rfc9163.original   rfc9163.txt 
HTTP E. Stark Internet Engineering Task Force (IETF) E. Stark
Internet-Draft Google Request for Comments: 9163 Google
Intended status: Experimental December 9, 2018 Category: Experimental June 2022
Expires: June 12, 2019 ISSN: 2070-1721
Expect-CT Extension for HTTP Expect-CT Extension for HTTP
draft-ietf-httpbis-expect-ct-08
Abstract Abstract
This document defines a new HTTP header field named Expect-CT, which This document defines a new HTTP header field named "Expect-CT",
allows web host operators to instruct user agents to expect valid which allows web host operators to instruct user agents (UAs) to
Signed Certificate Timestamps (SCTs) to be served on connections to expect valid Signed Certificate Timestamps (SCTs) to be served on
these hosts. Expect-CT allows web host operators to discover connections to these hosts. Expect-CT allows web host operators to
misconfigurations in their Certificate Transparency deployments. discover misconfigurations in their Certificate Transparency (CT)
Further, web host operaters can use Expect-CT to ensure that, if a UA deployments. Further, web host operators can use Expect-CT to ensure
which supports Expect-CT accepts a misissued certificate, that that if a UA that supports Expect-CT accepts a misissued certificate,
certificate will be discoverable in Certificate Transparency logs. that certificate will be discoverable in Certificate Transparency
logs.
Note to Readers
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at http://httpwg.github.io/
[2]; source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/expect-ct [3].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for examination, experimental implementation, and
evaluation.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document defines an Experimental Protocol for the Internet
and may be updated, replaced, or obsoleted by other documents at any community. This document is a product of the Internet Engineering
time. It is inappropriate to use Internet-Drafts as reference Task Force (IETF). It represents the consensus of the IETF
material or to cite them other than as "work in progress." 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.
This Internet-Draft will expire on June 12, 2019. 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 Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.1. Requirements Language
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology
2. Server and Client Behavior . . . . . . . . . . . . . . . . . 5 2. Server and Client Behavior
2.1. Response Header Field Syntax . . . . . . . . . . . . . . 5 2.1. Response Header Field Syntax
2.1.1. The report-uri Directive . . . . . . . . . . . . . . 6 2.1.1. The report-uri Directive
2.1.2. The enforce Directive . . . . . . . . . . . . . . . . 7 2.1.2. The enforce Directive
2.1.3. The max-age Directive . . . . . . . . . . . . . . . . 7 2.1.3. The max-age Directive
2.1.4. Examples . . . . . . . . . . . . . . . . . . . . . . 7 2.1.4. Examples
2.2. Host Processing Model . . . . . . . . . . . . . . . . . . 8 2.2. Host Processing Model
2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 8 2.2.1. HTTP-over-Secure-Transport Request Type
2.2.2. HTTP Request Type . . . . . . . . . . . . . . . . . . 8 2.2.2. HTTP Request Type
2.3. User Agent Processing Model . . . . . . . . . . . . . . . 8 2.3. User Agent Processing Model
2.3.1. Missing or Malformed Expect-CT Header Fields . . . . 9 2.3.1. Missing or Malformed Expect-CT Header Fields
2.3.2. Expect-CT Header Field Processing . . . . . . . . . . 9 2.3.2. Expect-CT Header Field Processing
2.3.3. Reporting . . . . . . . . . . . . . . . . . . . . . . 11 2.3.3. Reporting
2.4. Evaluating Expect-CT Connections for CT Compliance . . . 11 2.4. Evaluating Expect-CT Connections for CT Compliance
2.4.1. Skipping CT compliance checks . . . . . . . . . . . . 12 2.4.1. Skipping CT Compliance Checks
3. Reporting Expect-CT Failure . . . . . . . . . . . . . . . . . 12 3. Reporting Expect-CT Failure
3.1. Generating a violation report . . . . . . . . . . . . . . 12 3.1. Generating a Violation Report
3.2. Sending a violation report . . . . . . . . . . . . . . . 14 3.2. Sending a Violation Report
3.3. Receiving a violation report . . . . . . . . . . . . . . 15 3.3. Receiving a Violation Report
4. Usability Considerations . . . . . . . . . . . . . . . . . . 16 4. Usability Considerations
5. Authoring Considerations . . . . . . . . . . . . . . . . . . 16 5. Authoring Considerations
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16 6. Privacy Considerations
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 7. Security Considerations
7.1. Hostile header attacks . . . . . . . . . . . . . . . . . 17 7.1. Hostile Header Attacks
7.2. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 17 7.2. Maximum max-age
7.3. Amplification attacks . . . . . . . . . . . . . . . . . . 18 7.3. Amplification Attacks
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 8. IANA Considerations
8.1. Header Field Registry . . . . . . . . . . . . . . . . . . 18 8.1. Header Field Registry
8.2. Media Types Registry . . . . . . . . . . . . . . . . . . 18 8.2. Media Types Registry
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 9. References
9.1. Normative References . . . . . . . . . . . . . . . . . . 19 9.1. Normative References
9.2. Informative References . . . . . . . . . . . . . . . . . 21 9.2. Informative References
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Author's Address
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 21
A.1. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 21
A.2. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 22
A.3. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 22
A.4. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 22
A.5. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 22
A.6. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 22
A.7. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 22
A.8. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction 1. Introduction
This document defines a new HTTP header field that enables UAs to This document defines a new HTTP header field ([RFC9110],
identify web hosts that expect the presence of Signed Certificate Section 6.3) that enables UAs to identify web hosts that expect the
Timestamps (SCTs) [I-D.ietf-trans-rfc6962-bis] in subsequent presence of Signed Certificate Timestamps (SCTs) [RFC9162] in
Transport Layer Security (TLS) [RFC8446] connections. subsequent Transport Layer Security (TLS) [RFC8446] connections.
Web hosts that serve the Expect-CT HTTP header field are noted by the Web hosts that serve the Expect-CT header field are noted by the UA
UA as Known Expect-CT Hosts. The UA evaluates each connection to a as "Known Expect-CT Hosts". The UA evaluates each connection to a
Known Expect-CT Host for compliance with the UA's Certificate Known Expect-CT Host for compliance with the UA's Certificate
Transparency (CT) Policy. If the connection violates the CT Policy, 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/ 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 or fails the connection, depending on the configuration that the
Expect-CT Host has chosen. Expect-CT Host has chosen.
If misconfigured, Expect-CT can cause unwanted connection failures If misconfigured, Expect-CT can cause unwanted connection failures
(for example, if a host deploys Expect-CT but then switches to a (for example, if a host deploys Expect-CT but then switches to a
legitimate certificate that is not logged in Certificate Transparency legitimate certificate that is not logged in Certificate Transparency
logs, or if a web host operator believes their certificate to conform logs or if a web host operator believes their certificate to conform
to all UAs' CT policies but is mistaken). Web host operators are to all UAs' CT policies but is mistaken). Web host operators are
advised to deploy Expect-CT with precautions, by using the reporting advised to deploy Expect-CT with precautions by using the reporting
feature and gradually increasing the time interval during which the feature and gradually increasing the time interval during which the
UA regards the host as a Known Expect-CT Host. These precautions can UA regards the host as a Known Expect-CT Host. These precautions can
help web host operators gain confidence that their Expect-CT help web host operators gain confidence that their Expect-CT
deployment is not causing unwanted connection failures. deployment is not causing unwanted connection failures.
Expect-CT is a trust-on-first-use (TOFU) mechanism. The first time a 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 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 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, 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 Expect-CT provides value by 1) allowing UAs to detect the use of
unlogged certificates after the initial communication, and 2) unlogged certificates after the initial communication, and 2)
allowing web hosts to be confident that UAs are only trusting allowing web hosts to be confident that UAs are only trusting
publicly-auditable certificates. publicly auditable certificates.
Expect-CT is similar to HSTS [RFC6797] and HPKP [RFC7469]. HSTS Expect-CT is similar to HTTP Strict Transport Security (HSTS)
allows web sites to declare themselves accessible only via secure [RFC6797] and HTTP Public Key Pinning (HPKP) [RFC7469]. HSTS allows
connections, and HPKP allows web sites to declare their cryptographic websites to declare themselves accessible only via secure
identifies. Similarly, Expect-CT allows web sites to declare 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 themselves accessible only via connections that are compliant with CT
policy. Policy.
This Expect-CT specification is compatible with [RFC6962] and This Expect-CT specification is compatible with [RFC6962] and
[I-D.ietf-trans-rfc6962-bis], but not with future versions of [RFC9162], but not necessarily with future versions of Certificate
Certificate Transparency. Expect-CT header fields will be ignore Transparency. UAs will ignore Expect-CT header fields from web hosts
from web hosts which use future versions of Certificate Transparency, that use future versions of Certificate Transparency, unless a future
unless a future version of this document specifies how they should be version of this document specifies how they should be processed.
processed.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
1.2. Terminology 1.2. Terminology
Terminology is defined in this section. Terminology is defined in this section.
o "Certificate Transparency Policy" is a policy defined by the UA "Certificate Transparency Policy"
concerning the number, sources, and delivery mechanisms of Signed A policy defined by the UA concerning the number, sources, and
Certificate Timestamps that are associated with TLS connections. delivery mechanisms of Signed Certificate Timestamps that are
The policy defines the properties of a connection that must be met associated with TLS connections. The policy defines the
in order for the UA to consider it CT-qualified. properties of a connection that must be met in order for the UA to
consider it CT qualified.
o "Certificate Transparency Qualified" describes a TLS connection "Certificate Transparency Qualified"
for which the UA has determined that a sufficient quantity and Describes a TLS connection for which the UA has determined that a
quality of Signed Certificate Timestamps have been provided. sufficient quantity and quality of Signed Certificate Timestamps
have been provided.
o "CT-qualified" is an abbreviation for "Certificate Transparency "CT Qualified"
Qualified". An abbreviation for "Certificate Transparency Qualified".
o "CT Policy" is an abbreviation for "Certificate Transparency "CT Policy"
Policy". An abbreviation for "Certificate Transparency Policy".
o "Effective Expect-CT Date" is the time at which a UA observed a "Effective Expect-CT Date"
valid Expect-CT header field for a given host. The time at which a UA observed a valid Expect-CT header field for
a given host.
o "Expect-CT Host" is a conformant host implementing the HTTP server "Expect-CT Host"
aspects of Expect-CT. This means that an Expect-CT Host returns A conformant host implementing the HTTP server aspects of Expect-
the "Expect-CT" HTTP response header field in its HTTP response CT. This means that an Expect-CT Host returns the Expect-CT
messages sent over secure transport. The term "host" is response header field in its HTTP response messages sent over
equivalent to "server" in this specification. secure transport. The term "host" is equivalent to "server" in
this specification.
o "Known Expect-CT Host" is an Expect-CT Host that the UA has noted "Known Expect-CT Host"
as such. See Section 2.3.2.1 for particulars. An Expect-CT Host that the UA has noted as such. See
Section 2.3.2.1 for particulars.
o UA is an acronym for "user agent". For the purposes of this "User Agent (UA)"
specification, a UA is an HTTP client application typically For the purposes of this specification, a UA is an HTTP client
actively manipulated by a user [RFC7230]. application typically actively manipulated by a user [RFC9110].
o "Unknown Expect-CT Host" is an Expect-CT Host that the UA has not "Unknown Expect-CT Host"
noted. An Expect-CT Host that the UA has not noted.
2. Server and Client Behavior 2. Server and Client Behavior
2.1. Response Header Field Syntax 2.1. Response Header Field Syntax
The "Expect-CT" response header field is a new field defined in this 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 specification. It is used by a server to indicate that UAs should
evaluate connections to the host emitting the header field for CT evaluate connections to the host emitting the header field for CT
compliance (Section 2.4). compliance (Section 2.4).
Figure 1 describes the syntax (Augmented Backus-Naur Form) of the Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
header field, using the grammar defined in [RFC5234] and the rules header field, using the grammar defined in [RFC5234] and the rules
defined in Section 3.2 of [RFC7230]. The "#" ABNF extension is defined in Section 5 of [RFC9110]. The "#" ABNF extension is
specified in Section 7 of [RFC7230]. specified in Section 5.6.1 of [RFC9110].
Expect-CT = 1#expect-ct-directive Expect-CT = 1#expect-ct-directive
expect-ct-directive = directive-name [ "=" directive-value ] expect-ct-directive = directive-name [ "=" directive-value ]
directive-name = token directive-name = token
directive-value = token / quoted-string directive-value = token / quoted-string
Figure 1: Syntax of the Expect-CT header field Figure 1: Syntax of the Expect-CT Header Field
The directives defined in this specification are described below. The directives defined in this specification are described below.
The overall requirements for directives are: The overall requirements for directives are:
1. The order of appearance of directives is not significant. 1. The order of appearance of directives is not significant.
2. A given directive MUST NOT appear more than once in a given 2. A given directive MUST NOT appear more than once in a given
header field. Directives are either optional or required, as header field. Directives are either optional or required, as
stipulated in their definitions. stipulated in their definitions.
3. Directive names are case insensitive. 3. Directive names are case insensitive.
4. UAs MUST ignore any header fields containing directives, or other 4. UAs MUST ignore any header fields containing directives, or other
header field value data that do not conform to the syntax defined header field value data that does not conform to the syntax
in this specification. In particular, UAs MUST NOT attempt to defined in this specification. In particular, UAs MUST NOT
fix malformed header fields. attempt to fix malformed header fields.
5. If a header field contains any directive(s) the UA does not 5. If a header field contains any directive(s) the UA does not
recognize, the UA MUST ignore those directives. recognize, the UA MUST ignore those directives.
6. If the Expect-CT header field otherwise satisfies the above 6. If the Expect-CT header field otherwise satisfies the above
requirements (1 through 5), and Expect-CT is not disabled for requirements (1 through 5), and Expect-CT is not disabled for
local policy reasons (as discussed in Section 2.4.1), the UA MUST local policy reasons (as discussed in Section 2.4.1), the UA MUST
process the directives it recognizes. process the directives it recognizes.
2.1.1. The report-uri Directive 2.1.1. The report-uri Directive
The OPTIONAL "report-uri" directive indicates the URI to which the UA The OPTIONAL report-uri directive indicates the URI to which the UA
SHOULD report Expect-CT failures (Section 2.4). The UA POSTs the SHOULD report Expect-CT failures (Section 2.4). The UA POSTs the
reports to the given URI as described in Section 3. reports to the given URI as described in Section 3.
The "report-uri" directive is REQUIRED to have a directive value, for The report-uri directive is REQUIRED to have a directive value, for
which the syntax is defined in Figure 2. which the syntax is defined in Figure 2.
report-uri-value = absolute-URI report-uri-value = (DQUOTE absolute-URI DQUOTE) / absolute-URI
Figure 2: Syntax of the report-uri directive value Figure 2: Syntax of the report-uri Directive Value
"absolute-URI" is defined in Section 4.3 of [RFC3986]. The 'report-uri-value' MUST be quoted if it contains any character
not allowed in 'token'.
UAs MUST ignore "report-uri"s that do not use the HTTPS scheme. UAs absolute-URI is defined in Section 4.3 of [RFC3986].
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 UAs MUST ignore any report-uri that does not use the HTTPS scheme.
the host in the "report-uri" is a Known HSTS Host. 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 UAs SHOULD make their best effort to report Expect-CT failures to the
"report-uri", but they may fail to report in exceptional conditions. report-uri, but they may fail to report in exceptional conditions.
For example, if connecting to the "report-uri" itself incurs an For example, if connecting to the report-uri itself incurs an Expect-
Expect-CT failure or other certificate validation failure, the UA CT failure or other certificate validation failure, the UA MUST
MUST cancel the connection. Similarly, if Expect-CT Host A sets a cancel the connection. Similarly, if Expect-CT Host A sets a report-
"report-uri" referring to Expect-CT Host B, and if B sets a "report- uri referring to Expect-CT Host B, and if B sets a report-uri
uri" referring to A, and if both hosts fail to comply to the UA's CT 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 Policy, the UA SHOULD detect and break the loop by failing to send
reports to and about those hosts. reports to and about those hosts.
Note that the report-uri need not necessarily be in the same Internet 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 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 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 failures on the Expect-CT Host do not prevent reports from being
sent. sent.
UAs SHOULD limit the rate at which they send reports. For example, 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" it is unnecessary to send the same report to the same report-uri more
more than once in the same web browsing session. than once in the same web-browsing session.
2.1.2. The enforce Directive 2.1.2. The enforce Directive
The OPTIONAL "enforce" directive is a valueless directive that, if The OPTIONAL enforce directive is a valueless directive that, if
present (i.e., it is "asserted"), signals to the UA that compliance present (i.e., it is "asserted"), signals to the UA that compliance
to the CT Policy should be enforced (rather than report-only) and to the CT Policy should be enforced (rather than report-only) and
that the UA should refuse future connections that violate its CT that the UA should refuse future connections that violate its CT
Policy. When both the "enforce" directive and "report-uri" directive Policy. When both the enforce directive and report-uri directive (as
(as defined in Figure 2) are present, the configuration is referred defined in Figure 2) are present, the configuration is referred to as
to as an "enforce-and-report" configuration, signalling to the UA an "enforce-and-report" configuration, signaling to the UA that both
both that compliance to the CT Policy should be enforced and that compliance to the CT Policy should be enforced and violations should
violations should be reported. be reported.
2.1.3. The max-age Directive 2.1.3. The max-age Directive
The "max-age" directive specifies the number of seconds after the The max-age directive specifies the number of seconds after the
reception of the Expect-CT header field during which the UA SHOULD 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- regard the host from whom the message was received as a Known Expect-
CT Host. CT Host.
If a response contains an "Expect-CT" header field, then the response If a response contains an Expect-CT header field, then the response
MUST contain an "Expect-CT" header field with a "max-age" directive. MUST contain an Expect-CT header field with a max-age directive. (A
(A "max-age" directive need not appear in every "Expect-CT" header max-age directive need not appear in every Expect-CT header field in
field in the response.) The "max-age" directive is REQUIRED to have the response.) The max-age directive is REQUIRED to have a directive
a directive value, for which the syntax (after quoted-string value, for which the syntax (after quoted-string unescaping, if
unescaping, if necessary) is defined in Figure 3. necessary) is defined in Figure 3.
max-age-value = delta-seconds max-age-value = delta-seconds
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
Figure 3: Syntax of the max-age directive value Figure 3: Syntax of the max-age Directive Value
"delta-seconds" is used as defined in Section 1.2.1 of [RFC7234]. delta-seconds is used as defined in Section 1.3 of [RFC9111].
2.1.4. Examples 2.1.4. Examples
The following three examples demonstrate valid Expect-CT response The following three examples demonstrate valid Expect-CT response
header fields (where the second splits the directives into two field header fields (where the second splits the directives into two field
instances): instances):
Expect-CT: max-age=86400, enforce Expect-CT: max-age=86400, enforce
Expect-CT: max-age=86400,enforce Expect-CT: max-age=86400,enforce
Expect-CT: report-uri="https://foo.example/report" Expect-CT: report-uri="https://foo.example/report"
Expect-CT: max-age=86400,report-uri="https://foo.example/report" Expect-CT: max-age=86400,report-uri="https://foo.example/report"
Figure 4: Examples of valid Expect-CT response header fields Figure 4: Examples of Valid Expect-CT ResponseHeader Fields
2.2. Host Processing Model 2.2. Host Processing Model
This section describes the processing model that Expect-CT Hosts This section describes the processing model that Expect-CT Hosts
implement. The model has 2 parts: (1) the processing rules for HTTP implement. The model has 2 parts: (1) the processing rules for HTTP
request messages received over a secure transport (e.g., request messages received over a secure transport (e.g.,
authenticated, non-anonymous TLS); and (2) the processing rules for authenticated, non-anonymous TLS); and (2) the processing rules for
HTTP request messages received over non-secure transports, such as HTTP request messages received over non-secure transports, such as
TCP. TCP.
skipping to change at page 8, line 35 skipping to change at line 351
An Expect-CT Host includes an Expect-CT header field in its response. 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. 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 Establishing a given host as an Expect-CT Host, in the context of a
given UA, is accomplished as follows: given UA, is accomplished as follows:
1. Over the HTTP protocol running over secure transport, by 1. Over the HTTP protocol running over secure transport, by
correctly returning (per this specification) a valid Expect-CT correctly returning (per this specification) a valid Expect-CT
header field to the UA. header field to the UA.
2. Through other mechanisms, such as a client-side preloaded Expect- 2. Through other mechanisms such as a client-side preloaded Expect-
CT Host list. CT Host list.
2.2.2. HTTP Request Type 2.2.2. HTTP Request Type
Expect-CT Hosts SHOULD NOT include the Expect-CT header field in HTTP Expect-CT Hosts SHOULD NOT include the Expect-CT header field in HTTP
responses conveyed over non-secure transport. responses conveyed over non-secure transport.
2.3. User Agent Processing Model 2.3. User Agent Processing Model
The UA processing model relies on parsing domain names. Note that The UA processing model relies on parsing domain names. Note that
internationalized domain names SHALL be canonicalized by the UA internationalized domain names SHALL be canonicalized by the UA
according to the scheme in Section 10 of [RFC6797]. according to the scheme in Section 10 of [RFC6797].
The UA stores Known Expect-CT Hosts and their associated Expect-CT The UA stores Known Expect-CT Hosts and their associated Expect-CT
directives. This data is collectively known as a host's "Expect-CT" directives. This data is collectively known as a host's "Expect-CT
metadata". metadata".
2.3.1. Missing or Malformed Expect-CT Header Fields 2.3.1. Missing or Malformed Expect-CT Header Fields
If an HTTP response does not include an Expect-CT header field that 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 conforms to the grammar specified in Section 2.1, then the UA MUST
NOT update any Expect-CT metadata. NOT update any Expect-CT metadata.
2.3.2. Expect-CT Header Field Processing 2.3.2. Expect-CT Header Field Processing
If the UA receives an HTTP response over a secure transport that If the UA receives an HTTP response over a secure transport that
includes an Expect-CT header field conforming to the grammar includes an Expect-CT header field conforming to the grammar
specified in Section 2.1, the UA MUST evaluate the connection on 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 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 Policy, and then process the Expect-CT header field as follows. UAs
MUST ignore any Expect-CT header field received in an HTTP response MUST ignore any Expect-CT header field received in an HTTP response
conveyed over non-secure transport. conveyed over non-secure transport.
If the connection does not comply with the UA's CT Policy (i.e., the 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 connection is not CT qualified), then the UA MUST NOT update any
Expect-CT metadata. If the header field includes a "report-uri" Expect-CT metadata. If the header field includes a report-uri
directive, the UA SHOULD send a report to the specified "report-uri" directive, the UA SHOULD send a report to the specified report-uri
(Section 2.3.3). (Section 2.3.3).
If the connection complies with the UA's CT Policy (i.e., the If the connection complies with the UA's CT Policy (i.e., the
connection is CT-qualified), then the UA MUST either: connection is CT qualified), then the UA MUST either:
o Note the host as a Known Expect-CT Host if it is not already so * Note the host as a Known Expect-CT Host if it is not already so
noted (see Section 2.3.2.1), or noted (see Section 2.3.2.1) or
o Update the UA's cached information for the Known Expect-CT Host if * Update the UA's cached information for the Known Expect-CT Host if
the "enforce", "max-age", or "report-uri" header field value the enforce, max-age, or report-uri header field value directives
directives convey information different from that already convey information different from that already maintained by the
maintained by the UA. If the "max-age" directive has a value of UA. If the max-age directive has a value of 0, the UA MUST remove
0, the UA MUST remove its cached Expect-CT information if the host its cached Expect-CT information if the host was previously noted
was previously noted as a Known Expect-CT Host, and MUST NOT note as a Known Expect-CT Host and MUST NOT note this host as a Known
this host as a Known Expect-CT Host if it is not already noted. Expect-CT Host if it is not already noted.
If a UA receives an Expect-CT header field over a CT-compliant If a UA receives an Expect-CT header field over a CT-compliant
connection which uses a version of Certificate Transparency other connection that uses a version of Certificate Transparency other than
than [RFC6962] or [I-D.ietf-trans-rfc6962-bis], the UA MUST ignore [RFC6962] or [RFC9162], the UA MUST ignore the Expect-CT header field
the Expect-CT header field and clear any Expect-CT metadata and clear any Expect-CT metadata associated with the host.
associated with the host.
2.3.2.1. Noting Expect-CT 2.3.2.1. Noting Expect-CT
Upon receipt of the Expect-CT response header field over an error- Upon receipt of the Expect-CT response header field over an error-
free TLS connection (with X.509 certificate chain validation as free TLS connection (with X.509 certificate chain validation as
described in [RFC5280], as well as the validation described in described in [RFC5280], as well as the validation described in
Section 2.4), the UA MUST note the host as a Known Expect-CT Host, Section 2.4 of this document), the UA MUST note the host as a Known
storing the host's domain name and its associated Expect-CT Expect-CT Host, storing the host's domain name and its associated
directives in non-volatile storage. Expect-CT directives in non-volatile storage.
To note a host as a Known Expect-CT Host, the UA MUST set its Expect- 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 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 Section 2.3.2.2), using the metadata given in the most recently
received valid Expect-CT header field. received valid Expect-CT header field.
For forward compatibility, the UA MUST ignore any unrecognized For forward compatibility, the UA MUST ignore any unrecognized
Expect-CT header field directives, while still processing those Expect-CT header field directives while still processing those
directives it does recognize. Section 2.1 specifies the directives directives it does recognize. Section 2.1 specifies the directives
"enforce", "max-age", and "report-uri", but future specifications and enforce, max-age, and report-uri, but future specifications and
implementations might use additional directives. implementations might use additional directives.
2.3.2.2. Storage Model 2.3.2.2. Storage Model
If the substring matching the host production from the Request-URI If the substring matching the host production from the Request-URI
(of the message to which the host responded) does not exactly match (of the message to which the host responded) does not exactly match
an existing Known Expect-CT Host's domain name, per the matching an existing Known Expect-CT Host's domain name, per the matching
procedure for a Congruent Match specified in Section 8.2 of 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 [RFC6797], then the UA MUST add this host to the Known Expect-CT Host
cache. The UA caches: cache. The UA caches:
o the Expect-CT Host's domain name, * the Expect-CT Host's domain name.
o whether the "enforce" directive is present * whether the enforce directive is present.
o the Effective Expiration Date, which is the Effective Expect-CT * the Effective Expiration Date, which is the Effective Expect-CT
Date plus the value of the "max-age" directive. Alternatively, Date plus the value of the max-age directive. Alternatively, the
the UA MAY cache enough information to calculate the Effective UA MAY cache enough information to calculate the Effective
Expiration Date. The Effective Expiration Date is calculated from Expiration Date. The Effective Expiration Date is calculated from
when the UA observed the Expect-CT header field and is independent when the UA observed the Expect-CT header field and is independent
of when the response was generated. of when the response was generated.
o the value of the "report-uri" directive, if present. * the value of the report-uri directive, if present.
If any other metadata from optional or future Expect-CT header If any other metadata from optional or future Expect-CT header
directives are present in the Expect-CT header field, and the UA directives are present in the Expect-CT header field, and the UA
understands them, the UA MAY note them as well. 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 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 have noted erroneous Expect-CT Hosts (whether by accident or due to
attack) have some chance of recovering over time. If the server sets 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 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 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 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 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. 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 (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- value of 60 days instead of the 90-day value provided by the Expect-
CT host.) CT Host.)
2.3.3. Reporting 2.3.3. Reporting
If the UA receives, over a secure transport, an HTTP response that If the UA receives, over a secure transport, an HTTP response that
includes an Expect-CT header field with a "report-uri" directive, and 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 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 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 Expect-CT report for this connection, then the UA SHOULD send a
report to the specified "report-uri" as specified in Section 3. report to the specified report-uri as specified in Section 3.
2.4. Evaluating Expect-CT Connections for CT Compliance 2.4. Evaluating Expect-CT Connections for CT Compliance
When a UA sets up a TLS connection, the UA determines whether the 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 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 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 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 in its cache and not treat such hosts as Known
Expect-CT hosts. Expect-CT Hosts.
When a UA connects to a Known Expect-CT Host using a TLS connection, 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 if the TLS connection has no errors, then the UA will apply an
additional correctness check: compliance with a CT Policy. A UA additional correctness check: compliance with a CT Policy. A UA
should evaluate compliance with its CT Policy whenever connecting to should evaluate compliance with its CT Policy whenever connecting to
a Known Expect-CT Host. However, the check can be skipped for local 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 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 other checks cause the UA to terminate the connection before CT
compliance is evaluated. For example, a Public Key Pinning failure compliance is evaluated. For example, a Public Key Pinning failure
[RFC7469] could cause the UA to terminate the connection before CT [RFC7469] could cause the UA to terminate the connection before CT
compliance is checked. Similarly, if the UA terminates the compliance is checked. Similarly, if the UA terminates the
connection due to an Expect-CT failure, this could cause the UA to connection due to an Expect-CT failure, this could cause the UA to
skip subsequent correctness checks. When the CT compliance check is skip subsequent correctness checks. When the CT compliance check is
skipped or bypassed, Expect-CT reports (Section 3) will not be sent. 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 When CT compliance is evaluated for a Known Expect-CT Host, the UA
MUST evaluate compliance when setting up the TLS session, before MUST evaluate compliance when setting up the TLS session, before
beginning an HTTP conversation over the TLS channel. beginning an HTTP conversation over the TLS channel.
If a connection to a Known Expect-CT Host violates the UA's CT policy 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- (i.e., the connection is not CT qualified), and if the Known Expect-
CT Host's Expect-CT metadata indicates an "enforce" configuration, CT Host's Expect-CT metadata indicates an enforce configuration, the
the UA MUST treat the CT compliance failure as an error. The UA MAY UA MUST treat the CT compliance failure as an error. The UA MAY
allow the user to bypass the error, unless connection errors should allow the user to bypass the error unless connection errors should
have no user recourse due to other policies in effect (such as HSTS, have no user recourse due to other policies in effect (such as HSTS,
as described in Section 12.1 of [RFC6797]. as described in Section 12.1 of [RFC6797]).
If a connection to a Known Expect-CT Host violates the UA's CT 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 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 a report-uri, the UA SHOULD send an Expect-CT report to that report-
"report-uri" (Section 3). uri (Section 3).
2.4.1. Skipping CT compliance checks 2.4.1. Skipping CT Compliance Checks
It is acceptable for a UA to skip CT compliance checks for some hosts 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 according to local policy. For example, a UA MAY disable CT
compliance checks for hosts whose validated certificate chain compliance checks for hosts whose validated certificate chain
terminates at a user-defined trust anchor, rather than a trust anchor terminates at a user-defined trust anchor rather than a trust anchor
built-in to the UA (or underlying platform). built in to the UA (or underlying platform).
If the UA does not evaluate CT compliance, e.g., because the user has If the UA does not evaluate CT compliance, e.g., because the user has
elected to disable it, or because a presented certificate chain elected to disable it, or because a presented certificate chain
chains up to a user-defined trust anchor, UAs SHOULD NOT send Expect- chains up to a user-defined trust anchor, UAs SHOULD NOT send Expect-
CT reports. CT reports.
3. Reporting Expect-CT Failure 3. Reporting Expect-CT Failure
When the UA attempts to connect to a Known Expect-CT Host and the 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 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 failures to the report-uri, if any, in the Known Expect-CT Host's
Expect-CT metadata. Expect-CT metadata.
When the UA receives an Expect-CT response header field over a 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 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 an Expect-CT report for this connection, then the UA SHOULD report
Expect-CT failures to the configured "report-uri", if any. Expect-CT failures to the configured report-uri, if any.
3.1. Generating a violation report 3.1. Generating a Violation Report
To generate a violation report object, the UA constructs a JSON To generate a violation report object, the UA constructs a JSON
[RFC8259] object with the following keys and values: [RFC8259] object with the following keys and values:
o "date-time": the value for this key indicates the UTC time that "date-time"
the UA observed the CT compliance failure. The value is a string The value for this key indicates the UTC time that the UA observed
formatted according to Section 5.6, "Internet Date/Time Format", the CT compliance failure. The value is a string formatted
of [RFC3339]. according to Section 5.6 of [RFC3339], "Internet Date/Time
Format".
o "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.
o "port": the value is the port to which the UA made the original "hostname"
The value is the hostname to which the UA made the original
request that failed the CT compliance check. The value is request that failed the CT compliance check. The value is
provided as an integer. provided as a string.
o "scheme": the value is the scheme with which the UA made the "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 original request that failed the CT compliance check. The value
is provided as a string. This key is optional and is assumed to is provided as a string. This key is optional and is assumed to
be "https" if not present. be "https" if not present.
o "effective-expiration-date": the value indicates the Effective "effective-expiration-date"
Expiration Date (see Section 2.3.2.2) for the Expect-CT Host that The value indicates the Effective Expiration Date (see
failed the CT compliance check, in UTC. The value is provided as Section 2.3.2.2) for the Expect-CT Host that failed the CT
a string formatted according to Section 5.6 of [RFC3339] compliance check, in UTC. The value is provided as a string
("Internet Date/Time Format"). formatted according to Section 5.6 of [RFC3339], "Internet Date/
Time Format".
o "served-certificate-chain": the value is the certificate chain as "served-certificate-chain"
served by the Expect-CT Host during TLS session setup. The value The value is the certificate chain as served by the Expect-CT Host
is provided as an array of strings, which MUST appear in the order during TLS session setup. The value is provided as an array of
that the certificates were served; each string in the array is the strings, which MUST appear in the order that the certificates were
Privacy-Enhanced Mail (PEM) representation of each X.509 served; each string in the array is the Privacy-Enhanced Mail
certificate as described in [RFC7468]. (PEM) representation of each X.509 certificate as described in
[RFC7468].
o "validated-certificate-chain": the value is the certificate chain "validated-certificate-chain"
as constructed by the UA during certificate chain verification. The value is the certificate chain as constructed by the UA during
(This may differ from the value of the "served-certificate-chain" certificate chain verification. (This may differ from the value
key.) The value is provided as an array of strings, which MUST of the "served-certificate-chain" key.) The value is provided as
appear in the order matching the chain that the UA validated; each an array of strings, which MUST appear in the order matching the
string in the array is the Privacy-Enhanced Mail (PEM) chain that the UA validated; each string in the array is the PEM
representation of each X.509 certificate as described in representation of each X.509 certificate as described in
[RFC7468]. The first certificate in the chain represents the end- [RFC7468]. The first certificate in the chain represents the end-
entity certificate being verified. UAs that build certificate entity certificate being verified. UAs that build certificate
chains in more than one way during the validation process SHOULD chains in more than one way during the validation process SHOULD
send the last chain built. send the last chain built.
o "scts": the value represents the SCTs (if any) that the UA "scts"
received for the Expect-CT host and their validation statuses. The value represents the SCTs (if any) that the UA received for
The value is provided as an array of JSON objects. The SCTs may the Expect-CT Host and their validation statuses. The value is
appear in any order. Each JSON object in the array has the provided as an array of JSON objects. The SCTs may appear in any
following keys: order. Each JSON object in the array has the following keys:
* A "version" key, with an integer value. The UA MUST set this * 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 value to 1 if the SCT is in the format defined in Section 3.2
of [RFC6962] and "2" if it is in the format defined in of [RFC6962] or 2 if it is in the format defined in Section 4.5
Section 4.5 of [I-D.ietf-trans-rfc6962-bis]. of [RFC9162].
* The "status" key, with a string value that the UA MUST set to * The "status" key, with a string value that the UA MUST set to
one of the following values: "unknown" (indicating that the UA one of the following values: "unknown" (indicating that the UA
does not have or does not trust the public key of the log from does not have or does not trust the public key of the log from
which the SCT was issued), "valid" (indicating that the UA which the SCT was issued); "valid" (indicating that the UA
successfully validated the SCT as described in Section 5.2 of successfully validated the SCT as described in Section 5.2 of
[RFC6962] or Section 8.2.3 of [I-D.ietf-trans-rfc6962-bis]), or [RFC6962] or Section 8.1.3 of [RFC9162]); or "invalid"
"invalid" (indicating that the SCT validation failed because of (indicating that the SCT validation failed because of a bad
a bad signature or an invalid timestamp). signature or an invalid timestamp).
* The "source" key, with a string value that indicates from where * The "source" key, with a string value that indicates from where
the UA obtained the SCT, as defined in Section 3 of [RFC6962] the UA obtained the SCT, as defined in Section 3 of [RFC6962]
and Section 6 of [I-D.ietf-trans-rfc6962-bis]. The UA MUST set and Section 6 of [RFC9162]. The UA MUST set the value to one
the value to one of "tls-extension", "ocsp", or "embedded". of the following: "tls-extension", "ocsp", or "embedded".
These correspond to the three methods of delivering SCTs in the These correspond to the three methods of delivering SCTs in the
TLS handshake that are described in Section 3.3 of [RFC6962]. TLS handshake that are described in Section 3.3 of [RFC6962].
* The "serialized_sct" key, with a string value. If the value of * 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 the "version" key is 1, the UA MUST set this value to the
base64 encoded [RFC4648] serialized base64-encoded [RFC4648] serialized SignedCertificateTimestamp
"SignedCertificateTimestamp" structure from Section 3.2 of structure from Section 3.2 of [RFC6962]. The base64 encoding
[RFC6962]. The base64 encoding is defined in Section 4 of is defined in Section 4 of [RFC4648]. If the value of the
[RFC4648]. If the value of the "version" key is "2", the UA "version" key is 2, the UA MUST set this value to the
MUST set this value to the base64 encoded [RFC4648] serialized base64-encoded [RFC4648] serialized TransItem structure
"TransItem" structure representing the SCT, as defined in representing the SCT, as defined in Section 4.5 of [RFC9162].
Section 4.5 of [I-D.ietf-trans-rfc6962-bis].
o "failure-mode": the value indicates whether the Expect-CT report "failure-mode"
was triggered by an Expect-CT policy in enforce or report-only The value indicates whether the Expect-CT report was triggered by
mode. The value is provided as a string. The UA MUST set this an Expect-CT policy in enforce or report-only mode. The value is
value to "enforce" if the Expect-CT metadata indicates an provided as a string. The UA MUST set this value to "enforce" if
"enforce" configuration, and "report-only" otherwise. the Expect-CT metadata indicates an enforce configuration, and
"report-only" otherwise.
o "test-report": the value is set to true if the report is being "test-report"
sent by a testing client to verify that the report server behaves (optional) The value is set to true if the report is being sent by
correctly. The value is provided as a boolean, and MUST be set to 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 true if the report serves to test the server's behavior and can be
discarded. discarded.
3.2. Sending a violation report 3.2. Sending a Violation Report
The UA SHOULD report Expect-CT failures for Known Expect-CT Hosts: 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 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 with the UA's CT Policy and the host's Expect-CT metadata contains a
"report-uri". report-uri.
Additionally, the UA SHOULD report Expect-CT failures for hosts for Additionally, the UA SHOULD report Expect-CT failures for hosts for
which it does not have any stored Expect-CT metadata. That is, when 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 the UA connects to a host and receives an Expect-CT header field that
which contains the "report-uri" directive, the UA SHOULD report an contains the report-uri directive, the UA SHOULD report an Expect-CT
Expect-CT failure if the the connection does not comply with the UA's failure if the connection does not comply with the UA's CT Policy.
CT Policy.
The steps to report an Expect-CT failure are as follows. The steps to report an Expect-CT failure are as follows.
1. Prepare a JSON object "report object" with the single key 1. Prepare a JSON object report object with the single key "expect-
"expect-ct-report", whose value is the result of generating a ct-report", whose value is the result of generating a violation
violation report object as described in Section 3.1. report object as described in Section 3.1.
2. Let "report body" be the JSON stringification of "report object". 2. Let report body be the JSON stringification of report object.
3. Let "report-uri" be the value of the "report-uri" directive in 3. Let report-uri be the value of the report-uri directive in the
the Expect-CT header field. Expect-CT header field.
4. Send an HTTP POST request to "report-uri" with a "Content-Type" 4. Send an HTTP POST request to report-uri with a Content-Type
header field of "application/expect-ct-report+json", and an header field of application/expect-ct-report+json and an entity
entity body consisting of "report body". body consisting of report body.
The UA MAY perform other operations as part of sending the HTTP POST The UA MAY perform other operations as part of sending the HTTP POST
request, for example sending a CORS preflight as part of [FETCH]. 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 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- 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" level key to contain the report, replacing the "expect-ct-report"
key. Section 3.3 defines how report servers should handle report key. Section 3.3 defines how report servers should handle report
formats that they do not support. formats that they do not support.
3.3. Receiving a violation report 3.3. Receiving a Violation Report
Upon receiving an Expect-CT violation report, the report server MUST Upon receiving an Expect-CT violation report, the report server MUST
respond with a 2xx (Successful) status code if it can parse the respond with a 2xx (Successful) status code if it can parse the
request body as valid JSON, the report conforms to the format request body as valid JSON, the report conforms to the format
described in Section 3.1, and it recognizes the scheme, hostname, and described in Section 3.1, and it recognizes the scheme, hostname, and
port in the "scheme", "hostname", and "port" fields of the report. port in the "scheme", "hostname", and "port" fields of the report.
If the report body cannot be parsed, or the report does not conform If the report body cannot be parsed or does not conform to the format
to the format described in Section 3.1, or the report server does not described in Section 3.1, or the report server does not expect to
expect to receive reports for the scheme, hostname, or port in the receive reports for the scheme, hostname, or port in the report, then
report, then the report server MUST respond with a 400 Bad Request the report server MUST respond with a 400 Bad Request status code.
status code.
As described in Section 3.2, future versions of this specification As described in Section 3.2, future versions of this specification
may define new report formats that are sent with a different top- may define new report formats that are sent with a different top-
level key. If the report server does not recognize the report level key. If the report server does not recognize the report
format, the report server MUST respond with a 501 Not Implemented format, the report server MUST respond with a 501 Not Implemented
status code. status code.
If the report's "test-report" key is set to true, the server MAY 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 discard the report without further processing but MUST still return a
2xx (Successful) status code. If the "test-report" key is absent or 2xx (Successful) status code. If the "test-report" key is absent or
skipping to change at page 16, line 18 skipping to change at line 721
CT Policy, end users will experience denials of service. It is CT Policy, end users will experience denials of service. It is
advisable for UAs to explain to users why they cannot access the 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 Expect-CT Host, e.g., in a user interface that explains that the
host's certificate cannot be validated. host's certificate cannot be validated.
5. Authoring Considerations 5. Authoring Considerations
Expect-CT could be specified as a TLS extension or X.509 certificate Expect-CT could be specified as a TLS extension or X.509 certificate
extension instead of an HTTP response header field. Using an HTTP extension instead of an HTTP response header field. Using an HTTP
header field as the mechanism for Expect-CT introduces a layering header field as the mechanism for Expect-CT introduces a layering
mismatch: for example, the software that terminates TLS and validates mismatch; for example, the software that terminates TLS and validates
Certificate Transparency information might know nothing about HTTP. Certificate Transparency information might know nothing about HTTP.
Nevertheless, an HTTP header field was chosen primarily for ease of Nevertheless, an HTTP header field was chosen primarily for ease of
deployment. In practice, deploying new certificate extensions deployment. In practice, deploying new certificate extensions
requires certificate authorities to support them, and new TLS requires certificate authorities to support them, and new TLS
extensions require server software updates, including possibly to extensions require server software updates, including possibly to
servers outside of the site owner's direct control (such as in the servers outside of the site owner's direct control (such as in the
case of a third-party CDN). Ease of deployment is a high priority case of a third-party Content Delivery Network (CDN)). Ease of
for Expect-CT because it is intended as a temporary transition deployment is a high priority for Expect-CT because it is intended as
mechanism for user agents that are transitioning to universal a temporary transition mechanism for user agents that are
Certificate Transparency requirements. transitioning to universal Certificate Transparency requirements.
6. Privacy Considerations 6. Privacy Considerations
Expect-CT can be used to infer what Certificate Transparency policy a Expect-CT can be used to infer what Certificate Transparency Policy a
UA is using, by attempting to retrieve specially-configured websites UA is using by attempting to retrieve specially configured websites
which pass one user agents' policies but not another's. Note that that pass one user agent's policies but not another's. Note that
this consideration is true of UAs which enforce CT policies without this consideration is true of UAs that enforce CT policies without
Expect-CT as well. Expect-CT as well.
Additionally, reports submitted to the "report-uri" could reveal Additionally, reports submitted to the report-uri could reveal
information to a third party about which webpage is being accessed information to a third party about which web page is being accessed
and by which IP address, by using individual "report-uri" values for and by which IP address, by using individual report-uri values for
individually-tracked pages. This information could be leaked even if individually tracked pages. This information could be leaked even if
client-side scripting were disabled. client-side scripting were disabled.
Implementations store state about Known Expect-CT Hosts, and hence Implementations store state about Known Expect-CT Hosts and, hence,
which domains the UA has contacted. Implementations may choose to which domains the UA has contacted. Implementations may choose to
not store this state subject to local policy (e.g., in the private not store this state subject to local policy (e.g., in the private
browsing mode of a web browser). browsing mode of a web browser).
Violation reports, as noted in Section 3, contain information about Violation reports, as noted in Section 3, contain information about
the certificate chain that has violated the CT policy. In some the certificate chain that has violated the CT Policy. In some
cases, such as organization-wide compromise of the end-to-end cases, such as an organization-wide compromise of the end-to-end
security of TLS, this may include information about the interception security of TLS, this may include information about the interception
tools and design used by the organization that the organization would tools and design used by the organization that the organization would
otherwise prefer not be disclosed. otherwise prefer not be disclosed.
Because Expect-CT causes remotely-detectable behavior, it's advisable Because Expect-CT causes remotely detectable behavior, it's advisable
that UAs offer a way for privacy-sensitive end users to clear that UAs offer a way for privacy-sensitive end users to clear
currently noted Expect-CT hosts, and allow users to query the current currently noted Expect-CT Hosts and allow users to query the current
state of Known Expect-CT Hosts. state of Known Expect-CT Hosts.
7. Security Considerations 7. Security Considerations
7.1. Hostile header attacks 7.1. Hostile Header Attacks
When UAs support the Expect-CT header field, it becomes a potential When UAs support the Expect-CT header field, it becomes a potential
vector for hostile header attacks against site owners. If a site vector for hostile header attacks against site owners. If a site
owner uses a certificate issued by a certificate authority which does owner uses a certificate issued by a certificate authority that does
not embed SCTs nor serve SCTs via OCSP or TLS extension, a malicious not embed SCTs nor serve SCTs via the Online Certificate Status
server operator or attacker could temporarily reconfigure the host to Protocol (OCSP) or TLS extension, a malicious server operator or
comply with the UA's CT policy, and add the Expect-CT header field in attacker could temporarily reconfigure the host to comply with the
enforcing mode with a long "max-age". Implementing user agents would UA's CT Policy and add the Expect-CT header field in enforcing mode
note this as an Expect-CT Host (see Section 2.3.2.1). After having with a long max-age. Implementing user agents would note this as an
done this, the configuration could then be reverted to not comply Expect-CT Host (see Section 2.3.2.1). After having done this, the
with the CT policy, prompting failures. Note this scenario would configuration could then be reverted to not comply with the CT
require the attacker to have substantial control over the Policy, prompting failures. Note that this scenario would require
infrastructure in question, being able to obtain different the attacker to have substantial control over the infrastructure in
certificates, change server software, or act as a man-in-the-middle question, being able to obtain different certificates, change server
in connections. software, or act as a man in the middle in connections.
Site operators can mitigate this situation by one of: reconfiguring Site operators can mitigate this situation by one of the following:
their web server to transmit SCTs using the TLS extension defined in reconfiguring their web server to transmit SCTs using the TLS
Section 6.5 of [I-D.ietf-trans-rfc6962-bis], obtaining a certificate extension defined in Section 6.4 of [RFC9162]; obtaining a
from an alternative certificate authority which provides SCTs by one certificate from an alternative certificate authority that provides
of the other methods, or by waiting for the user agents' persisted SCTs by one of the other methods; or by waiting for the user agent's
notation of this as an Expect-CT host to reach its "max-age". User persisted notation of this as an Expect-CT Host to reach its max-age.
agents may choose to implement mechanisms for users to cure this User agents may choose to implement mechanisms for users to cure this
situation, as noted in Section 4. situation, as noted in Section 4.
7.2. Maximum max-age 7.2. Maximum max-age
There is a security trade-off in that low maximum values provide a 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 narrow window of protection for users that visit the Known Expect-CT
Host only infrequently, while high maximum values might result in a 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 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. simply an error on the part of the site owner.
There is probably no ideal maximum for the "max-age" directive. There is probably no ideal maximum for the max-age directive. Since
Since Expect-CT is primarily a policy-expansion and investigation Expect-CT is primarily a policy-expansion and investigation
technology rather than an end-user protection, a value on the order 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 of 30 days (2,592,000 seconds) may be considered a balance between
these competing security concerns. these competing security concerns.
7.3. Amplification attacks 7.3. Amplification Attacks
Another kind of hostile header attack uses the "report-uri" mechanism Another kind of hostile header attack uses the report-uri mechanism
on many hosts not currently exposing SCTs as a method to cause a 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- denial of service to the host receiving the reports. If some highly
trafficked websites emitted a non-enforcing Expect-CT header field trafficked websites emitted a non-enforcing Expect-CT header field
with a "report-uri", implementing UAs' reports could flood the with a report-uri, implementing UAs' reports could flood the
reporting host. It is noted in Section 2.1.1 that UAs should limit 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 the rate at which they emit reports, but an attacker may alter the
Expect-CT header's fields to induce UAs to submit different reports Expect-CT header fields to induce UAs to submit different reports to
to different URIs to still cause the same effect. different URIs to still cause the same effect.
8. IANA Considerations 8. IANA Considerations
8.1. Header Field Registry 8.1. Header Field Registry
This document registers the "Expect-CT" header field in the This document registers the "Expect-CT" header field in the
"Permanent Message Header Field Names" registry located at "Hypertext Transfer Protocol (HTTP) Field Name Registry" registry
https://www.iana.org/assignments/message-headers [4]. located at <https://www.iana.org/assignments/http-fields>.
Header field name: Expect-CT Header field name: Expect-CT
Applicable protocol: http Applicable protocol: http
Status: experimental Status: permanent
Author/Change controller: IETF Author/Change controller: IETF
Specification document(s): This document Specification document(s): This document
Related information: (empty) Related information: (empty)
8.2. Media Types Registry 8.2. Media Types Registry
The MIME media type for Expect-CT violation reports is "application/ This document registers the application/expect-ct-report+json media
expect-ct-report+json" (which uses the suffix established in type (which uses the suffix established in [RFC6839]) for Expect-CT
[RFC6839]). violation reports in the "Media Types" registry as follows.
Type name: application Type name: application
Subtype name: expect-ct-report+json Subtype name: expect-ct-report+json
Required parameters: n/a Required parameters: n/a
Optional parameters: n/a Optional parameters: n/a
Encoding considerations: binary Encoding considerations: binary
Security considerations: See Section 7 Security considerations: See Section 7
Interoperability considerations: n/a Interoperability considerations: n/a
Published specification: This document Published specification: This document
Applications that use this media type: UAs that implement Applications that use this media type: UAs that implement
Certificate Transparency compliance checks and reporting Certificate Transparency compliance checks and reporting
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Additional information: Additional information:
Deprecated alias names for this type: n/a Deprecated alias names for this type: n/a
Magic number(s): n/a Magic number(s): n/a
File extension(s): n/a File extension(s): n/a
Macintosh file type code(s): n/a Macintosh file type code(s): n/a
Person & email address to contact for further information: Emily Person & email address to contact for further information:
Stark (estark@google.com) Emily Stark (estark@google.com)
Intended usage: COMMON Intended usage: COMMON
Restrictions on usage: none Restrictions on usage: none
Author: Emily Stark (estark@google.com) Author: Emily Stark (estark@google.com)
Change controller: IETF Change controller: IETF
9. References 9. References
9.1. Normative References 9.1. Normative References
[I-D.ietf-trans-rfc6962-bis]
Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
Stradling, "Certificate Transparency Version 2.0", draft-
ietf-trans-rfc6962-bis-30 (work in progress), November
2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>. <https://www.rfc-editor.org/info/rfc3339>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
skipping to change at page 20, line 43 skipping to change at line 931
[RFC6839] Hansen, T. and A. Melnikov, "Additional Media Type [RFC6839] Hansen, T. and A. Melnikov, "Additional Media Type
Structured Syntax Suffixes", RFC 6839, Structured Syntax Suffixes", RFC 6839,
DOI 10.17487/RFC6839, January 2013, DOI 10.17487/RFC6839, January 2013,
<https://www.rfc-editor.org/info/rfc6839>. <https://www.rfc-editor.org/info/rfc6839>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>. <https://www.rfc-editor.org/info/rfc6962>.
[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>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX, [RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468, PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, <https://www.rfc-editor.org/info/rfc7468>. April 2015, <https://www.rfc-editor.org/info/rfc7468>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <https://www.rfc-editor.org/info/rfc7469>. 2015, <https://www.rfc-editor.org/info/rfc7469>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259, Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017, DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>. <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 9.2. Informative References
[FETCH] WHATWG, "Fetch - Living Standard", n.d., [FETCH] WHATWG, "Fetch - Living Standard",
<https://fetch.spec.whatwg.org>. <https://fetch.spec.whatwg.org>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
9.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/expect-ct
[4] https://www.iana.org/assignments/message-headers
Appendix A. Changes
A.1. Since -07
o Editorial changes
o Specify that the end-entity certificate appears first in the
"validated-certificate-chain" field of an Expect-CT report.
o Define how report format can be extended by future versions of
this specification.
o Add optional "scheme" key to report format.
o Specify exact status codes for report server errors.
o Limit report-uris to HTTPS only.
o Note that this version of Expect-CT is only compatible with RFC
6962 and 6962-bis, not any future versions of CT.
A.2. Since -06
o Editorial changes
A.3. Since -05
o Remove SHOULD requirement that UAs disallow certificate error
overrides for Known Expect-CT Hosts.
o Remove restriction that Expect-CT Hosts cannot be IP addresses.
o Editorial changes
A.4. Since -04
o Editorial changes
A.5. Since -03
o Editorial changes
A.6. Since -02
o Add concept of test reports and specify that servers must respond
with 2xx status codes to valid reports.
o Add "failure-mode" key to reports to allow report servers to
distinguish report-only from enforced failures.
A.7. Since -01
o Change SCT reporting format to support both RFC 6962 and 6962-bis
SCTs.
A.8. Since -00
o Editorial changes
o Change Content-Type header of reports to 'application/expect-ct-
report+json'
o Update header field syntax to match convention (issue #327)
o Reference RFC 6962-bis instead of RFC 6962
Author's Address Author's Address
Emily Stark Emily Stark
Google Google
Email: estark@google.com Email: estark@google.com
 End of changes. 137 change blocks. 
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