular NEA Server is likely to
need to handle multiple Posture Validators.
The NEA model allocates the following responsibilities to the Posture
Validator:
o Requesting attributes from a Posture Collector. The request may
include:
- Request Attributes that indicate to the Posture Collector to
fetch and provide Posture Attributes for particular
functionality on the endpoint.
o Receiving attributes from the Posture Collector. The response
from the Posture Collector may include:
- Posture Attributes collected for the requested functionality.
- Assertion Attributes that indicate the compliance result from
a prior assessment.
o Assessing the posture of endpoint features based on the
attributes received from the Collector.
o Communicating the posture assessment result to the Posture
Broker Server.
o Communicating the posture assessment results to the Posture
Collector; this attribute message may include:
- Result Attributes that communicate the posture assessment
result.
- Remediation Attributes that communicate the remediation
instructions to the Posture Collector.
o Monitoring out-of-band updates that trigger reassessment and
require notifications to be sent to the Posture Broker Server.
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o Providing cryptographic protection for attributes sent to the
Posture Collector and offering cryptographic verification of the
attributes received from the Posture Collector.
The above list describes the model's view of the possible
responsibilities of the Posture Validator. Note that this is not a
set of requirements for what each Posture Validator implementation
must support, nor is it an exhaustive list of all the things a
Posture Validator may do.
5.1.2.2. Posture Broker Server
The Posture Broker Server acts as a multiplexer and a de-multiplexer
for attribute messages. The Posture Broker Server parses the PB
messages received from the NEA Client and de-multiplexes them into PA
messages that it passes to the associated Posture Validators. The
Posture Broker Server multiplexes the PA messages (e.g., messages
containing (a) Request Attribute(s) from the relevant Posture
Validator(s)) into one or more PB messages and sends them to the NEA
Client via the Posture Transport protocol. The quantity and ordering
of Posture Validator responses (PA messages) and global assessment
decision multiplexed into the PB response message(s) can be
determined by the Posture Broker Server based on many factors
including policy or characteristics of the underlying network
transport (e.g., MTU).
The Posture Broker Server is also responsible for computing the
global assessment decision based on individual posture assessment
results from the various Posture Validators. This global assessment
decision is sent back to the NEA Client in Result Attributes within a
PB message. A particular NEA Server will have one Posture Broker
Server, and this Posture Broker Server will handle all the local and
remote Posture Validators.
The NEA model allocates the following responsibilities to the Posture
Broker Server:
o Maintaining a registry of Posture Validators and allowing for
Posture Validators to register and deregister.
o Multiplexing and de-multiplexing posture messages from and to
the relevant Posture Validators.
o Computing the global assessment decision based on posture
assessment results from the various Posture Validators and
compliance policy. This assessment decision is sent to the
Posture Broker Client in a PB message.
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5.1.2.3. Posture Transport Server
The Posture Transport Server is responsible for establishing a
reliable communication channel with the NEA Client for the message
dialog between the NEA Client and NEA Server. There might be more
than one Posture Transport Server on a particular NEA Server to
support different transport protocols. A particular Posture
Transport Server will typically handle requests from several Posture
Transport Clients and may require local configuration describing how
to reach the NEA Clients.
The NEA model allocates the following responsibilities to the Posture
Transport Server:
o Initiating and maintaining a communication channel with,
potentially, several NEA Clients.
o Providing cryptographic protection for the message dialog
between the NEA Client and NEA Server.
5.2. Protocols
The NEA reference model includes three layered protocols (PA, PB, and
PT) that allow for the exchange of attributes across the network.
While these protocols are intended to be used together to fulfill a
particular role in the model, they may offer overlapping
functionality. For example, each protocol should be capable of
protecting its information from attack (see section 8.2 for more
information).
5.2.1. Posture Attribute Protocol (PA)
PA is a protocol that carries one or more attributes between Posture
Collectors and their associated Posture Validator. The PA protocol
is a message-oriented lightweight wrapper around a set of attributes
being exchanged. This wrapper may indicate the purpose of attributes
within the message. Some of the types of messages expected include:
requests for posture information (Request Attributes), posture
information about the endpoint (Posture Attributes), results of an
assessment (Result Attributes), reusable compliance assertions
(Assertion Attributes), and instructions to remediate non-compliant
portions of the endpoint (Remediation Attributes). The PA protocol
also provides the requisite encoding and cryptographic protection for
the Posture Attributes.
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5.2.2. Posture Broker Protocol (PB)
PB is a protocol that carries aggregate attribute messages between
the Posture Collectors on the NEA Client and the corresponding
Posture Validators on the NEA Server involved in a particular
assessment. The PB protocol provides a session allowing for message
dialogs for every assessment. This PB session is then used to bind
multiple Posture Attribute requests and responses from the different
Posture Collectors and Posture Validators involved in a particular
assessment. The PB protocol may also carry the global assessment
decision in the Result Attribute from the Posture Broker Server to
the Posture Broker Client. PB may be used to carry additional types
of messages for use by the Posture Broker Client and Server (e.g.,
information about user preferred interface settings such as
language).
5.2.3. Posture Transport Protocol (PT)
PT is a transport protocol between the NEA Client and the NEA Server
responsible for carrying the messages generated by the PB protocol.
The PT protocol(s) transport(s) PB messages during the network
connection request or after network connectivity has been
established.
In scenarios where an initial assessment needs to occur during the
network connection, the PT protocol (e.g., EAP within 802.1X) may
have constrained use of the network, so deployments may choose to
limit the amount and/or size of the attributes exchanged. The NEA
Client and NEA Server should be able to detect when a potentially
constrained situation exists prior to the assessment based upon
properties of the underlying network protocol. Using this
information, NEA policy could dictate what aspects of the endpoint to
include in the initial assessment and potentially limit the PA
message attributes exchanged. This could be followed up by a full
reassessment after the endpoint is placed on the network.
Alternatively, deployments can choose not to limit their assessment
by configuring their network access technology to temporarily grant
restricted IP connectivity prior to the assessment and use an
unconstrained, high bandwidth IP-based transport during the
assessment. Some of the constraints that may exist for protocols
involved in the network connection phase include:
o Limited maximum transmission unit (MTU) size and ability to
negotiate larger MTUs,
o Inability to perform multiple roundtrips,
o Lack of support for piggybacking attributes for other protocols,
Sangster, et al. Informational [Page 19]RFC 5209 NEA Requirements June 2008
o Low bandwidth or high latency limitations precluding exchanges
of large amounts of data,
o Inability of servers to initiate messages except during the
network connection phase.
The PT protocol selection process needs to consider the impact of
selecting a particular PT and set of underlying protocols on the
deployment needs of PA and PB. PA and PB will be selected prior to
PT so the needs of PA and PB will be known. Certain underlying
protocol stacks may be too constrained to support adequate NEA
assessments during network connection.
The PT protocol provides reliable message delivery, mutual
authentication, and cryptographic protection for the PB messages as
specified by local policy.
5.3. Attributes
The PA protocol is responsible for the exchange of attributes between
a Posture Collector and Posture Validator. The PB protocol may also
carry the global assessment decision attributes from the Posture
Broker Server. Attributes are effectively the reserved word 'nouns'
of the posture assessment. The NEA Server is only able to ask for
information that has a corresponding attribute, thus bounding what
type of posture can be obtained. The NEA WG will define a common
(standard) set of attributes that are expected to be widely
applicable to Posture Collectors and thus used for maximum
interoperability, but Posture Collectors may support additional
vendor-specific attributes when necessary.
Depending on the deployment scenario, the purpose of the attributes
exchanged may be different (e.g., posture information vs. asserted
compliance). This section discusses the originator and expected
situation resulting in the use of each classification of attributes
in a PA message. These classifications are not intended to dictate
how the NEA WG will specify the attributes when defining the
attribute namespace or schema.
Sangster, et al. Informational [Page 20]RFC 5209 NEA Requirements June 2008
5.3.1. Attributes Normally Sent by NEA Client:
o Posture Attributes - Attributes and values sent to report
information about a particular aspect (based on semantic of the
attribute) of the system. These attributes are typically sent
in response to Request Attributes from the NEA Server. For
example, a set of Posture Attributes might describe the status
of the host-based firewall (e.g., if running, vendor, version).
The NEA Server would base its decision on comparing this type of
attribute against policy.
o Assertion Attributes - Attributes stating recent prior
compliance to policy in hopes of avoiding the need to recollect
the posture and send it to the NEA Server. Examples of
assertions include (a) NEA Server provided attributes (state)
describing a prior evaluation (e.g., opaque to endpoint, signed,
time stamped items stating specific results) or (b) NEA Client
identity information used by the NEA Server to locate state
about prior decisions (e.g., system-bound cookie). These might
be returned in lieu of, or in addition to, Posture Attributes.
5.3.2. Attributes Normally Sent by NEA Server:
o Request Attributes - Attributes that define the specific posture
information desired by the NEA Server. These attributes might
effectively form a template that the Posture Collector fills in
(subject to local policy restrictions) with the specific value
corresponding to each attribute. The resulting attributes are
typically Posture or Assertion Attributes from the NEA Client.
o Result Attributes - Attributes that contain the decisions of the
Posture Validators and/or Posture Broker Server. The level of
detail provided may vary from which individual attributes were
compliant or not through just the global assessment decision.
o Remediation Attributes - Attributes that explain to the NEA
Client and its user how to update the endpoint to become
compliant with the NEA Server policies. These attributes are
sent when the global assessment decision was that the endpoint
is not currently compliant. Remediation and Result Attributes
may both exist within a NEA Server attribute message.
o Assertion Attributes - Attributes containing NEA Server
assertions of compliance to a policy for future use by the NEA
Client. See section 5.3.1 for more information.
Sangster, et al. Informational [Page 21]RFC 5209 NEA Requirements June 2008
6. Use Cases
This section discusses several of the NEA use cases with intent to
describe and collectively bound the NEA problem space under
consideration. The use cases provide a context and general rationale
for the defined requirements. In order to ease understanding of each
use case and how it maps to the reference model, each use case will
be accompanied by a simple example and a discussion of how this
example relates to the NEA protocols. It should be emphasized that
the provided examples are not intended to indicate the only approach
to addressing the use case but rather are included to ease
understanding of how the flows might occur and impact the NEA
protocols.
We broadly classify the use cases into two categories, each with its
own set of trigger events:
o Initial assessment - evaluation of the posture of an endpoint
that has not recently been assessed and thus is not in
possession of any valid proof that it should be considered
compliant. This evaluation might be triggered by a request to
join a network, a request to use a service, or a desire to
understand the posture of a system.
o Reassessment - evaluation of the posture of an endpoint that has
previously been assessed. This evaluation could occur for a
variety of reasons including the NEA Client or Server
recognizing an occurrence affecting the endpoint that might
raise the endpoint's risk level. This could be as simple as it
having been a long time since the endpoint's prior reassessment.
6.1. Initial Assessment
An initial assessment occurs when a NEA Client or Server event occurs
that causes the evaluation of the posture of the endpoint for the
first time. Endpoints do not qualify for this category of use case
if they have been recently assessed and the NEA Client or Server has
maintained state (or proof) that the endpoint is compliant and
therefore does not need to have its posture evaluated again.
6.1.1. Triggered by Network Connection or Service Request
This use case focuses on assessments performed at the time an
endpoint attempts to join a network or request use of a service that
requires a posture evaluation. This use case is particularly
interesting because it allows the NEA Server to evaluate the posture
of an endpoint before allowing it access to the network or service.
Sangster, et al. Informational [Page 22]RFC 5209 NEA Requirements June 2008
This approach could be used to help detect endpoints with known
vulnerabilities and facilitate their repair before they are admitted
to the network and potentially exposed to threats on the network.
A variety of types of endpoint actions could result in this class of
assessment. For example, an assessment could be triggered by the
endpoint trying to access a highly protected network service (e.g.,
financial or HR application server) where heightened security
checking is required. A better known example could include
requesting entrance to a network that requires systems to meet
compliance policy. This example is discussed in more detail in the
following section.
6.1.1.1. Example
An IT employee returning from vacation boots his office desktop
computer that generates a request to join the wired enterprise
network. The network's security policy requires the system to
provide posture information in order to determine whether the
desktop's security features are enabled and up to date. The desktop
sends its patch, firewall, and anti-virus posture information. The
NEA Server determines that the system is lacking a recent security
patch designed to fix a serious vulnerability and the system is
placed on a restricted access network. The desktop follows the
provided remediation instructions to download and install the
necessary patch. Later, the desktop requests again to join the
network and this time is provided full access to the enterprise
network after a full assessment.
6.1.1.2. Possible Flows and Protocol Usage
The following describes typical message flows through the NEA
reference model for this example use case:
1. The IT employee's desktop computer connects to the network
through an access gateway in the wired enterprise network.
2. The Posture Broker Server on the NEA Server is instructed to
assess the endpoint joining the wired network.
3. Based upon compliance policy, the Posture Broker Server
contacts the operating system patch, host-based firewall, and
anti-virus Posture Validators to request the necessary posture.
Each Posture Validator creates a PA message containing the
desired attributes to be requested for assessment from the
desktop system.
Sangster, et al. Informational [Page 23]RFC 5209 NEA Requirements June 2008
4. The Posture Broker Server aggregates the PA messages from the
Posture Validators into a PB message. The Posture Broker
Server passes the PB message to the Posture Transport Server
that uses the PT protocol to send the PB message to the NEA
Client on the desktop computer.
5. The Posture Transport Client receives the message from the NEA
Server and passes it to the Posture Broker Client for message
delivery.
6. The Posture Broker Client de-multiplexes the PB message and
delivers the PA messages with the requests for attributes to
the firewall, operating system patch, and anti-virus Posture
Collectors.
7. Each Posture Collector involved consults local privacy policy
to determine what information is allowed to be disclosed and
then returns the requested attributes that are authorized in a
PA message to the Posture Broker Client.
8. The Posture Broker Client aggregates these PA messages into a
single PB message and sends it to the Posture Broker Server
using the Posture Transport Client to Server session.
9. The Posture Transport Server provides the PB message to the
Posture Broker Server that de-multiplexes the message and sends
the appropriate attributes to the corresponding Posture
Validator.
10. Each Posture Validator compares the values of the attributes it
receives with the expected values defined in its policy.
11. The anti-virus and firewall Posture Validators return
attributes to the Posture Broker Server stating the desktop
computer is compliant, but the operating system patch Posture
Validator returns non-compliant. The operating system patch
Posture Validator creates a PA message that contains attributes
with remediation instructions in addition to the attribute
indicating non-compliance result.
12. The Posture Broker Server aggregates the PA messages and sends
them in a PB message to the Posture Broker Client via the
Posture Transport Server and Posture Transport Client.
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13. The Posture Broker Client delivers the PA messages with the
results from the various Posture Validators to the Posture
Collectors including the PA message containing attributes with
remediation instructions to the operating system patch Posture
Collector. This Posture Collector then interacts with the user
to download and install the needed patches, potentially while
the endpoint remains quarantined.
14. Upon completion of the remediation, the above steps 1-10 are
repeated (triggered by the NEA Client repeating its request to
join the network).
15. This time each involved Posture Validator (including the
operating system patch Posture Validator) returns a compliant
status and the Posture Broker Server returns a compliant result
indicating a global success.
16. The Posture Broker Client receives the compliant result and the
IT employee's desktop is now on the network.
6.1.1.3. Impact on Requirements
The following are several different aspects of the use case example
that potentially need to be factored into the requirements.
o Posture assessment before endpoint allowed on network
o Endpoint sends attributes containing posture information
o NEA Server sends remediation instructions
o NEA Client causes a reassessment after remediation
6.1.2. Triggered by Endpoint
This use case highlights that an endpoint (possibly at the request of
a user) may wish to trigger an assessment of its posture to determine
whether its security protective mechanisms are running and up to
date.
6.1.2.1. Example
A student goes to the terminal room to work on a project. The
terminal room contains shared systems owned by the school that are on
the network. These systems have been previously used by other
students so their security posture is unknown. The student wishes to
check whether a system is currently in compliance with the school's
security policies prior to doing work, so she requests a posture
Sangster, et al. Informational [Page 25]RFC 5209 NEA Requirements June 2008
assessment. The NEA Server performs an initial assessment of the
system and determines it is compliant but the anti-virus protection
is not in use. The student receives an advisory response indicating
the system's anti-virus software is turned off but that otherwise it
complies with the school's policy. The student turns on the
anti-virus software, initiates a scan, and upon completion decides to
trust the system with her work.
6.1.2.2. Possible Flows and Protocol Usage
The following describes the message flows through the NEA reference
model for the student using a terminal room shared system example:
1. Student triggers the Posture Broker Client on the computer
system in the terminal room to initiate a posture assessment.
2. The Posture Broker Client establishes a session with the
Posture Broker Server that causes an assessment to be
triggered.
3. The Posture Broker Server detects the new session and consults
policy to determine that Posture Validators to involve in the
assessment. The Posture Broker Server decides to employ
several Posture Validators including the anti-virus Posture
Validator.
4. The Posture Validators involved create PA messages containing
requests for particular attributes containing information about
the desired terminal room computer based on the school's
security policy.
5. The Posture Broker Server assembles a PB message including each
of the PA messages from the Posture Validators.
6. The Posture Transport Server sends the PB message to the
Posture Transport Client where it is passed on to the Posture
Broker Client.
7. The Posture Broker Client on the student's computer
de-multiplexes the PA messages and delivers them to the
corresponding Posture Collectors.
8. The Posture Collectors consult privacy policy to decide what
information to share with the Server. If allowable, the
Collectors each return a PA message containing the requested
posture to the Posture Broker Client.
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9. The Posture Broker Client aggregates the returned PA messages
into a PB message and hands it to the Posture Transport Client
for transmission to the Posture Transport Server.
10. The Posture Broker Server separates and distributes the Posture
Collector PA messages to the associated Posture Validators.
11. The Posture Validators determine whether the attributes
containing the posture included in the PA message are compliant
with their policies and returns a posture assessment decision
to the Posture Broker Server. In this case, the anti-virus
Posture Validator returns a PA message indicating a
non-compliant result because the anti-virus software is not
running and includes attributes describing how to activate the
software.
12. The Posture Broker Server determines the overall compliance
decision based on all of the Validators' assessment results and
sends a PB message containing an attribute expressing the
global assessment decision and the anti-virus Validator's PA
message. In this case, the global assessment decision
indicates the system is compliant (despite the anti-virus
Validator's result) because the Posture Broker Server policy
allowed for the anti-virus to not be running as long as the
system was properly patched and running a firewall (which was
the case according to the other Posture Validators).
13. The Posture Transport Server sends the PB message to the
Posture Transport Client that provides the message to the
Posture Broker Client.
14. The Posture Broker Client on the terminal room computer
examines the PB message's global assessment decision attribute
and reports to the student that the system was deemed to be
compliant, but that an advisory was included.
15. The Posture Broker Client provides the PA message with the
remediation attributes to the anti-virus Posture Collector that
interacts with the user to explain how to turn on anti-virus to
improve the local protections.
16. The student turns on the anti-virus software and on completion
steps 1-10 are repeated.
17. This time the anti-virus Posture Validator returns a success
status and the Posture Broker Server returns a successful
global assessment decision in the PB message.
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18. The Posture Broker Client receives the successful global
assessment decision in the PB message and the student now uses
the computer for her assignment.
6.1.2.3. Impact on Requirements
The following are several different aspects of the use case example
that potentially need to be factored into the requirements.
o Voluntary endpoint requested initial assessment,
o Successful (compliant) global assessment decision included in PB
message with a PA message containing an advisory set of
attributes for remediation.
6.2. Posture Reassessment
Reassessment(s) of endpoints can happen anytime after being admitted
to the network after a successful initial NEA assessment. These
reassessments may be event-based, such as driven by posture changes
detected by the NEA Client, or changes detected by network
infrastructure such as detection of suspicious behavior or network
policy updates on the NEA Server. They may also be periodic (timer-
driven) to reassess the health of the endpoint.
6.2.1. Triggered by NEA Client
This use case allows for software on the endpoint or a user to
determine that a reassessment of the system is required. There are a
variety of reasons why such a reassessment might be beneficial
including: changes in its previously reported posture, detection of
potentially suspicious behavior, or even to enable the system to
periodically poll the NEA Server to assess its condition relative to
the latest policies.
6.2.1.1. Example
The desktops within a company's HR department have a history of poor
security practices and eventual compromise. The HR department
administrator decides to deploy software on each desktop to monitor
the use of security protective mechanisms to assure their use. One
day, an HR person accidentally turns off the desktop firewall. The
monitoring process detects the lack of a firewall and contacts the
NEA Server to request a reassessment of the firewall compliance. The
NEA Server returns a decision that the firewall must be reactivated
to stay on the network. The NEA Client explains the decision to the
user and how to reactivate the firewall. The HR person restarts the
firewall and initiates a request to rejoin the network.
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6.2.1.2. Possible Flows & Protocol Usage
The following describes the message flows through the NEA reference
model for the HR department example:
1. The desktop monitoring software that typically might act as a
Posture Collector triggers the Posture Broker Client to
initiate a posture reassessment. The Posture Broker Client
creates a PB message that contains a PA message indicating the
desktop firewall has been disabled.
2. The Posture Broker Client sends the PB message to the Posture
Broker Server.
3. The Posture Transport Client sends the PB message to the
Posture Transport Server over the PT protocol.
4. The Posture Broker Server receives the PB message and forwards
the PA message to the firewall Posture Validator for
evaluation.
5. The firewall Posture Validator determines that the endpoint is
no longer compliant because its firewall has been disabled.
6. The Posture Validator generates a PA message that contains
attributes indicating a non-compliant posture assessment result
and remediation instructions for how to reactivate the
firewall.
7. The Posture Validator communicates the PA message with the
posture assessment result to the Posture Broker Server to
respond back to the NEA Client.
8. The Posture Broker Server generates a PB message including a
global assessment decision of non-compliant and the PA message
from the firewall Posture Validator.
9. The Posture Transport Server transports the PB message to the
Posture Transport Client where it is passed to the Posture
Broker Client.
10. The Posture Broker Client processes the attribute containing
the global assessment decision received from the NEA Server and
displays the non-compliance messages to the user.
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11. The Posture Broker Client forwards the PA message to the
firewall Posture Collector; the Posture Collector displays the
remediation instructions for how to enable the desktop
firewall.
12. The user is prompted to initiate a reassessment after
completing the remediation.
13. Upon completion of the remediation, the NEA Client reinitiates
a request for reassessment and steps 1-4 are repeated. This
time the firewall Posture Validator determines the endpoint is
compliant and returns a successful posture assessment decision.
14. The Posture Broker Server generates a PB message with a global
assessment decision of compliant and returns this to the NEA
Client.
6.2.1.3. Impact on Requirements
The following are several different aspects of the use case example
that potentially need to be factored into the requirements.
o Voluntary, endpoint (software) initiated posture reassessment
request
o NEA Server requests specific firewall-oriented Posture
Attributes
o NEA Client (firewall Posture Collector) interacts with user to
remediate problem
6.2.2. Triggered by NEA Server
In many cases, especially for reassessment, the NEA Server may
initiate specific or complete reassessment of one or more endpoints
triggered by:
o Time (periodic)
o Event occurrence
o Policy updates
6.2.2.1. Example
An enterprise requires employees on the network to always stay up to
date with security critical operating system patches. A marketing
employee joins the network and performs an initial assessment. The
assessment determines the employee's laptop is compliant. Several
Sangster, et al. Informational [Page 30]RFC 5209 NEA Requirements June 2008
hours later, a major operating system vendor releases a set of
patches preventing a serious vulnerability that is being exploited on
the Internet.
The enterprise administrators make available the patches and change
the network policy to require them to be installed by 5 PM. This
policy change causes the NEA Server to request a reassessment to
determine which endpoints are impacted and lacking the patches. The
marketing employee's laptop is reassessed and determined to need the
patches. A remediation advisory is sent and presented to the
employee explaining how to obtain the patches and that they must be
installed by 5 PM. The marketing employee immediately downloads and
installs the patches and obtains an assertion that all patches are
now installed.
At 5 PM, the enterprise performs another reassessment of all impacted
endpoints to determine if they are now in compliance. The marketing
employee's laptop is reassessed and presents the assertion that it
has the patches installed and thus is determined to be compliant.
6.2.2.2. Possible Flows and Protocol Usage
The following describes the message flows through the NEA reference
model for the above example:
1. Marketing employee joins network and completes an initial
assessment resulting in a compliant decision.
2. The Enterprise Administrator configures an operating system
patch policy indicating that recent patches are required on all
endpoints by 5 PM to prevent serious vulnerabilities.
3. The NEA Server's operating system patch Posture Validator
becomes aware of this policy change and creates a PA message
requesting attributes describing OS patches in use and triggers
the Posture Broker Server to initiate a posture reassessment of
all endpoints connected to the network.
4. The Posture Broker creates a PB message that includes the PA
message from the operating system patch Posture Validator.
5. The Posture Broker Server gradually establishes a session with
each available NEA Client.
6. The Posture Broker Server sends the PB message to the Posture
Broker Client.
Sangster, et al. Informational [Page 31]RFC 5209 NEA Requirements June 2008
7. The Posture Transport Server carries the PB message to the
Posture Transport Client over the PT protocol.
8. The Posture Broker Client receives the PB message and forwards
the PA message to the operating system patch Posture Collector.
9. The operating system patch Posture Collector determines the OS
patches present on the endpoint and if authorized by its
disclosure policy creates a PA message containing the patch
information attributes.
10. The Posture Broker Client sends a PB message that includes the
operating system patch PA message.
11. The Posture Transport Client transports the PB message to the
Posture Transport Server where it is passed to the Posture
Broker Server.
12. The Posture Broker Server receives the PB message and delivers
the PA message to the operating system patch Posture Validator.
13. The operating system patch Posture Validator extracts the
attributes describing the current OS patches from the PA
message and uses the values to determine whether the endpoint
is compliant with the new policy. The Posture Validator
determines that the endpoint is not compliant since it does not
have the new OS patches installed.
14. The Posture Validator generates a PA message that includes
attributes stating the posture assessment decision is
non-compliant and attributes containing the remediation
instructions to enable the endpoint to download the required OS
patches.
15. The Posture Validator communicates the posture assessment
result to the Posture Broker Server along with its PA message.
16. The Posture Broker Server generates a global assessment
decision and sends a PB message with the decision and the
operating system patch Posture Validator's PA message.
17. The Posture Transport Server transports the PB message to the
Posture Transport Client where it is passed to the Posture
Broker Client.
18. The Posture Broker Client processes the Result Attribute
received from the NEA Server and displays the non-compliance
decision to the user.
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19. The Posture Broker Client forwards the PA message containing
the remediation instructions to the operating system patch
Posture Collector; the Posture Collector guides the user with
instructions on how to become compliant that include
downloading the appropriate OS patches to prevent the
vulnerability.
20. The marketing employee installs the required patches and now is
in compliance.
21. The NEA Client triggers a reassessment of the operating system
patches that causes a repeat of many of the steps above. This
time, in step 13 the operating system patch Posture Validator
determines the marketing employee's laptop is compliant. It
returns a reusable (e.g., signed and dated) set of attributes
that assert OS patch compliance to the latest policy. These OS
patch compliance assertions can be used in a future PA message
from the operating system patch Collector instead of
determining and providing the specific patch set posture as
before.
22. This time when the operating system patch Posture Collector
receives the PA message that contains reusable attributes
asserting compliance, it caches those attributes for future
use.
23. Later at 5 PM, the NEA Server triggers a gradual reassessment
to determine compliance to the patch advisory. When the
operating system patch Posture Collector receives the request
for posture information (like in step 9 above) it returns the
cached set of assertions (instead of specific OS patch
information) to indicate that the patches have been installed
instead of determining all the patches that have been installed
on the system.
24. When the operating system patch Posture Validator receives the
PA message containing the assertions, it is able to determine
that they are authentic and acceptable assertions instead of
specific posture. It returns a posture assessment decision of
compliant thus allowing the laptop to remain on the network.
6.2.2.3. Impact on Requirements
The following are several different aspects of the use case example
that potentially need to be factored into the requirements.
o Server-initiated reassessment required due to urgent patch
availability
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o NEA Client submits reusable assertion attributes instead of
posture that patch is installed
o NEA Server capable of recognizing previously issued assertion
attributes are sufficient instead of posture
7. Requirements
This section describes the requirements that will be used by the NEA
WG to assess and compare candidate protocols for PA, PB, and PT.
These requirements frequently express features that a candidate
protocol must be capable of offering so that a deployer can decide
whether to make use of that feature. This section does not state
requirements about what features of each protocol must be used during
a deployment.
For example, a requirement (MUST, SHOULD, or MAY) might exist for
cryptographic security protections to be available from each protocol
but this does not require that a deployer make use of all or even any
of them should they deem their environment to offer other protections
that are sufficient.
7.1. Common Protocol Requirements
The following are the common requirements that apply to the PA, PB,
and PT protocols in the NEA reference model:
C-1 NEA protocols MUST support multiple round trips between the NEA
Client and NEA Server in a single assessment.
C-2 NEA protocols SHOULD provide a way for both the NEA Client and
the NEA Server to initiate a posture assessment or reassessment
as needed.
C-3 NEA protocols including security capabilities MUST be capable of
protecting against active and passive attacks by intermediaries
and endpoints including prevention from replay based attacks.
C-4 The PA and PB protocols MUST be capable of operating over any PT
protocol. For example, the PB protocol must provide a transport
independent interface allowing the PA protocol to operate
without change across a variety of network protocol environments
(e.g., EAP/802.1X, TLS, and Internet Key Exchange Protocol
version 2 (IKEv2)).
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C-5 The selection process for NEA protocols MUST evaluate and prefer
the reuse of existing open standards that meet the requirements
before defining new ones. The goal of NEA is not to create
additional alternative protocols where acceptable solutions
already exist.
C-6 NEA protocols MUST be highly scalable; the protocols MUST
support many Posture Collectors on a large number of NEA Clients
to be assessed by numerous Posture Validators residing on
multiple NEA Servers.
C-7 The protocols MUST support efficient transport of a large number
of attribute messages between the NEA Client and the NEA Server.
C-8 NEA protocols MUST operate efficiently over low bandwidth or
high latency links.
C-9 For any strings intended for display to a user, the protocols
MUST support adapting these strings to the user's language
preferences.
C-10 NEA protocols MUST support encoding of strings in UTF-8 format
[UTF8].
C-11 Due to the potentially different transport characteristics
provided by the underlying candidate PT protocols, the NEA
Client and NEA Server MUST be capable of becoming aware of and
adapting to the limitations of the available PT protocol. For
example, some PT protocol characteristics that might impact the
operation of PA and PB include restrictions on: which end can
initiate a NEA connection, maximum data size in a message or
full assessment, upper bound on number of roundtrips, and
ordering (duplex) of messages exchanged. The selection process
for the PT protocols MUST consider the limitations the candidate
PT protocol would impose upon the PA and PB protocols.
7.2. Posture Attribute (PA) Protocol Requirements
The Posture Attribute (PA) protocol defines the transport and data
model to carry posture and validation information between a
particular Posture Collector associated with the NEA Client and a
Posture Validator associated with a NEA Server. The PA protocol
carries collections of standard attributes and vendor-specific
attributes. The PA protocol itself is carried inside the PB
protocol.
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The following requirements define the desired properties that form
the basis for comparison and evaluation of candidate PA protocols.
These requirements do not mandate the use of these properties, but
merely that the candidate protocols are capable of offering the
property if it should be needed.
PA-1 The PA protocol MUST support communication of an extensible set
of NEA standards defined attributes. These attributes will be
distinguishable from non-standard attributes.
PA-2 The PA protocol MUST support communication of an extensible set
of vendor-specific attributes. These attributes will be
segmented into uniquely identified vendor-specific namespaces.
PA-3 The PA protocol MUST enable a Posture Validator to make one or
more requests for attributes from a Posture Collector within a
single assessment. This enables the Posture Validator to
reassess the posture of a particular endpoint feature or to
request additional posture including from other parts of the
endpoint.
PA-4 The PA protocol MUST be capable of returning attributes from a
Posture Validator to a Posture Collector. For example, this
might enable the Posture Collector to learn the specific reason
for a failed assessment and to aid in remediation and
notification of the system owner.
PA-5 The PA protocol SHOULD provide authentication, integrity, and
confidentiality protection for attributes communicated between a
Posture Collector and Posture Validator. This enables
end-to-end security across a NEA deployment that might involve
traversal of several systems or trust boundaries.
PA-6 The PA protocol MUST be capable of carrying attributes that
contain non-binary and binary data including encrypted content.
7.3. Posture Broker (PB) Protocol Requirements
The PB protocol supports multiplexing of Posture Attribute messages
(based on PA protocol) between the Posture Collectors on the NEA
Client to and from the Posture Validators on the NEA Server (in
either direction).
The PB protocol carries the global assessment decision made by the
Posture Broker Server, taking into account the results of the Posture
Validators involved in the assessment, to the Posture Broker Client.
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The PB protocol also aggregates and transports advisories and
notifications such as remediation instructions (e.g., patch
references) from one or more Posture Validators.
The requirements for the PB protocol are:
PB-1 The PB protocol MUST be capable of carrying attributes from the
Posture Broker Server to the Posture Broker Client. This
enables the Posture Broker Client to learn the posture
assessment decision and if appropriate to aid in remediation and
notification of the endpoint owner.
PB-2 The PB protocol MUST NOT interpret the contents of PA messages
being carried, i.e., the data it is carrying must be opaque to
it.
PB-3 The PB protocol MUST carry unique identifiers that are used by
the Posture Brokers to route (deliver) PA messages between
Posture Collectors and Posture Validators. Such message routing
should facilitate dynamic registration or deregistration of
Posture Collectors and Validators. For example, a dynamically
registered anti-virus Posture Validator should be able to
subscribe to receive messages from its respective anti-virus
Posture Collector on NEA Clients.
PB-4 The PB protocol MUST be capable of supporting a half-duplex PT
protocol. However this does not preclude PB from operating
full-duplex when running over a full-duplex PT.
PB-5 The PB protocol MAY support authentication, integrity and
confidentiality protection for the attribute messages it carries
between a Posture Broker Client and Posture Broker Server. This
provides security protection for a message dialog of the
groupings of attribute messages exchanged between the Posture
Broker Client and Posture Broker Server. Such protection is
orthogonal to PA protections (which are end to end) and allows
for simpler Posture Collector and Validators to be implemented,
and for consolidation of cryptographic operations possibly
improving scalability and manageability.
PB-6 The PB protocol MUST support grouping of attribute messages
optimize transport of messages and minimize round trips.
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7.4. Posture Transport (PT) Protocol Requirements
The Posture Transport (PT) protocol carries PB protocol messages
between the Posture Transport Client and the Posture Transport
Server. PT is responsible for providing a protected transport for
the PB protocol. The PT protocol may itself be transported by one or
more concatenated sessions using lower layer protocols, such as
802.1X, RADIUS [RADIUS], TLS, or IKE.
This section defines the requirements that candidate PT protocols
must be capable of supporting.
PT-1 The PT protocol MUST NOT interpret the contents of PB messages
being transported, i.e., the data it is carrying must be opaque
to it.
PT-2 The PT protocol MUST be capable of supporting mutual
authentication, integrity, confidentiality, and replay
protection of the PB messages between the Posture Transport
Client and the Posture Transport Server.
PT-3 The PT protocol MUST provide reliable delivery for the PB
protocol. This includes the ability to perform fragmentation
and reassembly, detect duplicates, and reorder to provide
in-sequence delivery, as required.
PT-4 The PT protocol SHOULD be able to run over existing network
access protocols such as 802.1X and IKEv2.
PT-5 The PT protocol SHOULD be able to run between a NEA Client and
NEA Server over TCP or UDP (similar to Lightweight Directory
Access Protocol (LDAP)).
8. Security Considerations
This document defines the functional requirements for the PA, PB, and
PT protocols used for Network Endpoint Assessment. As such, it does
not define a specific protocol stack or set of technologies, so this
section will highlight security issues that may apply to NEA in
general or to particular aspects of the NEA reference model.
Note that while a number of topics are outside the scope of the NEA
WG and thus this specification (see section 3.1), it is important
that those mechanisms are protected from attack. For example, the
methods of triggering an assessment or reassessment are out of scope
but should be appropriately protected from attack (e.g., an attacker
hiding the event indicating a NEA Server policy change has occurred).
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NEA intends to facilitate detection and corrective actions for
cooperating endpoints to become compliant with network compliance
policies. For example, it is envisioned that these policies will
allow deployers to detect out-of-date, inactive, or absent security
mechanisms on the endpoint that might leave it more vulnerable to
known attacks. If an endpoint is more vulnerable to compromise, then
it is riskier to have this endpoint present on the network with other
valuable assets. By proactively assessing cooperating endpoints
before their entrance to the network, deployers can improve their
resilience to attack prior to network access. Similarly,
reassessments of cooperating endpoints on the network may be helpful
in assuring that security mechanisms remain in use and are up to date
with the latest policies.
NEA fully recognizes that not all endpoints will be cooperating by
providing their valid posture (or any posture at all). This might
occur if malware is influencing the NEA Client or policies, and thus
a trustworthy assessment isn't possible. Such a situation could
result in the admission of an endpoint that introduces threats to the
network and other endpoints despite passing the NEA compliance
assessment.
8.1. Trust
Network Endpoint Assessment involves assessing the posture of
endpoints entering or already on the network against compliance
policies to assure they are adequately protected. Therefore, there
must be an implied distrusting of endpoints until there is reason to
believe (based on posture information) that they are protected from
threats addressed by compliance policy and can be trusted to not
propagate those threats to other endpoints. On the network provider
side, the NEA Client normally is expected to trust the network
infrastructure systems to not misuse any disclosed posture
information (see section 9) and any remediation instructions provided
to the endpoint. The NEA Client normally also needs to trust that
the NEA Server will only request information required to determine
whether the endpoint is safe to access the network assets.
Between the NEA Client and Server there exists a network that is not
assumed to be trustworthy. Therefore, little about the network is
implicitly trusted beyond its willingness and ability to transport
the exchanged messages in a timely manner. The amount of trust given
to each component of the NEA reference model is deployment specific.
The NEA WG intends to provide security mechanisms to reduce the
amount of trust that must be assumed by a deployer. The following
sections will discuss each area in more detail.
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8.1.1. Endpoint
For NEA to properly operate, the endpoint needs to be trusted to
accurately represent the requested security posture of the endpoint
to the NEA Server. By NEA WG charter, the NEA reference model does
not explicitly specify how to detect or prevent lying endpoints that
intentionally misrepresent their posture. Similarly, the detection
of malware (e.g., root kits) that are able to trick the Posture
Collectors into returning incorrect information is the subject for
research and standardization outside the IETF (e.g., Trusted
Computing Group [TCG]) and is not specifically addressed by the
model. However, if such mechanisms are used in a deployment, the NEA
reference model should be able to accommodate these technologies by
allowing them to communicate over PA to Posture Validators or work
orthogonally to protect the NEA Client from attack and assure the
ability of Posture Collectors to view the actual posture.
Besides having to trust the integrity of the NEA Client and its
ability to accurately collect and report Posture Attributes about the
endpoint, we try to limit other assumed trust. Most of the usage
models for NEA expect the posture information to be sent to the NEA
Server for evaluation and decision making. When PA and/or PT level
security protections are used, the endpoint needs to trust the
integrity and potentially confidentiality of the trust anchor
information (e.g., public key certificates) used by the Posture
Collector and/or Posture Transport Client. However, NEA
implementations may choose to send or pre-provision some policies to
the endpoint for evaluation that would assume more trust in the
endpoint. In this case, the NEA Server must trust the endpoint's
policy storage, evaluation, and reporting mechanisms to not falsify
the results of the posture evaluation.
Generally the endpoint should not trust network communications (e.g.,
inbound connection requests) unless this trust has been specifically
authorized by the user or owner defined policy or action. The NEA
reference model assumes the entire NEA Client is local to the
endpoint. Unsolicited communications originating from the network
should be inspected by normal host-based security protective
mechanisms (e.g., firewalls, security protocols, Intrusion
Detection/Prevention System (IDS/IPS), etc.). Communications
associated with a NEA assessment or reassessment requires some level
of trust particularly when initiated by the NEA Server
(reassessment). The degree of trust can be limited by use of strong
security protections on the messages as dictated by the network
deployer and the endpoint user/owner policy.
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8.1.2. Network Communications
Between the NEA Client and Server, there may exist a variety of types
of devices to facilitate the communication path. Some of the devices
may serve as intermediaries (e.g., simple L2 switches) so they may
have the opportunity to observe and change the message dialogs.
The intermediary devices may fall into a few major categories that
impact our degree of trust in their operation. First, some
intermediary devices may act as message forwarders or carriers for PT
(e.g., L2 switches, L3 routers). For these devices we trust them not
to drop the messages or actively attempt to disrupt (e.g., denial of
service (DoS)) the NEA deployment.
Second, some intermediary devices may be part of the access control
layer of the network and as such, we trust them to enforce policies
including remediation, isolation, and access controls given to them
as a result on a NEA assessment. These devices may also fill other
types of roles described in this section.
Third, some devices may act as a termination point or proxy for the
PT carrier protocol. Frequently, it is expected that the carrier
protocol for PT will terminate on the NEA Client and Server so will
be co-resident with the PT endpoints. If this expectation is not
present in a deployment, we must trust the termination device to
accurately proxy the PT messages without alteration into the next
carrier protocol (e.g., if inner EAP method messages are transitioned
from an EAP [EAP] tunnel to a RADIUS session).
Fourth, many networks include infrastructure such as IDS/IPS devices
that monitor and take corrective action when suspicious behavior is
observed on the network. These devices may have a relationship with
the NEA Server that is not within scope for this specification.
Devices trusted by the NEA Server to provide security information
that might affect the NEA Server's decisions are trusted to operate
properly and not cause the NEA Server to make incorrect decisions.
Finally, other types of intermediary devices may exist on the network
between the NEA Client and Server that are present to service other
network functions beside NEA. These devices might be capable of
passively eavesdropping on the network, archiving information for
future purposes (e.g., replay or privacy invasion), or more actively
attacking the NEA protocols. Because these devices do not play a
role in facilitating NEA, it is essential that NEA deployers not be
forced to trust them for NEA to reliably operate. Therefore, it is
required that NEA protocols offer security protections to assure
these devices can't steal, alter, spoof or otherwise damage the
reliability of the message dialogs.
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8.1.3. NEA Server
The NEA Server (including potentially remote systems providing
posture validation services) is generally trusted to apply the
specified assessment policies and must be protected from compromise.
It is essential that NEA Server deployments properly safeguard these
systems from a variety of attacks from the network and endpoints to
assure their proper operation.
While there is a need to trust the NEA Server operation to some
degree, rigorous security architecture, analysis, monitoring, and
review should assure its network footprint and internal workings are
protected from attack. The network footprint would include
communications over the network that might be subject to attack such
as policy provisioning from the policy authoring systems and general
security and system management protocols. Some examples of internal
workings include protections from malware attacking the intra-NEA
Server communications, NEA Server internal logic, or policy stores
(particularly those that would change the resulting decisions or
enforcements). The NEA Server needs to trust the underlying NEA and
lower layer network protocols to properly behave and safeguard the
exchanged messages with the endpoint. The NEA reference model does
not attempt to address integrity protection of the operating system
or other software supporting the NEA Server.
One interesting example is where some components of the NEA Server
physically reside in different systems. This might occur when a
Posture Validator (or a remote backend server used by a local Posture
Validator) exists on another system from the Posture Broker Server.
Similarly, the Posture Broker Server might exist on a separate system
from the Posture Transport Server. When there is a physical
separation, the communications between the remote components of the
NEA Server must ensure that the PB session and PA message dialogs are
resistant to active and passive attacks, in particular, guarded
against eavesdropping, forgery and replay. Similarly, the Posture
Validators may also wish to minimize their trust in the Posture
Broker Server beyond its ability to properly send and deliver PA
messages. The Posture Validators could employ end-to-end PA security
to verify the authenticity and protect the integrity and/or
confidentiality of the PA messages exchanged.
When PA security is used, each Posture Validator must be able to
trust the integrity and potentially confidentiality of its trust
anchor policies.
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8.2. Protection Mechanisms at Multiple Layers
Inherent in the requirements is a desire for NEA candidate protocols
throughout the reference model to be capable of providing strong
security mechanisms as dictated by the particular deployment. In
some cases, these mechanisms may appear to provide overlapping or
redundant protections. These apparent overlaps may be used in
combination to offer a defense in depth approach to security.
However, because of the layering of the protocols, each set of
protections offers slightly different benefits and levels of
granularity.
For example, a deployer may wish to encrypt traffic at the PT layer
to protect against some forms of traffic analysis or interception by
an eavesdropper. Additionally, the deployer may also selectively
encrypt messages containing the posture of an endpoint to achieve
end-to-end confidentiality to its corresponding Posture Validator.
In particular, this might be desired when the Posture Validator is
not co-located with the NEA Server so the information will traverse
additional network segments after the PT protections have been
enforced or so that the Posture Validator can authenticate the
corresponding Posture Collector (or vice versa).
Different use cases and environments for the NEA technologies will
likely influence the selection of the strength and security
mechanisms employed during an assessment. The goal of the NEA
requirements is to encourage the selection of technologies and
protocols that are capable of providing the necessary protections for
a wide variety of types of assessment.
8.3. Relevant Classes of Attack
A variety of attacks are possible against the NEA protocols and
assessment technologies. This section does not include a full
security analysis, but wishes to highlight a few attacks that
influenced the requirement definition and should be considered by
deployers selecting use of protective mechanisms within the NEA
reference model.
As discussed, there are a variety of protective mechanisms included
in the requirements for candidate NEA protocols. Different use cases
and environments may cause deployers to decide not to use some of
these mechanisms; however, this should be done with an understanding
that the deployment may become vulnerable to some classes of attack.
As always, a balance of risk vs. performance, usability,
manageability, and other factors should be taken into account.
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The following types of attacks are applicable to network protocols
defined in the reference model and thus should be considered by
deployers.
8.3.1. Man-in-the-Middle (MITM)
MITM attacks against a network protocol exist when a third party can
insert itself between two communicating entities without detection
and gain benefit from involvement in their message dialog. For
example, a malware infested system might wish to join the network
replaying posture observed from a clean endpoint entering the
network. This might occur by the system inserting itself into and
actively proxying an assessment message dialog. The impact of the
damage caused by the MITM can be limited or prevented by selection of
appropriate protocol protective mechanisms.
For example, the requirement for PT to be capable of supporting
mutual authentication prior to any endpoint assessment message
dialogs prevents the attacker from inserting itself as an active
participant (proxy) within the communications without detection
(assuming the attacker lacks credentials convincing either party it
is legitimate). Reusable credentials should not be exposed on the
network to assure the MITM doesn't have a way to impersonate either
party. The PT requirement for confidentiality-protected (encrypted)
communications linked to the above authentication prevents a passive
MITM from eavesdropping by observing the message dialog and keeping a
record of the conformant posture values for future use. The PT
requirement for replay prevention stops a passive MITM from later
establishing a new session (or hijacking an existing session) and
replaying previously observed message dialogs.
If a non-compliant, active MITM is able to trick a clean endpoint to
give up its posture information, and the MITM has legitimate
credentials, it might be able to appear to a NEA Server as having
compliant posture when it does not. For example, a non-compliant
MITM could connect and authenticate to a NEA Server and as the NEA
Server requests posture information, the MITM could request the same
posture from the clean endpoint. If the clean endpoint trusts the
MITM to perform a reassessment and is willing to share the requested
posture, the MITM could obtain the needed posture from the clean
endpoint and send it to the NEA Server. In order to address this
form of MITM attack, the NEA protocols would need to offer a strong
(cryptographic) binding between the posture information and the
authenticated session to the NEA Server so the NEA Server knows the
posture originated from the endpoint that authenticated. Such a
strong binding between the posture's origin and the authenticating
endpoint may be feasible so should be preferred by the NEA WG.
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8.3.2. Message Modification
Without message integrity protection, an attacker capable of
intercepting a message might be capable of modifying its contents and
causing an incorrect decision to be made. For example, the attacker
might change the Posture Attributes to always reflect incorrect
values and thus prevent a compliant system from joining the network.
Unless the NEA Server could detect this change, the attacker could
prevent admission to large numbers of clean systems. Conversely, the
attacker could allow a malware infested machine to be admitted by
changing the sent Posture Attributes to reflect compliant values,
thus hiding the malware from the Posture Validator. The attacker
could also infect compliant endpoints by sending malicious
remediation instructions that, when performed, would introduce
malware on the endpoint or deactivate security mechanisms.
In order to protect against such attacks, the PT includes a
requirement for strong integrity protection (e.g., including a
protected hash like a Hashed Message Authentication Code (HMAC)
[HMAC] of the message) so any change to a message would be detected.
PA includes a similar requirement to enable end-to-end integrity
protection of the attributes, extending the protection all the way to
the Posture Validator even if it is located on another system behind
the NEA Server.
It is important that integrity protection schemes leverage fresh
secret information (not known by the attacker) that is bound to the
authenticated session such as an HMAC using a derived fresh secret
associated with the session. Inclusion of freshness information
allows the parties to protect against some forms of message replay
attacks using secret information from prior sessions.
8.3.3. Message Replay or Attribute Theft
An attacker might listen to the network, recording message dialogs or
attributes from a compliant endpoint for later reuse to the same NEA
Server or just to build an inventory of software running on other
systems watching for known vulnerabilities. The NEA Server needs to
be capable of detecting the replay of posture and/or the model must
assure that the eavesdropper cannot obtain the information in the
first place. For this reason, the PT protocol is required to provide
confidentiality and replay prevention.
The cryptographic protection from disclosure of the PT, PB, or PA
messages prevents the passive listener from observing the exchanged
messages and thus prevents theft of the information for future use.
However, an active attacker might be able to replay the encrypted
message if there is no strong link to the originating party or
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session. By linking the encrypted message dialog to the
authentication event and leveraging per-transaction freshness and
keying exchanges, this prevents a replay of the encrypted
transaction.
8.3.4. Other Types of Attack
This section doesn't claim to present an exhaustive list of attacks
against the NEA reference model. Several types of attack will become
easier to understand and analyze once the NEA WG has created
specifications describing the specific selected technologies and
protocols to be used within NEA. One such area is Denial of Service
(DoS). At this point in time, it is not practical to try to define
all of the potential exposures present within the NEA protocols, so
such an analysis should be included in the Security Considerations
sections of the selected NEA protocols.
However, it is important that the NEA Server be resilient to DoS
attacks as an outage might affect large numbers of endpoints wishing
to join or remain on the network. The NEA reference model expects
that the PT protocol would have some amount of DoS resilience and
that the PA and PB protocols would need to build upon that base with
their own protections. To help narrow the window of attack by
unauthenticated parties, it is envisioned that NEA Servers would
employ PT protocols that enable an early mutual authentication of the
requesting endpoint as one technique for filtering out attacks.
Attacks occurring after the authentication would at least come from
sources possessing valid credentials and could potentially be held
accountable. Similarly, NEA protocols should offer strong replay
protection to prevent DoS-based attacks based on replayed sessions
and messages. Posture assessment should be strongly linked with the
Posture Transport authentications that occurred to assure the posture
came from the authenticated party. Cryptographic mechanisms and
other potentially resource intensive operations should be used
sparingly until the validity of the request can be established. This
and other resource/protocol based attacks can be evaluated once the
NEA technologies and their cryptographic use have been selected.
9. Privacy Considerations
While there are a number of beneficial uses of the NEA technology for
organizations that own and operate networks offering services to
similarly owned endpoints, these same technologies might enhance the
potential for abuse and invasion of personal privacy if misused.
This section will discuss a few of the potential privacy concerns
raised by the deployment of this technology and offer some guidance
to implementers.
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The NEA technology enables greater visibility into the configuration
of an endpoint from the network. Such transparency enables the
network to take into consideration the strength of the endpoint's
security mechanisms when making access control decisions to network
resources. However, this transparency could also be used to enforce
restrictive policies to the detriment of the user by limiting their
choice of software or prying into past or present uses of the
endpoint.
The scope of the NEA WG was limited to specifying protocols targeting
the use cases where the endpoints and network are owned by the same
party or the endpoint owner has established a clear expectation of
disclosure/compliance with the network owner. This is a familiar
model for governments, institutions, and a wide variety of
enterprises that provide endpoints to their employees to perform
their jobs. In many of these situations, the endpoint is purchased
and owned by the enterprise and they often reserve the right to audit
and possibly dictate the allowable uses of the device. The NEA
technologies allow them to automate the inspection of the contents of
an endpoint and this information may be linked to the access control
mechanisms on the network to limit endpoint use should the endpoint
not meet minimal compliance levels.
In these environments, the level of personal privacy the employee
enjoys may be significantly reduced subject to local laws and
customs. However, in situations where the endpoint is owned by the
user or where local laws protect the rights of the user even when
using endpoints owned by another party, it is critical that the NEA
implementation enable the user to control what endpoint information
is shared with the network. Such controls imposed by the user might
prevent or limit their ability to access certain networks or
protected resources, but this must be a user choice.
9.1. Implementer Considerations
The NEA WG is not defining NEA Client policy content standards nor
defining requirements on aspects of an implementation outside of the
network protocols; however, the following guidance is provided to
encourage privacy friendly implementations for broader use than just
the enterprise-oriented setting described above.
NEA Client implementations are encouraged to offer an opt-in policy
to users prior to sharing their endpoint's posture information. The
opt-in mechanism should be on a per-user, per-NEA Server basis so
each user can control which networks can access any posture
information on their system. For those networks that are allowed to
assess the endpoint, the user should be able to specify granular
restrictions on what particular types and specific attributes Posture
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Collectors are allowed to disclose. Posture Validator
implementations are discouraged from having the default behavior of
using wild carded requests for posture potentially leading to
overexposure of information (see section 9.2). Instead Posture
Validators, by default, should only request the specific attributes
that are required to perform their assessment.
Requests for attributes that are not explicitly allowed (or
specifically disallowed) to be shared should result in a user
notification and/or log record so the user can assess whether the
service is doing something undesirable or whether the user is willing
to share this additional information in order to gain access. Some
products might consider policy-driven support for prompting the user
for authorization with a specific description of the posture
information being requested prior to sending it to the NEA Server.
It is envisioned that the owner of the endpoint is able to specify
disclosure policies that may override or influence the user's
policies on the attributes visible to the network. If the owner
disclosure policy allows for broader posture availability than the
user policy, the implementation should provide a feedback mechanism
to the user so they understand the situation and can choose whether
to use the endpoint in those circumstances.
In such a system, it is important that the user's policy authoring
interface is easy to understand and clearly articulates the current
disclosure policy of the system including any influences from the
owner policy. Users should be able to understand what posture is
available to the network and the general impact of this information
being known. In order to minimize the list of restrictions
enumerated, use of a conservative default disclosure policy such as
"that which is not explicitly authorized for disclosure is not
allowed" might make sense to avoid unintentional leakage of
information.
NEA Server implementations should provide newly subscribing endpoints
with a disclosure statement that clearly states:
o What information is required
o How this information will be used and protected
o What local privacy policies are applicable
This information will empower subscribing users to decide whether the
disclosure of this information is acceptable considering local laws
and customs.
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9.2. Minimizing Attribute Disclosure
One important issue in the design of the NEA reference model and
protocols is enabling endpoints to disclose minimal information
required to establish compliance with network policies. There are
several models that could be considered as to how the disclosed
attribute set is established. Each model has privacy related
benefits and issues that should be considered by product developers.
This section summarizes three potential models for how attribute
disclosure might be provided within NEA products and some privacy
implications potentially associated with each model.
The first model is easy to implement and deploy but has privacy and
potentially latency and scalability implications. This approach
effectively defaults the local policy to send all known NEA Posture
Attributes when an assessment occurs. While this might simplify
deployment, it exposes a lot of information that is potentially not
relevant to the security assessment of the system and may introduce
privacy issues. For example, is it really important that the
enterprise know whether Firefox is being used on a system instead of
other browsers during the security posture assessment?
The second model involves an out-of-band provisioning of the
disclosure policy to all endpoints. This model may involve the
enterprise establishing policy that a particular list of attributes
must be provided when a NEA exchange occurs. Endpoint privacy policy
may filter this attribute list, but such changes could cause the
endpoint not to be given network or resource access. This model
simplifies the network exchange as the endpoint always sends the
filtered list of attributes when challenged by a particular network.
However, this approach requires an out-of-band management protocol to
establish and manage the NEA disclosure policies of all systems.
The third model avoids the need for pre-provisioning of a disclosure
policy by allowing the NEA Server to specifically request what
attributes are required. This is somewhat analogous to the policy
being provisioned during the NEA exchanges so is much easier to
manage. This model allows for the NEA Server to iteratively ask for
attributes based on the values of prior attributes. Note, even in
this model the NEA protocols are not expected to be a general purpose
query language, but rather allow the NEA Server to request specific
attributes as only the defined attributes are possible to request.
For example, an enterprise might ask about the OS version in the
initial message dialog and after learning the system is running Linux
ask for a different set of attributes specific to Linux than it would
if the endpoint was a Windows system. It is envisioned that this
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approach might minimize the set of attributes sent over the network
if the assessment is of a complex system (such as trying to
understand what patches are missing from an OS).
In each model, the user could create a set of per-network privacy
filter policies enforced by the NEA Client to prevent the disclosure
of attributes felt to be personal in nature or not relevant to a
particular network. Such filters would protect the privacy of the
user but might result in the user not being allowed access to the
desired asset (or network) or being provided limited access.
10. References
10.1. Normative References
[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
10.2. Informative References
[802.1X] IEEE Standards for Local and Metropolitan Area Networks:
Port based Network Access Control, IEEE Std 802.1X-2001,
June 2001.
[CNAC] Cisco, Cisco's Network Admission Control Main Web Site,
http://www.cisco.com/go/nac
[EAP] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[IPSEC] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[NAP] Microsoft, Network Access Protection Main Web Site,
http://www.microsoft.com/nap
[RADIUS] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865,
June 2000.
Sangster, et al. Informational [Page 50]RFC 5209 NEA Requirements June 2008
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[TCG] Trusted Computing Group, Main TCG Web Site,
http://www.trustedcomputinggroup.org/
[TNC] Trusted Computing Group, Trusted Network Connect Main Web
Site, https://www.trustedcomputinggroup.org/groups/network/
11. Acknowledgments
The authors of this document would like to acknowledge the NEA
Working Group members who have contributed to previous requirements
and problem statement documents that influenced the direction of this
specification: Kevin Amorin, Parvez Anandam, Diana Arroyo, Uri
Blumenthal, Alan DeKok, Lauren Giroux, Steve Hanna, Thomas Hardjono,
Tim Polk, Ravi Sahita, Joe Salowey, Chris Salter, Mauricio Sanchez,
Yaron Sheffer, Jeff Six, Susan Thompson, Gary Tomlinson, John
Vollbrecht, Nancy Winget, Han Yin, and Hao Zhou.
Sangster, et al. Informational [Page 51]RFC 5209 NEA Requirements June 2008
Authors' Addresses
Paul Sangster
Symantec Corporation
6825 Citrine Dr
Carlsbad, CA 92009 USA
Phone: +1 760 438-5656
EMail: Paul_Sangster@symantec.com
Hormuzd Khosravi
Intel
2111 NE 25th Avenue
Hillsboro, OR 97124 USA
Phone: +1 503 264 0334
EMail: hormuzd.m.khosravi@intel.com
Mahalingam Mani
Avaya Inc.
1033 McCarthy Blvd.
Milpitas, CA 95035 USA
Phone: +1 408 321-4840
EMail: mmani@avaya.com
Kaushik Narayan
Cisco Systems Inc.
10 West Tasman Drive
San Jose, CA 95134
Phone: +1 408 526-8168
EMail: kaushik@cisco.com
Joseph Tardo
Nevis Networks
295 N. Bernardo Ave., Suite 100
Mountain View, CA 94043 USA
EMail: joseph.tardo@nevisnetworks.com
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Sangster, et al. Informational [Page 53]
