Internet Engineering Task Force (IETF) E. Stark
Request for Comments: 9163 Google
Category: Experimental June 2022
ISSN: 2070-1721
Expect-CT Extension for HTTP
Abstract
This document defines a new HTTP header field named "Expect-CT",
which allows web host operators to instruct user agents (UAs) to
expect valid Signed Certificate Timestamps (SCTs) to be served on
connections to these hosts. Expect-CT allows web host operators to
discover misconfigurations in their Certificate Transparency (CT)
deployments. Further, web host operators can use Expect-CT to ensure
that if a UA that supports Expect-CT accepts a misissued certificate,
that certificate will be discoverable in Certificate Transparency
logs.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. 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). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see 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/rfc9163.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
1.2. Terminology
2. Server and Client Behavior
2.1. Response Header Field Syntax
2.1.1. The report-uri Directive
2.1.2. The enforce Directive
2.1.3. The max-age Directive
2.1.4. Examples
2.2. Host Processing Model
2.2.1. HTTP-over-Secure-Transport Request Type
2.2.2. HTTP Request Type
2.3. User Agent Processing Model
2.3.1. Missing or Malformed Expect-CT Header Fields
2.3.2. Expect-CT Header Field Processing
2.3.3. Reporting
2.4. Evaluating Expect-CT Connections for CT Compliance
2.4.1. Skipping CT Compliance Checks
3. Reporting Expect-CT Failure
3.1. Generating a Violation Report
3.2. Sending a Violation Report
3.3. Receiving a Violation Report
4. Usability Considerations
5. Authoring Considerations
6. Privacy Considerations
7. Security Considerations
7.1. Hostile Header Attacks
7.2. Maximum max-age
7.3. Amplification Attacks
8. IANA Considerations
8.1. Header Field Registry
8.2. Media Types Registry
9. References
9.1. Normative References
9.2. Informative References
Author's Address
1. Introduction
This document defines a new HTTP header field ([RFC9110],
Section 6.3) that enables UAs to identify web hosts that expect the
presence of Signed Certificate Timestamps (SCTs) [RFC9162] in
subsequent Transport Layer Security (TLS) [RFC8446] connections.
Web hosts that serve the Expect-CT header field are noted by the UA
as "Known Expect-CT Hosts". The UA evaluates each connection to a
Known Expect-CT Host for compliance with the UA's Certificate
Transparency (CT) Policy. If the connection violates the CT Policy,
the UA sends a report to a URI configured by the Expect-CT Host and/
or fails the connection, depending on the configuration that the
Expect-CT Host has chosen.
If misconfigured, Expect-CT can cause unwanted connection failures
(for example, if a host deploys Expect-CT but then switches to a
legitimate certificate that is not logged in Certificate Transparency
logs or if a web host operator believes their certificate to conform
to all UAs' CT policies but is mistaken). Web host operators are
advised to deploy Expect-CT with precautions by using the reporting
feature and gradually increasing the time interval during which the
UA regards the host as a Known Expect-CT Host. These precautions can
help web host operators gain confidence that their Expect-CT
deployment is not causing unwanted connection failures.
Expect-CT is a trust-on-first-use (TOFU) mechanism. The first time a
UA connects to a host, it lacks the information necessary to require
SCTs for the connection. Thus, the UA will not be able to detect and
thwart an attack on the UA's first connection to the host. Still,
Expect-CT provides value by 1) allowing UAs to detect the use of
unlogged certificates after the initial communication, and 2)
allowing web hosts to be confident that UAs are only trusting
publicly auditable certificates.
Expect-CT is similar to HTTP Strict Transport Security (HSTS)
[RFC6797] and HTTP Public Key Pinning (HPKP) [RFC7469]. HSTS allows
websites to declare themselves accessible only via secure
connections, and HPKP allows websites to declare their cryptographic
identifies. Similarly, Expect-CT allows websites to declare
themselves accessible only via connections that are compliant with CT
Policy.
This Expect-CT specification is compatible with [RFC6962] and
[RFC9162], but not necessarily with future versions of Certificate
Transparency. UAs will ignore Expect-CT header fields from web hosts
that use future versions of Certificate Transparency, unless a future
version of this document specifies how they should be processed.
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.
1.2. Terminology
Terminology is defined in this section.
"Certificate Transparency Policy"
A policy defined by the UA concerning the number, sources, and
delivery mechanisms of Signed Certificate Timestamps that are
associated with TLS connections. The policy defines the
properties of a connection that must be met in order for the UA to
consider it CT qualified.
"Certificate Transparency Qualified"
Describes a TLS connection for which the UA has determined that a
sufficient quantity and quality of Signed Certificate Timestamps
have been provided.
"CT Qualified"
An abbreviation for "Certificate Transparency Qualified".
"CT Policy"
An abbreviation for "Certificate Transparency Policy".
"Effective Expect-CT Date"
The time at which a UA observed a valid Expect-CT header field for
a given host.
"Expect-CT Host"
A conformant host implementing the HTTP server aspects of Expect-
CT. This means that an Expect-CT Host returns the Expect-CT
response header field in its HTTP response messages sent over
secure transport. The term "host" is equivalent to "server" in
this specification.
"Known Expect-CT Host"
An Expect-CT Host that the UA has noted as such. See
Section 2.3.2.1 for particulars.
"User Agent (UA)"
For the purposes of this specification, a UA is an HTTP client
application typically actively manipulated by a user [RFC9110].
"Unknown Expect-CT Host"
An Expect-CT Host that the UA has not noted.
2. Server and Client Behavior
2.1. Response Header Field Syntax
The Expect-CT response header field is a new field defined in this
specification. It is used by a server to indicate that UAs should
evaluate connections to the host emitting the header field for CT
compliance (Section 2.4).
Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
header field, using the grammar defined in [RFC5234] and the rules
defined in Section 5 of [RFC9110]. The "#" ABNF extension is
specified in Section 5.6.1 of [RFC9110].
Expect-CT = 1#expect-ct-directive
expect-ct-directive = directive-name [ "=" directive-value ]
directive-name = token
directive-value = token / quoted-string
Figure 1: Syntax of the Expect-CT Header Field
The directives defined in this specification are described below.
The overall requirements for directives are:
1. The order of appearance of directives is not significant.
2. A given directive MUST NOT appear more than once in a given
header field. Directives are either optional or required, as
stipulated in their definitions.
3. Directive names are case insensitive.
4. UAs MUST ignore any header fields containing directives, or other
header field value data that does not conform to the syntax
defined in this specification. In particular, UAs MUST NOT
attempt to fix malformed header fields.
5. If a header field contains any directive(s) the UA does not
recognize, the UA MUST ignore those directives.
6. If the Expect-CT header field otherwise satisfies the above
requirements (1 through 5), and Expect-CT is not disabled for
local policy reasons (as discussed in Section 2.4.1), the UA MUST
process the directives it recognizes.
2.1.1. The report-uri Directive
The OPTIONAL report-uri directive indicates the URI to which the UA
SHOULD report Expect-CT failures (Section 2.4). The UA POSTs the
reports to the given URI as described in Section 3.
The report-uri directive is REQUIRED to have a directive value, for
which the syntax is defined in Figure 2.
report-uri-value = (DQUOTE absolute-URI DQUOTE) / absolute-URI
Figure 2: Syntax of the report-uri Directive Value
The 'report-uri-value' MUST be quoted if it contains any character
not allowed in 'token'.
absolute-URI is defined in Section 4.3 of [RFC3986].
UAs MUST ignore any report-uri that does not use the HTTPS scheme.
UAs MUST check Expect-CT compliance when the host in the report-uri
is a Known Expect-CT Host; similarly, UAs MUST apply HSTS [RFC6797]
if the host in the report-uri is a Known HSTS Host.
UAs SHOULD make their best effort to report Expect-CT failures to the
report-uri, but they may fail to report in exceptional conditions.
For example, if connecting to the report-uri itself incurs an Expect-
CT failure or other certificate validation failure, the UA MUST
cancel the connection. Similarly, if Expect-CT Host A sets a report-
uri referring to Expect-CT Host B, and if B sets a report-uri
referring to A, and if both hosts fail to comply to the UA's CT
Policy, the UA SHOULD detect and break the loop by failing to send
reports to and about those hosts.
Note that the report-uri need not necessarily be in the same Internet
domain or web origin as the host being reported about. Hosts are in
fact encouraged to use a separate host as the report-uri so that CT
failures on the Expect-CT Host do not prevent reports from being
sent.
UAs SHOULD limit the rate at which they send reports. For example,
it is unnecessary to send the same report to the same report-uri more
than once in the same web-browsing session.
2.1.2. The enforce Directive
The OPTIONAL enforce directive is a valueless directive that, if
present (i.e., it is "asserted"), signals to the UA that compliance
to the CT Policy should be enforced (rather than report-only) and
that the UA should refuse future connections that violate its CT
Policy. When both the enforce directive and report-uri directive (as
defined in Figure 2) are present, the configuration is referred to as
an "enforce-and-report" configuration, signaling to the UA that both
compliance to the CT Policy should be enforced and violations should
be reported.
2.1.3. The max-age Directive
The max-age directive specifies the number of seconds after the
reception of the Expect-CT header field during which the UA SHOULD
regard the host from whom the message was received as a Known Expect-
CT Host.
If a response contains an Expect-CT header field, then the response
MUST contain an Expect-CT header field with a max-age directive. (A
max-age directive need not appear in every Expect-CT header field in
the response.) The max-age directive is REQUIRED to have a directive
value, for which the syntax (after quoted-string unescaping, if
necessary) is defined in Figure 3.
max-age-value = delta-seconds
delta-seconds = 1*DIGIT
Figure 3: Syntax of the max-age Directive Value
delta-seconds is used as defined in Section 1.3 of [RFC9111].
2.1.4. Examples
The following three examples demonstrate valid Expect-CT response
header fields (where the second splits the directives into two field
instances):
Expect-CT: max-age=86400, enforce
Expect-CT: max-age=86400,enforce
Expect-CT: report-uri="https://foo.example/report"
Expect-CT: max-age=86400,report-uri="https://foo.example/report"
Figure 4: Examples of Valid Expect-CT ResponseHeader Fields
2.2. Host Processing Model
This section describes the processing model that Expect-CT Hosts
implement. The model has 2 parts: (1) the processing rules for HTTP
request messages received over a secure transport (e.g.,
authenticated, non-anonymous TLS); and (2) the processing rules for
HTTP request messages received over non-secure transports, such as
TCP.
2.2.1. HTTP-over-Secure-Transport Request Type
An Expect-CT Host includes an Expect-CT header field in its response.
The header field MUST satisfy the grammar specified in Section 2.1.
Establishing a given host as an Expect-CT Host, in the context of a
given UA, is accomplished as follows:
1. Over the HTTP protocol running over secure transport, by
correctly returning (per this specification) a valid Expect-CT
header field to the UA.
2. Through other mechanisms such as a client-side preloaded Expect-
CT Host list.
2.2.2. HTTP Request Type
Expect-CT Hosts SHOULD NOT include the Expect-CT header field in HTTP
responses conveyed over non-secure transport.
2.3. User Agent Processing Model
The UA processing model relies on parsing domain names. Note that
internationalized domain names SHALL be canonicalized by the UA
according to the scheme in Section 10 of [RFC6797].
The UA stores Known Expect-CT Hosts and their associated Expect-CT
directives. This data is collectively known as a host's "Expect-CT
metadata".
2.3.1. Missing or Malformed Expect-CT Header Fields
If an HTTP response does not include an Expect-CT header field that
conforms to the grammar specified in Section 2.1, then the UA MUST
NOT update any Expect-CT metadata.
2.3.2. Expect-CT Header Field Processing
If the UA receives an HTTP response over a secure transport that
includes an Expect-CT header field conforming to the grammar
specified in Section 2.1, the UA MUST evaluate the connection on
which the header field was received for compliance with the UA's CT
Policy, and then process the Expect-CT header field as follows. UAs
MUST ignore any Expect-CT header field received in an HTTP response
conveyed over non-secure transport.
If the connection does not comply with the UA's CT Policy (i.e., the
connection is not CT qualified), then the UA MUST NOT update any
Expect-CT metadata. If the header field includes a report-uri
directive, the UA SHOULD send a report to the specified report-uri
(Section 2.3.3).
If the connection complies with the UA's CT Policy (i.e., the
connection is CT qualified), then the UA MUST either:
* Note the host as a Known Expect-CT Host if it is not already so
noted (see Section 2.3.2.1) or
* Update the UA's cached information for the Known Expect-CT Host if
the enforce, max-age, or report-uri header field value directives
convey information different from that already maintained by the
UA. If the max-age directive has a value of 0, the UA MUST remove
its cached Expect-CT information if the host was previously noted
as a Known Expect-CT Host and MUST NOT note this host as a Known
Expect-CT Host if it is not already noted.
If a UA receives an Expect-CT header field over a CT-compliant
connection that uses a version of Certificate Transparency other than
[RFC6962] or [RFC9162], the UA MUST ignore the Expect-CT header field
and clear any Expect-CT metadata associated with the host.
2.3.2.1. Noting Expect-CT
Upon receipt of the Expect-CT response header field over an error-
free TLS connection (with X.509 certificate chain validation as
described in [RFC5280], as well as the validation described in
Section 2.4 of this document), the UA MUST note the host as a Known
Expect-CT Host, storing the host's domain name and its associated
Expect-CT directives in non-volatile storage.
To note a host as a Known Expect-CT Host, the UA MUST set its Expect-
CT metadata in its Known Expect-CT Host cache (as specified in
Section 2.3.2.2), using the metadata given in the most recently
received valid Expect-CT header field.
For forward compatibility, the UA MUST ignore any unrecognized
Expect-CT header field directives while still processing those
directives it does recognize. Section 2.1 specifies the directives
enforce, max-age, and report-uri, but future specifications and
implementations might use additional directives.
2.3.2.2. Storage Model
If the substring matching the host production from the Request-URI
(of the message to which the host responded) does not exactly match
an existing Known Expect-CT Host's domain name, per the matching
procedure for a Congruent Match specified in Section 8.2 of
[RFC6797], then the UA MUST add this host to the Known Expect-CT Host
cache. The UA caches:
* the Expect-CT Host's domain name.
* whether the enforce directive is present.
* the Effective Expiration Date, which is the Effective Expect-CT
Date plus the value of the max-age directive. Alternatively, the
UA MAY cache enough information to calculate the Effective
Expiration Date. The Effective Expiration Date is calculated from
when the UA observed the Expect-CT header field and is independent
of when the response was generated.
* the value of the report-uri directive, if present.
If any other metadata from optional or future Expect-CT header
directives are present in the Expect-CT header field, and the UA
understands them, the UA MAY note them as well.
UAs MAY set an upper limit on the value of max-age so that UAs that
have noted erroneous Expect-CT Hosts (whether by accident or due to
attack) have some chance of recovering over time. If the server sets
a max-age greater than the UA's upper limit, the UA may behave as if
the server set the max-age to the UA's upper limit. For example, if
the UA caps max-age at 5,184,000 seconds (60 days), and an Expect-CT
Host sets a max-age directive of 90 days in its Expect-CT header
field, the UA may behave as if the max-age were effectively 60 days.
(One way to achieve this behavior is for the UA to simply store a
value of 60 days instead of the 90-day value provided by the Expect-
CT Host.)
2.3.3. Reporting
If the UA receives, over a secure transport, an HTTP response that
includes an Expect-CT header field with a report-uri directive, and
the connection does not comply with the UA's CT Policy (i.e., the
connection is not CT qualified), and the UA has not already sent an
Expect-CT report for this connection, then the UA SHOULD send a
report to the specified report-uri as specified in Section 3.
2.4. Evaluating Expect-CT Connections for CT Compliance
When a UA sets up a TLS connection, the UA determines whether the
host is a Known Expect-CT Host according to its Known Expect-CT Host
cache. An Expect-CT Host is "expired" if the Effective Expiration
Date refers to a date in the past. The UA MUST ignore any expired
Expect-CT Hosts in its cache and not treat such hosts as Known
Expect-CT Hosts.
When a UA connects to a Known Expect-CT Host using a TLS connection,
if the TLS connection has no errors, then the UA will apply an
additional correctness check: compliance with a CT Policy. A UA
should evaluate compliance with its CT Policy whenever connecting to
a Known Expect-CT Host. However, the check can be skipped for local
policy reasons (as discussed in Section 2.4.1) or in the event that
other checks cause the UA to terminate the connection before CT
compliance is evaluated. For example, a Public Key Pinning failure
[RFC7469] could cause the UA to terminate the connection before CT
compliance is checked. Similarly, if the UA terminates the
connection due to an Expect-CT failure, this could cause the UA to
skip subsequent correctness checks. When the CT compliance check is
skipped or bypassed, Expect-CT reports (Section 3) will not be sent.
When CT compliance is evaluated for a Known Expect-CT Host, the UA
MUST evaluate compliance when setting up the TLS session, before
beginning an HTTP conversation over the TLS channel.
If a connection to a Known Expect-CT Host violates the UA's CT Policy
(i.e., the connection is not CT qualified), and if the Known Expect-
CT Host's Expect-CT metadata indicates an enforce configuration, the
UA MUST treat the CT compliance failure as an error. The UA MAY
allow the user to bypass the error unless connection errors should
have no user recourse due to other policies in effect (such as HSTS,
as described in Section 12.1 of [RFC6797]).
If a connection to a Known Expect-CT Host violates the UA's CT
Policy, and if the Known Expect-CT Host's Expect-CT metadata includes
a report-uri, the UA SHOULD send an Expect-CT report to that report-
uri (Section 3).
2.4.1. Skipping CT Compliance Checks
It is acceptable for a UA to skip CT compliance checks for some hosts
according to local policy. For example, a UA MAY disable CT
compliance checks for hosts whose validated certificate chain
terminates at a user-defined trust anchor rather than a trust anchor
built in to the UA (or underlying platform).
If the UA does not evaluate CT compliance, e.g., because the user has
elected to disable it, or because a presented certificate chain
chains up to a user-defined trust anchor, UAs SHOULD NOT send Expect-
CT reports.
3. Reporting Expect-CT Failure
When the UA attempts to connect to a Known Expect-CT Host and the
connection is not CT qualified, the UA SHOULD report Expect-CT
failures to the report-uri, if any, in the Known Expect-CT Host's
Expect-CT metadata.
When the UA receives an Expect-CT response header field over a
connection that is not CT qualified, if the UA has not already sent
an Expect-CT report for this connection, then the UA SHOULD report
Expect-CT failures to the configured report-uri, if any.
3.1. Generating a Violation Report
To generate a violation report object, the UA constructs a JSON
[RFC8259] object with the following keys and values:
"date-time"
The value for this key indicates the UTC time that the UA observed
the CT compliance failure. The value is a string formatted
according to Section 5.6 of [RFC3339], "Internet Date/Time
Format".
"hostname"
The value is the hostname to which the UA made the original
request that failed the CT compliance check. The value is
provided as a string.
"port"
The value is the port to which the UA made the original request
that failed the CT compliance check. The value is provided as an
integer.
"scheme"
(optional) The value is the scheme with which the UA made the
original request that failed the CT compliance check. The value
is provided as a string. This key is optional and is assumed to
be "https" if not present.
"effective-expiration-date"
The value indicates the Effective Expiration Date (see
Section 2.3.2.2) for the Expect-CT Host that failed the CT
compliance check, in UTC. The value is provided as a string
formatted according to Section 5.6 of [RFC3339], "Internet Date/
Time Format".
"served-certificate-chain"
The value is the certificate chain as served by the Expect-CT Host
during TLS session setup. The value is provided as an array of
strings, which MUST appear in the order that the certificates were
served; each string in the array is the Privacy-Enhanced Mail
(PEM) representation of each X.509 certificate as described in
[RFC7468].
"validated-certificate-chain"
The value is the certificate chain as constructed by the UA during
certificate chain verification. (This may differ from the value
of the "served-certificate-chain" key.) The value is provided as
an array of strings, which MUST appear in the order matching the
chain that the UA validated; each string in the array is the PEM
representation of each X.509 certificate as described in
[RFC7468]. The first certificate in the chain represents the end-
entity certificate being verified. UAs that build certificate
chains in more than one way during the validation process SHOULD
send the last chain built.
"scts"
The value represents the SCTs (if any) that the UA received for
the Expect-CT Host and their validation statuses. The value is
provided as an array of JSON objects. The SCTs may appear in any
order. Each JSON object in the array has the following keys:
* A "version" key, with an integer value. The UA MUST set this
value to 1 if the SCT is in the format defined in Section 3.2
of [RFC6962] or 2 if it is in the format defined in Section 4.5
of [RFC9162].
* The "status" key, with a string value that the UA MUST set to
one of the following values: "unknown" (indicating that the UA
does not have or does not trust the public key of the log from
which the SCT was issued); "valid" (indicating that the UA
successfully validated the SCT as described in Section 5.2 of
[RFC6962] or Section 8.1.3 of [RFC9162]); or "invalid"
(indicating that the SCT validation failed because of a bad
signature or an invalid timestamp).
* The "source" key, with a string value that indicates from where
the UA obtained the SCT, as defined in Section 3 of [RFC6962]
and Section 6 of [RFC9162]. The UA MUST set the value to one
of the following: "tls-extension", "ocsp", or "embedded".
These correspond to the three methods of delivering SCTs in the
TLS handshake that are described in Section 3.3 of [RFC6962].
* The "serialized_sct" key, with a string value. If the value of
the "version" key is 1, the UA MUST set this value to the
base64-encoded [RFC4648] serialized SignedCertificateTimestamp
structure from Section 3.2 of [RFC6962]. The base64 encoding
is defined in Section 4 of [RFC4648]. If the value of the
"version" key is 2, the UA MUST set this value to the
base64-encoded [RFC4648] serialized TransItem structure
representing the SCT, as defined in Section 4.5 of [RFC9162].
"failure-mode"
The value indicates whether the Expect-CT report was triggered by
an Expect-CT policy in enforce or report-only mode. The value is
provided as a string. The UA MUST set this value to "enforce" if
the Expect-CT metadata indicates an enforce configuration, and
"report-only" otherwise.
"test-report"
(optional) The value is set to true if the report is being sent by
a testing client to verify that the report server behaves
correctly. The value is provided as a boolean and MUST be set to
true if the report serves to test the server's behavior and can be
discarded.
3.2. Sending a Violation Report
The UA SHOULD report Expect-CT failures for Known Expect-CT Hosts:
that is, when a connection to a Known Expect-CT Host does not comply
with the UA's CT Policy and the host's Expect-CT metadata contains a
report-uri.
Additionally, the UA SHOULD report Expect-CT failures for hosts for
which it does not have any stored Expect-CT metadata; that is, when
the UA connects to a host and receives an Expect-CT header field that
contains the report-uri directive, the UA SHOULD report an Expect-CT
failure if the connection does not comply with the UA's CT Policy.
The steps to report an Expect-CT failure are as follows.
1. Prepare a JSON object report object with the single key "expect-
ct-report", whose value is the result of generating a violation
report object as described in Section 3.1.
2. Let report body be the JSON stringification of report object.
3. Let report-uri be the value of the report-uri directive in the
Expect-CT header field.
4. Send an HTTP POST request to report-uri with a Content-Type
header field of application/expect-ct-report+json and an entity
body consisting of report body.
The UA MAY perform other operations as part of sending the HTTP POST
request, such as sending a Cross-Origin Resource Sharing (CORS)
preflight as part of [FETCH].
Future versions of this specification may need to modify or extend
the Expect-CT report format. They may do so by defining a new top-
level key to contain the report, replacing the "expect-ct-report"
key. Section 3.3 defines how report servers should handle report
formats that they do not support.
3.3. Receiving a Violation Report
Upon receiving an Expect-CT violation report, the report server MUST
respond with a 2xx (Successful) status code if it can parse the
request body as valid JSON, the report conforms to the format
described in Section 3.1, and it recognizes the scheme, hostname, and
port in the "scheme", "hostname", and "port" fields of the report.
If the report body cannot be parsed or does not conform to the format
described in Section 3.1, or the report server does not expect to
receive reports for the scheme, hostname, or port in the report, then
the report server MUST respond with a 400 Bad Request status code.
As described in Section 3.2, future versions of this specification
may define new report formats that are sent with a different top-
level key. If the report server does not recognize the report
format, the report server MUST respond with a 501 Not Implemented
status code.
If the report's "test-report" key is set to true, the server MAY
discard the report without further processing but MUST still return a
2xx (Successful) status code. If the "test-report" key is absent or
set to false, the server SHOULD store the report for processing and
analysis by the owner of the Expect-CT Host.
4. Usability Considerations
When the UA detects a Known Expect-CT Host in violation of the UA's
CT Policy, end users will experience denials of service. It is
advisable for UAs to explain to users why they cannot access the
Expect-CT Host, e.g., in a user interface that explains that the
host's certificate cannot be validated.
5. Authoring Considerations
Expect-CT could be specified as a TLS extension or X.509 certificate
extension instead of an HTTP response header field. Using an HTTP
header field as the mechanism for Expect-CT introduces a layering
mismatch; for example, the software that terminates TLS and validates
Certificate Transparency information might know nothing about HTTP.
Nevertheless, an HTTP header field was chosen primarily for ease of
deployment. In practice, deploying new certificate extensions
requires certificate authorities to support them, and new TLS
extensions require server software updates, including possibly to
servers outside of the site owner's direct control (such as in the
case of a third-party Content Delivery Network (CDN)). Ease of
deployment is a high priority for Expect-CT because it is intended as
a temporary transition mechanism for user agents that are
transitioning to universal Certificate Transparency requirements.
6. Privacy Considerations
Expect-CT can be used to infer what Certificate Transparency Policy a
UA is using by attempting to retrieve specially configured websites
that pass one user agent's policies but not another's. Note that
this consideration is true of UAs that enforce CT policies without
Expect-CT as well.
Additionally, reports submitted to the report-uri could reveal
information to a third party about which web page is being accessed
and by which IP address, by using individual report-uri values for
individually tracked pages. This information could be leaked even if
client-side scripting were disabled.
Implementations store state about Known Expect-CT Hosts and, hence,
which domains the UA has contacted. Implementations may choose to
not store this state subject to local policy (e.g., in the private
browsing mode of a web browser).
Violation reports, as noted in Section 3, contain information about
the certificate chain that has violated the CT Policy. In some
cases, such as an organization-wide compromise of the end-to-end
security of TLS, this may include information about the interception
tools and design used by the organization that the organization would
otherwise prefer not be disclosed.
Because Expect-CT causes remotely detectable behavior, it's advisable
that UAs offer a way for privacy-sensitive end users to clear
currently noted Expect-CT Hosts and allow users to query the current
state of Known Expect-CT Hosts.
7. Security Considerations
7.1. Hostile Header Attacks
When UAs support the Expect-CT header field, it becomes a potential
vector for hostile header attacks against site owners. If a site
owner uses a certificate issued by a certificate authority that does
not embed SCTs nor serve SCTs via the Online Certificate Status
Protocol (OCSP) or TLS extension, a malicious server operator or
attacker could temporarily reconfigure the host to comply with the
UA's CT Policy and add the Expect-CT header field in enforcing mode
with a long max-age. Implementing user agents would note this as an
Expect-CT Host (see Section 2.3.2.1). After having done this, the
configuration could then be reverted to not comply with the CT
Policy, prompting failures. Note that this scenario would require
the attacker to have substantial control over the infrastructure in
question, being able to obtain different certificates, change server
software, or act as a man in the middle in connections.
Site operators can mitigate this situation by one of the following:
reconfiguring their web server to transmit SCTs using the TLS
extension defined in Section 6.5 of [RFC9162]; obtaining a
certificate from an alternative certificate authority that provides
SCTs by one of the other methods; or by waiting for the user agent's
persisted notation of this as an Expect-CT Host to reach its max-age.
User agents may choose to implement mechanisms for users to cure this
situation, as noted in Section 4.
7.2. Maximum max-age
There is a security trade-off in that low maximum values provide a
narrow window of protection for users that visit the Known Expect-CT
Host only infrequently, while high maximum values might result in a
denial of service to a UA in the event of a hostile header attack or
simply an error on the part of the site owner.
There is probably no ideal maximum for the max-age directive. Since
Expect-CT is primarily a policy-expansion and investigation
technology rather than an end-user protection, a value on the order
of 30 days (2,592,000 seconds) may be considered a balance between
these competing security concerns.
7.3. Amplification Attacks
Another kind of hostile header attack uses the report-uri mechanism
on many hosts not currently exposing SCTs as a method to cause a
denial of service to the host receiving the reports. If some highly
trafficked websites emitted a non-enforcing Expect-CT header field
with a report-uri, implementing UAs' reports could flood the
reporting host. It is noted in Section 2.1.1 that UAs should limit
the rate at which they emit reports, but an attacker may alter the
Expect-CT header fields to induce UAs to submit different reports to
different URIs to still cause the same effect.
8. IANA Considerations
8.1. Header Field Registry
This document registers the "Expect-CT" header field in the
"Hypertext Transfer Protocol (HTTP) Field Name Registry" registry
located at <https://www.iana.org/assignments/http-fields>.
Header field name: Expect-CT
Applicable protocol: http
Status: permanent
Author/Change controller: IETF
Specification document(s): This document
Related information: (empty)
8.2. Media Types Registry
This document registers the application/expect-ct-report+json media
type (which uses the suffix established in [RFC6839]) for Expect-CT
violation reports in the "Media Types" registry as follows.
Type name: application
Subtype name: expect-ct-report+json
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: binary
Security considerations: See Section 7
Interoperability considerations: n/a
Published specification: This document
Applications that use this media type: UAs that implement
Certificate Transparency compliance checks and reporting
Additional information:
Deprecated alias names for this type: n/a
Magic number(s): n/a
File extension(s): n/a
Macintosh file type code(s): n/a
Person & email address to contact for further information:
Emily Stark (estark@google.com)
Intended usage: COMMON
Restrictions on usage: none
Author: Emily Stark (estark@google.com)
Change controller: IETF
9. References
9.1. Normative References
[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>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[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>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
<https://www.rfc-editor.org/info/rfc6797>.
[RFC6839] Hansen, T. and A. Melnikov, "Additional Media Type
Structured Syntax Suffixes", RFC 6839,
DOI 10.17487/RFC6839, January 2013,
<https://www.rfc-editor.org/info/rfc6839>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, <https://www.rfc-editor.org/info/rfc7468>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <https://www.rfc-editor.org/info/rfc7469>.
[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>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC9111] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/info/rfc9111>.
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://www.rfc-editor.org/info/rfc9162>.
9.2. Informative References
[FETCH] WHATWG, "Fetch - Living Standard",
<https://fetch.spec.whatwg.org>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Author's Address
Emily Stark
Google
Email: estark@google.com
