RFC 9163 | Expect-CT | June 2022 |
Stark | Experimental | [Page] |
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.¶
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 (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.¶
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.¶
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.¶
Terminology is defined in this section.¶
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].¶
The directives defined in this specification are described below. The overall requirements for directives are:¶
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.¶
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.¶
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.¶
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.¶
delta-seconds
is used as defined in Section 1.3 of [RFC9111].¶
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.¶
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:¶
Expect-CT Hosts SHOULD NOT include the Expect-CT header field in HTTP responses conveyed over non-secure transport.¶
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".¶
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.¶
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:¶
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.¶
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.¶
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:¶
enforce
directive is present.¶
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.¶
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.)¶
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.¶
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).¶
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.¶
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.¶
To generate a violation report object
, the UA constructs a JSON
[RFC8259] object with the following
keys and values:¶
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].¶
enforce
configuration, and
"report-only" otherwise.¶
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.¶
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.¶
report body
be the JSON stringification of report object
.¶
report-uri
be the value of the report-uri
directive in the Expect-CT
header field.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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>.¶
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.¶