Internet Engineering Task Force (IETF) A. Popov
Request for Comments: 8473 M. Nystroem
Category: Standards Track Microsoft Corp.
ISSN: 2070-1721 D. Balfanz, Ed.
N. Harper
Google Inc.
J. Hodges
Kings Mountain Systems
October 2018
Token Binding over HTTP
Abstract
This document describes a collection of mechanisms that allow HTTP
servers to cryptographically bind security tokens (such as cookies
and OAuth tokens) to TLS connections.
We describe both first-party and federated scenarios. In a first-
party scenario, an HTTP server is able to cryptographically bind the
security tokens that it issues to a client -- and that the client
subsequently returns to the server -- to the TLS connection between
the client and the server. Such bound security tokens are protected
from misuse, since the server can generally detect if they are
replayed inappropriately, e.g., over other TLS connections.
Federated Token Bindings, on the other hand, allow servers to
cryptographically bind security tokens to a TLS connection that the
client has with a different server than the one issuing the token.
This document is a companion document to "The Token Binding Protocol
Version 1.0" (RFC 8471).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8473.
Popov, et al. Standards Track [Page 1]RFC 8473 Token Binding over HTTP October 2018
Copyright Notice
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The Sec-Token-Binding HTTP Request Header Field . . . . . . . 4
2.1. HTTPS Token Binding Key-Pair Scoping . . . . . . . . . . 5
3. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . . . 6
4. First-Party Use Cases . . . . . . . . . . . . . . . . . . . . 7
5. Federation Use Cases . . . . . . . . . . . . . . . . . . . . 7
5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. HTTP Redirects . . . . . . . . . . . . . . . . . . . . . 10
5.4. Negotiated Key Parameters . . . . . . . . . . . . . . . . 12
5.5. Federation Example . . . . . . . . . . . . . . . . . . . 13
6. Implementation Considerations . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7.1. Security Token Replay . . . . . . . . . . . . . . . . . . 16
7.2. Sensitivity of the Sec-Token-Binding Header . . . . . . . 16
7.3. Securing Federated Sign-On Protocols . . . . . . . . . . 17
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
8.1. Scoping of Token Binding Key Pairs . . . . . . . . . . . 20
8.2. Lifetime of Token Binding Key Pairs . . . . . . . . . . . 20
8.3. Correlation . . . . . . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . 23
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
Popov, et al. Standards Track [Page 2]RFC 8473 Token Binding over HTTP October 2018
1. Introduction
The Token Binding protocol [RFC8471] defines a Token Binding ID for a
TLS connection between a client and a server. The Token Binding ID
of a TLS connection is constructed using the public key of a
private-public key pair. The client proves possession of the
corresponding private key. This Token Binding key pair is
long-lived. That is, subsequent TLS connections between the same
client and server have the same Token Binding ID, unless specifically
reset, e.g., by the user. When issuing a security token (e.g., an
HTTP cookie or an OAuth token [RFC6749]) to a client, the server can
include the Token Binding ID in the token, thus cryptographically
binding the token to TLS connections between that particular client
and server, and inoculating the token against abuse (reuse, attempted
impersonation, etc.) by attackers.
While the Token Binding protocol [RFC8471] defines a message format
for establishing a Token Binding ID, it does not specify how this
message is embedded in higher-level protocols. The purpose of this
specification is to define how TokenBindingMessages are embedded in
HTTP (both versions 1.1 [RFC7230] and 2 [RFC7540]). Note that
TokenBindingMessages are only defined if the underlying transport
uses TLS. This means that Token Binding over HTTP is only defined
when HTTP is layered on top of TLS (commonly referred to as HTTPS
[RFC2818]).
HTTP clients establish a Token Binding ID with a server by including
a special HTTP header field in HTTP requests. The HTTP header field
value is a base64url-encoded TokenBindingMessage.
A TokenBindingMessage allows a client to establish multiple Token
Binding IDs with the server by including multiple TokenBinding
structures. By default, a client will establish a Provided Token
Binding ID with the server, indicating a Token Binding ID that the
client will persistently use with the server. Under certain
conditions, the client can also include a Referred Token Binding ID
in the TokenBindingMessage, indicating a Token Binding ID that the
client is using with a different server than the one that the
TokenBindingMessage is sent to. This is useful in federation
scenarios.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Popov, et al. Standards Track [Page 3]
RFC 8473 Token Binding over HTTP October 2018
2. The Sec-Token-Binding HTTP Request Header Field
Once a client and server have negotiated the Token Binding protocol
with HTTP/1.1 or HTTP/2 (see [RFC8471] and [RFC8472]), clients MUST
include a Sec-Token-Binding header field in their HTTP requests and
MUST include only one such header field per HTTP request. Also, the
Sec-Token-Binding header field MUST NOT be included in HTTP
responses. The ABNF of the Sec-Token-Binding header field is (per
the style of [RFC7230]; see also Section 8.3 of [RFC7231]):
Sec-Token-Binding = EncodedTokenBindingMessage
The header field name is Sec-Token-Binding, and its single value,
EncodedTokenBindingMessage, is a base64url encoding of a single
TokenBindingMessage, as defined in [RFC8471]. The base64url encoding
uses the URL and filename safe character set described in Section 5
of [RFC4648], with all trailing padding characters (i.e., "=")
omitted and without the inclusion of any line breaks, whitespace, or
other additional characters.
For example:
Sec-Token-Binding: AIkAAgBBQFzK4_bhAqLDwRQxqJWte33d7hZ0hZWHwk-miKPg4E\
9fcgs7gBPoz-9RfuDfN9WCw6keHEw1ZPQMGs9CxpuHm-YAQM_j\
aOwwej6a-cQBGU7CJpUHOvXG4VvjNq8jDsvta9Y8_bPEPj25Gg\
mKiPjhJEtZA6mJ_9SNifLvVBTi7fR9wSAAAA
(Note that the backslashes and line breaks are provided to ease
readability; they are not part of the actual encoded message.)
If the server receives more than one Sec-Token-Binding header field
in an HTTP request, then the server MUST reject the message with a
400 (Bad Request) HTTP status code. Additionally, the
Sec-Token-Binding header field:
o SHOULD NOT be stored by origin servers on PUT requests,
o MAY be listed by a server in a Vary response header field, and
o MUST NOT be used in HTTP trailers.
The TokenBindingMessage MUST contain exactly one TokenBinding
structure with a TokenBindingType value of provided_token_binding,
which MUST be signed with the Token Binding private key used by the
client for connections between itself and the server that the HTTP
request is sent to (clients use different Token Binding key pairs for
different servers; see Section 2.1 below). The Token Binding ID
Popov, et al. Standards Track [Page 4]RFC 8473 Token Binding over HTTP October 2018
established by this TokenBinding is called a "Provided Token
Binding ID".
The TokenBindingMessage MAY also contain exactly one TokenBinding
structure with a TokenBindingType value of referred_token_binding, as
specified in Section 5.3. In addition to the latter, or rather than
the latter, the TokenBindingMessage MAY contain other TokenBinding
structures. This is specific to the use case in question; such use
cases are outside the scope of this specification.
A TokenBindingMessage is validated by the server as described in
Section 4.2 ("Server Processing Rules") of [RFC8471]. If validation
fails and a Token Binding is rejected, any associated bound tokens
MUST also be rejected by the server. HTTP requests containing
invalid tokens MUST be rejected. In this case, the server
application MAY return HTTP status code 400 (Bad Request) or proceed
with an application-specific "invalid token" response (e.g.,
directing the client to re-authenticate and present a different
token), or terminate the connection.
In HTTP/2, the client SHOULD use header compression [RFC7541] to
avoid the overhead of repeating the same header field in subsequent
HTTP requests.
2.1. HTTPS Token Binding Key-Pair Scoping
HTTPS is used in conjunction with various application protocols and
application contexts, in various ways. For example, general-purpose
web browsing is one such HTTP-based application context. Within that
context, HTTP cookies [RFC6265] are typically utilized for state
management, including client authentication. A related, though
distinct, example of other HTTP-based application contexts is where
OAuth tokens [RFC6749] are utilized to manage authorization for
third-party application access to resources. The token-scoping rules
of these two examples can differ: the scoping rules for cookies are
concisely specified in [RFC6265], whereas OAuth is a framework and
defines various token types with various scopings, some of which are
determined by the encompassing application.
The scoping of Token Binding key pairs generated by web browsers for
the purpose of binding HTTP cookies MUST be no wider than the
granularity of a "registered domain" (also known as "effective
top-level domain + 1", or "eTLD+1"). An origin's "registered domain"
is the origin's host's public suffix plus the label to its left
(where the term "public suffix" is defined in the "NOTE:" paragraph
in Section 5.3 of [RFC6265] as "a domain that is controlled by a
public registry"). For example, for "https://www.example.com", the
public suffix (eTLD) is "com", and the registered domain (eTLD+1) is
Popov, et al. Standards Track [Page 5]RFC 8473 Token Binding over HTTP October 2018
"example.com". User Agents SHOULD use an up-to-date public suffix
list, such as the one maintained by Mozilla [PSL].
This means that in practice the scope of a Token Binding key pair is
no larger than the scope of a cookie allowed by a web browser. If a
web browser restricts cookies to a narrower scope than registered
domains, the scope of Token Binding key pairs MAY also be narrower.
This applies to the use of Token Binding key pairs in first-party use
cases, as well as in federation use cases defined in this
specification (Section 5).
Key pairs used to bind other application tokens, such as OAuth tokens
or "OpenID Connect" ID Tokens [OpenID.Core], SHOULD adhere to the
above eTLD+1 scoping requirement for those tokens being employed in
first-party or federation scenarios. Applications other than web
browsers MAY use different key-pair scoping rules. See also
Section 8.1 below.
Scoping rules for other HTTP-based application contexts are outside
the scope of this specification.
3. TLS Renegotiation
Token Binding over HTTP/1.1 [RFC7230] can be performed in combination
with TLS renegotiation. In this case, renegotiation MUST only occur
between a client's HTTP request and the server's response, the client
MUST NOT send any pipelined requests, and the client MUST NOT
initiate renegotiation. (That is, the client may only send a
renegotiation ClientHello in response to the server's HelloRequest.)
These conditions ensure that both the client and the server can
clearly identify which TLS Exported Keying Material value [RFC5705]
to use when generating or verifying the TokenBindingMessage. This
also prevents a TokenBindingMessage from being split across TLS
renegotiation boundaries due to TLS message fragmentation; see
Section 6.2.1 of [RFC5246].
(Note: This document deals with TLS 1.2 and therefore refers to
RFC 5246 (which has been obsoleted by RFC 8446); [TOKENBIND-TLS13]
addresses Token Binding in TLS 1.3.)
Popov, et al. Standards Track [Page 6]
RFC 8473 Token Binding over HTTP October 2018
4. First-Party Use Cases
In a first-party use case (also known as a "same-site" use case), an
HTTP server issues a security token such as a cookie (or similar) to
a client and expects the client to return the security token at a
later time, e.g., in order to authenticate. Binding the security
token to the TLS connection between the client and the server
protects the security token from misuse, since the server can detect
if the security token is replayed inappropriately, e.g., over other
TLS connections.
See Section 5 of [RFC8471] for general guidance regarding the binding
of security tokens and their subsequent validation.
5. Federation Use Cases
5.1. Introduction
For privacy reasons, clients use different Token Binding key pairs to
establish Provided Token Binding IDs with different servers. As a
result, a server cannot bind a security token (such as an OAuth token
or an OpenID Connect ID Token [OpenID.Core]) to a TLS connection that
the client has with a different server. This is, however, a common
requirement in federation scenarios: for example, an Identity
Provider may wish to issue an identity token to a client and
cryptographically bind that token to the TLS connection between the
client and a Relying Party.
In this section, we describe mechanisms to achieve this. The common
idea among these mechanisms is that a server (called the "Token
Consumer" in this document) signals to the client that it should
reveal the Provided Token Binding ID that is used between the client
and itself to another server (called the "Token Provider" in this
document). Also common across the mechanisms is how the Token
Binding ID is revealed to the Token Provider: the client uses the
Token Binding protocol [RFC8471] and includes a TokenBinding
structure in the Sec-Token-Binding HTTP header field defined above.
What differs between the various mechanisms is how the Token Consumer
signals to the client that it should reveal the Token Binding ID to
the Token Provider. Below, we specify one such mechanism, which is
suitable for redirect-based interactions between Token Consumers and
Token Providers.
Popov, et al. Standards Track [Page 7]
RFC 8473 Token Binding over HTTP October 2018
Client Token Consumer Token Provider
+--------+ +----+ +-----+
| Client | | TC | | TP |
+--------+ +----+ +-----+
| | |
| | |
| | |
| Client interacts w/TC | |
| using TokenBindingID TBID1: | |
| TBMSG[[provided_token_binding,| |
| TBID1, signature]] | |
|------------------------------>| |
| | |
| Client interacts w/TP |
| using TokenBindingID TBID2: |
| TBMSG[[provided_token_binding, |
| TBID2, signature]] |
|----------------------------------------------------->|
| |
| | |
| TC signals permission to | |
| reveal TBID1 to TP | |
|<------------------------------| |
| | |
| |
| Client interacts w/TP |
| using TokenBindingID TBID1 and TBID2: |
| TBMSG[[provided_token_binding, |
| TBID2, signature], |
| [referred_token_binding, |
| TBID1, signature]] |
|----------------------------------------------------->|
| |
| | |
| | |
5.2. Overview
In a federated sign-on protocol, an Identity Provider issues an
identity token to a client, which sends the identity token to a
Relying Party to authenticate itself. Examples of this include
OpenID Connect (in which the identity token is called an "ID Token")
and the Security Assertion Markup Language (SAML)
[OASIS.saml-core-2.0-os] (in which the identity token is a SAML
assertion).
Popov, et al. Standards Track [Page 8]RFC 8473 Token Binding over HTTP October 2018
To better protect the security of the identity token, the Identity
Provider may wish to bind the identity token to the TLS connection
between the client and the Relying Party, thus ensuring that only
said client can use the identity token. The Relying Party will
compare the Token Binding ID (or a cryptographic hash of it) in the
identity token with the Token Binding ID (or a hash thereof) of the
TLS connection between this Relying Party and the client.
This is an example of a federation scenario, which more generally can
be described as follows:
o A Token Consumer causes the client to issue a token request to the
Token Provider. The goal is for the client to obtain a token and
then use it with the Token Consumer.
o The client delivers the token request to the Token Provider.
o The Token Provider issues the token. The token is issued for the
specific Token Consumer who requested it (thus preventing
malicious Token Consumers from using tokens with other Token
Consumers). The token is, however, typically a bearer token,
meaning that any client can use it with the Token Consumer -- not
just the client to which it was issued.
o Therefore, in the previous step, the Token Provider may want to
include in the token the Token Binding ID (or a cryptographic hash
of it) that the client uses when communicating with the Token
Consumer, thus binding the token to the client's Token Binding key
pair. The client proves possession of the private key when
communicating with the Token Consumer through the Token Binding
protocol [RFC8471] and uses the corresponding public key of this
key pair as a component of the Token Binding ID. Comparing the
Token Binding ID from the token to the Token Binding ID
established with the client allows the Token Consumer to verify
that the token was sent to it by the legitimate client.
o To allow the Token Provider to include the Token Binding ID in the
token, the Token Binding ID between the client and the Token
Consumer must therefore be communicated to the Token Provider
along with the token request. Communicating a Token Binding ID
involves proving possession of a private key and is described in
the Token Binding protocol [RFC8471].
The client will perform this last operation only if the Token
Consumer requests the client to do so.
Popov, et al. Standards Track [Page 9]RFC 8473 Token Binding over HTTP October 2018
Below, we specify how Token Consumers can signal this request in
redirect-based federation protocols. Note that this assumes that the
federated sign-on flow starts at the Token Consumer or, at the very
least, includes a redirect from the Token Consumer to the Token
Provider. It is outside the scope of this document to specify
similar mechanisms for flows that do not include such redirects.
5.3. HTTP Redirects
When a Token Consumer redirects the client to a Token Provider as a
means to deliver the token request, it SHOULD include an
Include-Referred-Token-Binding-ID HTTP response header field in its
HTTP response. The ABNF of the Include-Referred-Token-Binding-ID
header is (per the style of [RFC7230]; see also Section 8.3 of
[RFC7231]):
Include-Referred-Token-Binding-ID = "true"
Where the header field name is "Include-Referred-Token-Binding-ID"
and the field value of "true" is case insensitive. For example:
Include-Referred-Token-Binding-ID: true
Including this response header field signals to the client that it
should reveal, to the Token Provider, the Token Binding ID used
between itself and the Token Consumer. In the absence of this
response header field, the client will not disclose any information
about the Token Binding used between the client and the Token
Consumer to the Token Provider.
As illustrated in Section 5.5, when a client receives this header
field, it should take the TokenBindingID [RFC8471] of the provided
TokenBinding from the referrer and create a referred TokenBinding
with it to include in the TokenBindingMessage in the redirect
request. In other words, the Token Binding message in the redirect
request to the Token Provider now includes one provided binding and
one referred binding, the latter constructed from the binding between
the client and the Token Consumer.
When a client receives the Include-Referred-Token-Binding-ID header,
it includes the referred Token Binding even if both the Token
Provider and the Token Consumer fall under the same eTLD+1 and the
provided and Referred Token Binding IDs are the same.
The referred Token Binding is sent only in the initial request
resulting from the HTTP response that included the
Include-Referred-Token-Binding-ID header. Should the response to
that initial request be a further redirect, the original referred
Popov, et al. Standards Track [Page 10]RFC 8473 Token Binding over HTTP October 2018
Token Binding is no longer included in subsequent requests. (A new
referred Token Binding may be included if the redirecting endpoint
itself responded with an Include-Referred-Token-Binding-ID response
header.)
If the Include-Referred-Token-Binding-ID header field is
received in response to a request that did not include the
Sec-Token-Binding header field, the client MUST ignore the
Include-Referred-Token-Binding-ID header field.
This header field only has meaning if the HTTP status code is a
redirection code (300-399) and MUST be ignored by the client for any
other status codes. As described in Section 2, if the client
supports the Token Binding protocol and has negotiated the Token
Binding protocol with both the Token Consumer and the Token Provider,
it sends the Sec-Token-Binding header field to the Token Provider
with each HTTP request.
The TokenBindingMessage included in the redirect request to the Token
Provider SHOULD contain a TokenBinding with a TokenBindingType value
of referred_token_binding. If included, this TokenBinding MUST be
signed with the Token Binding private key used by the client for
connections between itself and the Token Consumer (more specifically,
the server that issued the Include-Referred-Token-Binding-ID response
header field). The Token Binding ID established by this TokenBinding
is called a "Referred Token Binding ID".
As described above, the TokenBindingMessage MUST additionally contain
a Provided Token Binding ID, i.e., a TokenBinding structure with a
TokenBindingType value of provided_token_binding, which MUST be
signed with the Token Binding private key used by the client for
connections between itself and the Token Provider (more specifically,
the server that the token request is being sent to).
If, for some deployment-specific reason, the initial Token Provider
("TP1") needs to redirect the client to another Token Provider
("TP2") rather than directly back to the Token Consumer, it can be
accommodated using the header fields defined in this specification in
the following fashion ("the redirect-chain approach"):
Initially, the client is redirected to TP1 by the Token Consumer
("TC"), as described above. Upon receiving a client's request
that contains a TokenBindingMessage that in turn contains both
provided and referred TokenBindings (for TP1 and TC,
respectively), TP1 responds to the client with a redirect response
that (1) contains the Include-Referred-Token-Binding-ID header
field and (2) directs the client to send a request to TP2. This
causes the client to follow the same pattern and send a request
Popov, et al. Standards Track [Page 11]RFC 8473 Token Binding over HTTP October 2018
containing a TokenBindingMessage that contains both provided and
referred TokenBindings (for TP2 and TP1, respectively) to TP2.
Note that this pattern can continue to additional Token Providers.
In this case, TP2 issues a security token, bound to the client's
TokenBinding with TP1, and sends a redirect response to the client
pointing to TP1. TP1 in turn constructs a security token for the
Token Consumer, bound to the TC's referred TokenBinding that had
been conveyed earlier, and sends a redirect response pointing to
the TC, containing the bound security token, to the client.
The above is intended as only a non-normative example. Details are
specific to deployment contexts. Other approaches are possible but
are outside the scope of this specification.
5.4. Negotiated Key Parameters
The TLS extension for Token Binding protocol negotiation [RFC8472]
allows the server and client to negotiate the parameters (signature
algorithm, length) of the Token Binding key pair. It is possible
that the Token Binding ID used between the client and the Token
Consumer, and the Token Binding ID used between the client and the
Token Provider, use different key parameters. The client MUST use
the key parameters negotiated with the Token Consumer in the
referred_token_binding TokenBinding of the TokenBindingMessage, even
if those key parameters are different from the ones negotiated with
the server that the header field is sent to.
Token Providers SHOULD support all the Token Binding key parameters
specified in [RFC8471]. If a Token Provider does not support the key
parameters specified in the referred_token_binding TokenBinding in
the TokenBindingMessage, it MUST NOT issue a bound token.
Popov, et al. Standards Track [Page 12]RFC 8473 Token Binding over HTTP October 2018
5.5. Federation Example
The diagram below shows a typical HTTP redirect-based web browser
single sign-on (SSO) profile (Section 4.1 of
[OASIS.saml-prof-2.0-os]) (no artifact, no callbacks), featuring the
binding of, for example, a TLS Token Binding ID into an OpenID
Connect ID Token.
Legend:
+------------+------------------------------------------------------+
| EKM: | TLS Exported Keying Material [RFC5705] |
| | |
| {EKMn}Ksm: | EKM for server "n", signed by the private key of |
| | TBID "m", where "n" must represent the server |
| | receiving the ETBMSG. If a conveyed TB's type is |
| | provided_token_binding, then m = n, else if TB's |
| | type is referred_token_binding, then m != n. For |
| | example, see step 1b in the diagram below. |
| | |
| ETBMSG: | "Sec-Token-Binding" HTTP header field conveying an |
| | EncodedTokenBindingMessage, in turn conveying |
| | TokenBinding (TB)struct(s), e.g., ETBMSG[[TB]] or |
| | ETBMSG[[TB1],[TB2]] |
| | |
| ID Token: | the ID Token in OpenID Connect. It is the semantic |
| | equivalent of a SAML "authentication assertion". |
| | "ID Token w/TBIDn" denotes a "token bound" ID Token |
| | containing TBIDn. |
| | |
| Ks and Kp: | private (aka secret) key and public key, |
| | respectively, of the client-side Token Binding key |
| | pair |
| | |
| OIDC: | OpenID Connect |
| | |
| TB: | TokenBinding struct containing a signed EKM, TBID, |
| | and TB type, e.g., |
| | [{EKM1}Ks1,TBID1,provided_token_binding] |
| | |
| TBIDn: | Token Binding ID for client and server n's token- |
| | bound TLS association. TBIDn contains Kpn. |
+------------+------------------------------------------------------+
Popov, et al. Standards Track [Page 13]RFC 8473 Token Binding over HTTP October 2018
Client, aka Token Consumer, aka Token Provider, aka
User Agent OpenID Client, OpenID Provider,
OIDC Relying Party, OIDC Provider,
SAML Relying Party SAML Identity Provider
[ server "1" ] [ server "2" ]
+--------+ +----+ +-----+
| Client | | TC | | TP |
+--------+ +----+ +-----+
| | |
| | |
| | |
| 0. Client interacts w/TC | |
| over HTTPS, establishes Ks1 and Kp1, TBID1 |
| ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] |
|------------------------------>| |
| | |
| | |
| | |
| 1a. OIDC ID Token request, aka| |
| "Authentication Request", conveyed with |
| an HTTP response header field of |
| Include-Referred-Token-Binding-ID:true. |
| Any security-relevant cookies | |
| should contain TBID1. | |
+<- - - - - - - - - - - - - - - - | |
. | (redirect to TP via 301, 302, | |
. | 303, 307, or 308) | |
. | | |
+------------------------------------------------------->|
| 1b. opens HTTPS w/TP, |
| establishes Ks2, Kp2, TBID2; |
| sends a GET or POST with |
| ETBMSG[[{EKM2}Ks2,TBID2,provided_token_binding], |
| [{EKM2}Ks1,TBID1,referred_token_binding]] |
| as well as the ID Token request |
| | |
| | |
| | |
| 2. user authentication (if applicable; |
| methods vary; particulars are out of scope) |
|<====================================================>|
| (TP generates ID Token for TC containing TBID1; may |
| also set cookie(s) containing TBID2 and/or TBID1; |
| details vary; particulars are out of scope) |
| | |
| | |
Popov, et al. Standards Track [Page 14]RFC 8473 Token Binding over HTTP October 2018
| | |
| 3a. ID Token containing Kp1, issued for TC, |
| conveyed via OIDC "Authentication Response" |
+<- - - - - - - - - - - - - - - - - - - - - - - - - - - -|
. | (redirect to TC) | |
. | | |
. | | |
+-------------------------------->| |
| 3b. HTTPS GET or POST with |
| ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] |
| conveying an Authentication Response containing |
| an ID Token w/TBID1, issued for TC |
| | |
| | |
| | |
| 4. user is signed on; any security-relevant cookie(s)|
| that is set SHOULD contain TBID1 |
|<------------------------------| |
| | |
| | |
6. Implementation Considerations
HTTPS-based applications may have multi-party use cases other than,
or in addition to, the HTTP redirect-based signaling and conveyance
of referred Token Bindings, as presented above in Section 5.3.
Thus, Token Binding implementations should provide APIs for such
applications to generate Token Binding messages containing Token
Binding IDs of various application-specified Token Binding types, to
be conveyed by the Sec-Token-Binding header field.
However, Token Binding implementations MUST only convey Token Binding
IDs to servers if signaled to do so by an application. Signaling
mechanisms other than the Include-Referred-Token-Binding-ID HTTP
response header field are possible, but these mechanisms are outside
the scope of this specification.
NOTE: See Section 8 ("Privacy Considerations") for privacy guidance
regarding the use of this functionality.
Popov, et al. Standards Track [Page 15]RFC 8473 Token Binding over HTTP October 2018
7. Security Considerations
7.1. Security Token Replay
The goal of the federated Token Binding mechanisms is to prevent
attackers from exporting and replaying tokens used in protocols
between the client and the Token Consumer, thereby impersonating
legitimate users and gaining access to protected resources. Although
bound tokens can still be replayed by any malware present in clients
(which may be undetectable to a server), in order to export bound
tokens to other machines and successfully replay them, attackers also
need to export the corresponding Token Binding private keys. Token
Binding private keys are therefore high-value assets and SHOULD be
strongly protected, ideally by generating them in a hardware security
module that prevents key export.
This consideration is a special case of the scenario described in
Section 7.1 ("Security Token Replay") of [RFC8471].
7.2. Sensitivity of the Sec-Token-Binding Header
The purpose of the Token Binding protocol is to convince the server
that the client that initiated the TLS connection controls a certain
key pair. For the server to correctly draw this conclusion after
processing the Sec-Token-Binding header field, certain secrecy and
integrity requirements must be met.
For example, the client must keep its Token Binding private key
secret. If the private key is not secret, then another actor in the
system could create a valid Token Binding header field and thereby
impersonate the client. This can render the main purpose of the
protocol -- to bind bearer tokens to certain clients -- moot.
Consider, for example, an attacker who obtained (perhaps through a
network intrusion) an authentication cookie that a client uses with a
certain server. Consider further that the server bound that cookie
to the client's Token Binding ID precisely to thwart misuse of the
cookie. If the attacker were to come into possession of the client's
private key, they could then establish a TLS connection with the
server and craft a Sec-Token-Binding header field that matches the
binding present in the cookie, thus successfully authenticating as
the client and gaining access to the client's data at the server.
The Token Binding protocol, in this case, did not successfully bind
the cookie to the client.
Likewise, we need integrity protection of the Sec-Token-Binding
header field. A client should not be tricked into sending to a
server a Sec-Token-Binding header field that contains Token Bindings
signed with any Token Binding keys that the client does not control.
Popov, et al. Standards Track [Page 16]RFC 8473 Token Binding over HTTP October 2018
Consider an attacker A that somehow has knowledge of the Exported
Keying Material (EKM) for a TLS connection between a client C and a
server S. (While that is somewhat unlikely, it is also not entirely
out of the question, since the client might not treat the EKM as a
secret -- after all, a pre-image-resistant hash function has been
applied to the TLS master secret, making it impossible for someone
knowing the EKM to recover the TLS master secret. Such
considerations might lead some clients to not treat the EKM as a
secret.) Such an attacker A could craft a Sec-Token-Binding header
field with A's key pair over C's EKM. If the attacker could now
trick C into sending such a header field to S, it would appear to S
as if C controls a certain key pair, when in fact it does not (the
attacker A controls the key pair).
If A has a pre-existing relationship with S (e.g., perhaps has an
account on S), it now appears to the server S as if A is connecting
to it, even though it is really C. (If the server S does not simply
use Token Binding IDs to identify clients but also uses bound
authentication cookies, then A would also have to trick C into
sending one of A's cookies to S, which it can do through a variety of
means -- inserting cookies through JavaScript APIs, setting cookies
through related-domain attacks, etc.) In other words, in this
scenario, A can trick C into logging into A's account on S. This
could lead to a loss of privacy for C, since A presumably has some
other way to also access the account and can thus indirectly observe
C's behavior (for example, if S has a feature that lets account
holders see their activity history on S).
Therefore, we need to protect the integrity of the Sec-Token-Binding
header field. One eTLD+1 should not be able to set the
Sec-Token-Binding header field (through a Document Object Model (DOM)
API [W3C.REC-DOM-Level-3-Core-20040407] or otherwise) that the User
Agent uses with another eTLD+1. Employing the "Sec-" header field
prefix helps to meet this requirement by denoting the header field
name as a "forbidden header name"; see [fetch-spec].
7.3. Securing Federated Sign-On Protocols
As explained above, in a federated sign-on scenario, a client will
prove possession of two different Token Binding private keys to a
Token Provider: one private key corresponds to the "provided" Token
Binding ID (which the client normally uses with the Token Provider),
and the other is the Token Binding private key corresponding to the
"referred" Token Binding ID (which the client normally uses with the
Token Consumer). The Token Provider is expected to issue a token
that is bound to the Referred Token Binding ID.
Popov, et al. Standards Track [Page 17]
RFC 8473 Token Binding over HTTP October 2018
Both proofs (that of the provided Token Binding private key and that
of the referred Token Binding private key) are necessary. To show
this, consider the following scenario:
o The client has an authentication token with the Token Provider
that is bound to the client's Token Binding ID used with that
Token Provider.
o The client wants to establish a secure (i.e., free of men-in-the-
middle) authenticated session with the Token Consumer but has not
yet done so (in other words, we are about to run the federated
sign-on protocol).
o A man-in-the-middle is allowed to intercept the connection between
the client and the Token Consumer or between the client and the
Token Provider (or both).
The goal is to detect the presence of the man-in-the-middle in these
scenarios.
First, consider a man-in-the-middle between the client and the Token
Provider. Recall that we assume that the client possesses a bound
authentication token (e.g., cookie) for the Token Provider. The
man-in-the-middle can intercept and modify any message sent by the
client to the Token Provider and any message sent by the Token
Provider to the client. (This means, among other things, that the
man-in-the-middle controls the JavaScript running at the client in
the origin of the Token Provider.) It is not, however, in possession
of the client's Token Binding private key. Therefore, it can choose
to either (1) replace the Token Binding ID in requests from the
client to the Token Provider and create a Sec-Token-Binding header
field that matches the TLS connection between the man-in-the-middle
and the Token Provider or (2) leave the Sec-Token-Binding header
field unchanged. If it chooses the latter, the signature in the
Token Binding message (created by the original client on the EKM for
the connection between the client and the man-in-the-middle) will not
match a signature on the EKM between the man-in-the-middle and the
Token Provider. If it chooses the former (and creates its own
signature, using its own Token Binding private key, over the EKM for
the connection between itself, the man-in-the-middle, and the Token
Provider), then the Token Binding message will match the connection
between the man-in-the-middle and the Token Provider, but the Token
Binding ID in the message will not match the Token Binding ID that
the client's authentication token is bound to. Either way, the
man-in-the-middle is detected by the Token Provider, but only if the
proof of possession of the provided Token Binding private key is
required in the protocol (as is done above).
Popov, et al. Standards Track [Page 18]RFC 8473 Token Binding over HTTP October 2018
Next, consider the presence of a man-in-the-middle between the client
and the Token Consumer. That man-in-the-middle can intercept and
modify any message sent by the client to the Token Consumer and any
message sent by the Token Consumer to the client. The Token Consumer
is the party that redirects the client to the Token Provider. In
this case, the man-in-the-middle controls the redirect URL and can
tamper with any redirect URL issued by the Token Consumer (as well as
with any JavaScript running in the origin of the Token Consumer).
The goal of the man-in-the-middle is to trick the Token Provider into
issuing a token bound to its Token Binding ID and not to the Token
Binding ID of the legitimate client. To thwart this goal of the
man-in-the-middle, the client's Referred Token Binding ID must be
communicated to the Token Provider in a manner that cannot be
affected by the man-in-the-middle (who, as mentioned above, can
modify redirect URLs and JavaScript at the client). Including the
referred TokenBinding structure in the Sec-Token-Binding header field
(as opposed to, say, including the Referred Token Binding ID in an
application-level message as part of the redirect URL) is one way to
assure that the man-in-the-middle between the client and the Token
Consumer cannot affect the communication of the Referred Token
Binding ID to the Token Provider.
Therefore, the Sec-Token-Binding header field in the federated
sign-on use case contains both a proof of possession of the provided
Token Binding key and a proof of possession of the referred Token
Binding key.
Note that the presence of Token Binding does not relieve the Token
Provider and Token Consumer from performing various checks to ensure
the security of clients during the use of federated sign-on
protocols. These include the following:
o The Token Provider should not issue tokens to Token Consumers that
have been shown to act maliciously. To aid in this, the
federation protocol should identify the Token Consumer to the
Token Provider (e.g., through OAuth client IDs or similar
mechanisms), and the Token Provider should ensure that tokens are
indeed issued to the Token Consumer identified in the token
request (e.g., by verifying that the redirect URI is associated
with the OAuth client ID).
Popov, et al. Standards Track [Page 19]RFC 8473 Token Binding over HTTP October 2018
o The Token Consumer should verify that the tokens were issued for
it and not for some other Token Consumer. To aid in this, the
federation protocol should include an audience parameter in the
token response or apply equivalent mechanisms (the implicit OAuth
flow requires Token Consumers to identify themselves when they
exchange OAuth authorization codes for OAuth refresh tokens,
leaving it up to the Token Provider to verify that the OAuth
authorization was delivered to the correct Token Consumer).
8. Privacy Considerations
8.1. Scoping of Token Binding Key Pairs
Clients use different Token Binding key pairs for different servers,
so as to not allow Token Binding to become a tracking tool across
different servers. However, the scoping of the Token Binding key
pairs to servers varies according to the scoping rules of the
application protocol (Section 4.1 of [RFC8471]).
In the case of HTTP cookies, servers may use Token Binding to secure
their cookies. These cookies can be attached to any subdomain of
effective top-level domains (eTLDs), and clients therefore should use
the same Token Binding key pair across such subdomains. This will
ensure that any server capable of receiving the cookie will see the
same Token Binding ID from the client and thus be able to verify the
Token Binding of the cookie. See Section 2.1 above.
If the client application is not a web browser, it may have
additional knowledge about the relationship between different
servers. For example, the client application might be aware of the
fact that two servers play the roles of Relying Party and Identity
Provider, respectively, in a federated sign-on protocol and that they
therefore share the identity of the user. In such cases, it is
permissible to use different Token Binding key-pair scoping rules,
such as using the same Token Binding key pair for both the Relying
Party and the Identity Provider. Absent such special knowledge,
conservative key-pair scoping rules should be used, assuring that
clients use different Token Binding key pairs with different servers.
8.2. Lifetime of Token Binding Key Pairs
Token Binding key pairs do not have an expiration time. This means
that they can potentially be used by a server to track a user for an
extended period of time (similar to a long-lived cookie). HTTPS
clients such as web User Agents SHOULD therefore provide a user
interface for discarding Token Binding key pairs (similar to the
controls provided for deleting cookies).
Popov, et al. Standards Track [Page 20]RFC 8473 Token Binding over HTTP October 2018
If a User Agent provides modes such as private browsing mode in which
the user is promised that browsing state such as cookies are
discarded after the session is over, the User Agent MUST also discard
Token Binding key pairs from such modes after the session is over.
Generally speaking, users should be given the same level of control
over the lifetime of Token Binding key pairs as they have over
cookies or other potential tracking mechanisms.
8.3. Correlation
An application's various communicating endpoints that receive Token
Binding IDs for TLS connections other than their own obtain
information about the application's other TLS connections. (In this
context, "an application" is a combination of client-side and
server-side components, communicating over HTTPS, where the client
side may be web-browser-based, native-application-based, or both.)
These other Token Binding IDs can serve as correlation handles for
the endpoints of the other connections. If the receiving endpoints
are otherwise aware of these other connections, then no additional
information is being exposed. For instance, if in a redirect-based
federation protocol the Identity Provider and Relying Party already
possess URLs for one another, then also having Token Binding IDs for
these connections does not provide additional correlation
information. If not, by providing the other Token Binding IDs,
additional information is then exposed that can be used to correlate
the other endpoints. In such cases, a privacy analysis of enabled
correlations and their potential privacy impacts should be performed
as part of the application design decisions of how, and whether, to
utilize Token Binding.
Also, Token Binding implementations must take care to only reveal
Token Binding IDs to other endpoints if signaled to do so by the
application associated with a Token Binding ID; see Section 6
("Implementation Considerations").
Finally, care should be taken to ensure that unrelated applications
do not obtain information about each other's Token Bindings. For
instance, a Token Binding implementation shared between multiple
applications on a given system should prevent unrelated applications
from obtaining each other's Token Binding information. This may be
accomplished by using techniques such as application isolation and
key segregation, depending upon system capabilities.
Popov, et al. Standards Track [Page 21]RFC 8473 Token Binding over HTTP October 2018
9. IANA Considerations
Below is the Internet Assigned Numbers Authority (IANA) "Permanent
Message Header Field Names" registration information per [RFC3864].
Header Field name: Sec-Token-Binding
Protocol: HTTP
Status: standard
Reference: This document
Header Field name: Include-Referred-Token-Binding-ID
Protocol: HTTP
Status: standard
Reference: This document
10. References
10.1. Normative References
[PSL] Mozilla, "Public Suffix List",
<https://publicsuffix.org/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
Popov, et al. Standards Track [Page 22]RFC 8473 Token Binding over HTTP October 2018
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8471] Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges,
"The Token Binding Protocol Version 1.0", RFC 8471,
DOI 10.17487/RFC8471, October 2018,
<https://www.rfc-editor.org/info/rfc8471>.
[RFC8472] Popov, A., Ed., Nystroem, M., and D. Balfanz, "Transport
Layer Security (TLS) Extension for Token Binding Protocol
Negotiation", RFC 8472, DOI 10.17487/RFC8472, October
2018, <https://www.rfc-editor.org/info/rfc8472>.
10.2. Informative References
[fetch-spec]
WhatWG, "Fetch", Living Standard,
<https://fetch.spec.whatwg.org/>.
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocols for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard
saml-core-2.0-os, March 2005, <http://docs.oasis-open.org/
security/saml/v2.0/saml-core-2.0-os.pdf>.
Popov, et al. Standards Track [Page 23]RFC 8473 Token Binding over HTTP October 2018
[OASIS.saml-prof-2.0-os]
Hughes, J., Ed., Cantor, S., Ed., Hodges, J., Ed., Hirsch,
F., Ed., Mishra, P., Ed., Philpott, R., Ed., and E. Maler,
Ed., "Profiles for the OASIS Security Assertion Markup
Language (SAML) V2.0", OASIS Standard
OASIS.saml-profiles-2.0-os, March 2005,
<http://docs.oasis-open.org/security/
saml/v2.0/saml-profiles-2.0-os.pdf>.
[OpenID.Core]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0 incorporating
errata set 1", November 2014,
<http://openid.net/specs/openid-connect-core-1_0.html>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[TOKENBIND-TLS13]
Harper, N., "Token Binding for Transport Layer Security
(TLS) Version 1.3 Connections", Work in Progress,
draft-ietf-tokbind-tls13-01, May 2018.
[W3C.REC-DOM-Level-3-Core-20040407]
Le Hors, A., Ed., Le Hegaret, P., Ed., Wood, L., Ed.,
Nicol, G., Ed., Robie, J., Ed., Champion, M., Ed., and S.
Byrne, Ed., "Document Object Model (DOM) Level 3 Core
Specification", World Wide Web Consortium Recommendation
REC-DOM-Level-3-Core-20040407, April 2004,
<https://www.w3.org/TR/2004/
REC-DOM-Level-3-Core-20040407>.
Popov, et al. Standards Track [Page 24]RFC 8473 Token Binding over HTTP October 2018
Acknowledgements
This document incorporates comments and suggestions offered by Eric
Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony
Nadalin, Michael B. Jones, Bill Cox, Brian Campbell, and others.
This document was produced under the chairmanship of John Bradley and
Leif Johansson. The area directors included Eric Rescorla, Kathleen
Moriarty, and Stephen Farrell.
Authors' Addresses
Andrei Popov
Microsoft Corp.
United States of America
Email: andreipo@microsoft.com
Magnus Nystroem
Microsoft Corp.
United States of America
Email: mnystrom@microsoft.com
Dirk Balfanz (editor)
Google Inc.
United States of America
Email: balfanz@google.com
Nick Harper
Google Inc.
United States of America
Email: nharper@google.com
Jeff Hodges
Kings Mountain Systems
United States of America
Email: Jeff.Hodges@KingsMountain.com
Popov, et al. Standards Track [Page 25]
