HTTP Working Group E. Stark
Internet-Draft Google
Intended status: Experimental August 14, 2017
Expires: February 15, 2018
Expect-CT Extension for HTTP
draft-ietf-httpbis-expect-ct-latest
Abstract
This document defines a new HTTP header, named Expect-CT, that allows
web host operators to instruct user agents to expect valid Signed
Certificate Timestamps (SCTs) to be served on connections to these
hosts. When configured in enforcement mode, user agents (UAs) will
remember that hosts expect SCTs and will refuse connections that do
not conform to the UA's Certificate Transparency policy. When
configured in report-only mode, UAs will report the lack of valid
SCTs to a URI configured by the host, but will allow the connection.
By turning on Expect-CT, web host operators can discover
misconfigurations in their Certificate Transparency deployments and
ensure that misissued certificates accepted by UAs are 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on February 15, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Server and Client Behavior . . . . . . . . . . . . . . . . . 5
2.1. Response Header Field Syntax . . . . . . . . . . . . . . 5
2.1.1. The report-uri Directive . . . . . . . . . . . . . . 6
2.1.2. The enforce Directive . . . . . . . . . . . . . . . . 6
2.1.3. The max-age Directive . . . . . . . . . . . . . . . . 7
2.1.4. Examples . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Server Processing Model . . . . . . . . . . . . . . . . . 7
2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 8
2.2.2. HTTP Request Type . . . . . . . . . . . . . . . . . . 8
2.3. User Agent Processing Model . . . . . . . . . . . . . . . 8
2.3.1. Expect-CT Header Field Processing . . . . . . . . . . 8
2.3.2. HTTP-Equiv Element Attribute . . . . . . . . . 9
2.3.3. Noting Expect-CT . . . . . . . . . . . . . . . . . . 9
2.3.4. Storage Model . . . . . . . . . . . . . . . . . . . . 9
2.4. Evaluating Expect-CT Connections for CT Compliance . . . 10
3. Reporting Expect-CT Failure . . . . . . . . . . . . . . . . . 11
3.1. Generating a violation report . . . . . . . . . . . . . . 11
3.2. Sending a violation report . . . . . . . . . . . . . . . 13
4. Security Considerations . . . . . . . . . . . . . . . . . . . 14
4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 14
4.2. Avoiding amplification attacks . . . . . . . . . . . . . 14
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Usability Considerations . . . . . . . . . . . . . . . . . . 15
8. Authoring Considerations . . . . . . . . . . . . . . . . . . 15
8.1. HTTP Header . . . . . . . . . . . . . . . . . . . . . . . 16
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9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 16
9.2. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
This document defines a new HTTP header that enables UAs to identify
web hosts that expect the presence of Signed Certificate Timestamps
(SCTs) [I-D.ietf-trans-rfc6962-bis] in future Transport Layer
Security (TLS) [RFC5246] connections.
Web hosts that serve the Expect-CT HTTP header 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 caution, by using the reporting
feature and gradually increasing the interval where the UA remembers
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.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
1.2. Terminology
Terminology is defined in this section.
Certificate Transparency Policy is a policy defined by the UA
concerning the number, sources, and delivery mechanisms of Signed
Certificate Timestamps that are served on TLS connections. The
policy defines the properties of a connection that must be met in
order for the UA to consider it CT-qualified.
Certificate Transparency Qualified describes a TLS connection for
which the UA has determined that a sufficient quantity and quality
of Signed Certificate Timestamps have been provided.
CT-qualified See Certificate Transparency Qualified.
CT Policy See Certificate Transparency Policy.
Effective Expect-CT Date is the time at which a UA observed a valid
Expect-CT header for a given host.
Expect-CT Host See HTTP Expect-CT Host.
HTTP Expect-CT is the overall name for the combined UA- and server-
side security policy defined by this specification.
HTTP Expect-CT Host is a conformant host implementing the HTTP
server aspects of HTTP Expect-CT. This means that an Expect-CT
Host returns the "Expect-CT" HTTP response header field in its
HTTP response messages sent over secure transport.
Known Expect-CT Host is an Expect-CT Host that the UA has noted as
such. See Section 2.3.3 for particulars.
UA is an acronym for "user agent". For the purposes of this
specification, a UA is an HTTP client application typically
actively manipulated by a user [RFC7230].
Unknown Expect-CT Host is an Expect-CT Host that the UA has not
noted.
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2. Server and Client Behavior
2.1. Response Header Field Syntax
The "Expect-CT" header field is a new response header defined in this
specification. It is used by a server to indicate that UAs should
evaluate connections to the host emitting the header for CT
compliance (Section 2.4).
Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
header field, using the grammar defined in RFC 5234 [RFC5234] and the
rules defined in Section 3.2 of RFC 7230 [RFC7230].
Expect-CT = #expect-ct-directive
expect-ct-directive = directive-name [ "=" directive-value ]
directive-name = token
directive-value = token / quoted-string
Figure 1: Syntax of the Expect-CT header field
Optional white space ("OWS") is used as defined in Section 3.2.3 of
RFC 7230 [RFC7230]. "token" and "quoted-string" are used as defined
in Section 3.2.6 of RFC 7230 [RFC7230].
The directives defined in this specification are described below.
The overall requirements for directives are:
1. The order of appearance of directives is not significant.
2. A given directive MUST NOT appear more than once in a given
header field. Directives are either optional or required, as
stipulated in their definitions.
3. Directive names are case insensitive.
4. UAs MUST ignore any header fields containing directives, or other
header field value data, that do not conform to the syntax
defined in this specification. In particular, UAs must not
attempt to fix malformed header fields.
5. If a header field contains any directive(s) the UA does not
recognize, the UA MUST ignore those directives.
6. If the Expect-CT header field otherwise satisfies the above
requirements (1 through 5), the UA MUST process the directives it
recognizes.
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2.1.1. The report-uri Directive
The OPTIONAL "report-uri" directive indicates the URI to which the UA
SHOULD report Expect-CT failures (Section 2.4). The UA POSTs the
reports to the given URI as described in Section 3.
The "report-uri" directive is REQUIRED to have a directive value, for
which the syntax is defined in Figure 2.
report-uri-value = absolute-URI
Figure 2: Syntax of the report-uri directive value
"absolute-URI" is defined in Section 4.3 of RFC 3986 [RFC3986].
Hosts may set "report-uri"s that use HTTP or HTTPS. If the scheme in
the "report-uri" is one that uses TLS (e.g., HTTPS), UAs MUST check
Expect-CT compliance when the host in the "report-uri" is a Known
Expect-CT Host; similarly, UAs MUST apply HSTS if the host in the
"report-uri" is a Known HSTS Host.
Note that the report-uri need not necessarily be in the same Internet
domain or web origin as the host being reported about.
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 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.
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.
2.1.2. The enforce Directive
The OPTIONAL "enforce" directive is a valueless directive that, if
present (i.e., it is "asserted"), signals to the UA that compliance
to the CT Policy should be enforced (rather than report-only) and
that the UA should refuse future connections that violate its CT
Policy. When both the "enforce" directive and "report-uri" directive
(as defined in Figure 2) are present, the configuration is referred
to as an "enforce-and-report" configuration, signalling to the UA
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both that compliance to the CT Policy should be enforced and that
violations should be reported.
2.1.3. The max-age Directive
The "max-age" directive specifies the number of seconds after the
reception of the Expect-CT header field during which the UA SHOULD
regard the host from whom the message was received as a Known Expect-
CT Host.
The "max-age" directive is REQUIRED to be present within an "Expect-
CT" header field. The "max-age" directive is REQUIRED to have a
directive value, for which the syntax (after quoted-string
unescaping, if necessary) is defined in Figure 3.
max-age-value = delta-seconds
delta-seconds = 1*DIGIT
Figure 3: Syntax of the max-age directive value
"delta-seconds" is used as defined in Section 1.2.1 of RFC 7234
[RFC7234].
2.1.4. Examples
The following examples demonstrate valid Expect-CT response header
fields:
Expect-CT: max-age=86400,enforce
Expect-CT: max-age=86400, enforce, 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
2.2. Server Processing Model
This section describes the processing model that Expect-CT Hosts
implement. The model has 2 parts: (1) the processing rules for HTTP
request messages received over a secure transport (e.g.,
authenticated, non-anonymous TLS); and (2) the processing rules for
HTTP request messages received over non-secure transports, such as
TCP.
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2.2.1. HTTP-over-Secure-Transport Request Type
When replying to an HTTP request that was conveyed over a secure
transport, an Expect-CT Host SHOULD include in its response exactly
one Expect-CT header field. The header field MUST satisfy the
grammar specified in Section 2.1.
Establishing a given host as an Expect-CT Host, in the context of a
given UA, is accomplished as follows:
1. Over the HTTP protocol running over secure transport, by
correctly returning (per this specification) at least one valid
Expect-CT header field to the UA.
2. Through other mechanisms, such as a client-side preloaded Expect-
CT Host list.
2.2.2. HTTP Request Type
Expect-CT Hosts SHOULD NOT include the Expect-CT header field in HTTP
responses conveyed over non-secure transport. UAs MUST ignore any
Expect-CT header received in an HTTP response conveyed over non-
secure transport.
2.3. User Agent Processing Model
The UA processing model relies on parsing domain names. Note that
internationalized domain names SHALL be canonicalized according to
the scheme in Section 10 of [RFC6797].
2.3.1. Expect-CT Header Field Processing
If the UA receives, over a secure transport, an HTTP response 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 was received for compliance with the UA's CT Policy,
and then process the Expect-CT header field as follows.
If the connection complies with the UA's CT Policy (i.e. the
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
noted (see Section 2.3.3), or
o Update the UA's cached information for the Known Expect-CT Host if
the "enforce", "max-age", or "report-uri" header field value
directives convey information different from that already
maintained by the UA. If the "max-age" directive has a value of
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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 the connection does not comply with the UA's CT Policy (i.e. is
not CT-qualified), then the UA MUST NOT note this host as a Known
Expect-CT Host.
If the header field includes 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.
The UA MUST ignore any Expect-CT header field not conforming to the
grammar specified in Section 2.1.
2.3.2. HTTP-Equiv Element Attribute
UAs MUST NOT heed "http-equiv="Expect-CT"" attribute settings on
"" elements [W3C.REC-html401-19991224] in received content.
2.3.3. Noting Expect-CT
Upon receipt of the Expect-CT response header field over an error-
free TLS connection (including the validation adding in Section 2.4),
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. The domain name and associated Expect-CT
directives are collectively known as "Expect-CT metadata".
To note a host as a Known Expect-CT Host, the UA MUST set its Expect-
CT metadata given in the most recently received valid Expect-CT
header, as specified in Section 2.3.4.
For forward compatibility, the UA MUST ignore any unrecognized
Expect-CT header directives, while still processing those directives
it does recognize. Section 2.1 specifies the directives "enforce",
"max-age", and "report-uri", but future specifications and
implementations might use additional directives.
2.3.4. Storage Model
Known Expect-CT Hosts are identified only by domain names, and never
IP addresses. If the substring matching the host production from the
Request-URI (of the message to which the host responded)
syntactically matches the IP-literal or IPv4address productions from
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Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a
Known Expect-CT Host.
Otherwise, if the substring does not congruently match an existing
Known Expect-CT Host's domain name, per the matching procedure
specified in Section 8.2 of [RFC6797], then the UA MUST add this host
to the Known Expect-CT Host cache. The UA caches:
o the Expect-CT Host's domain name,
o whether the "enforce" directive is present
o the Effective Expiration Date, which is the Effective Expect-CT
Date plus the value of the "max-age" directive. Alternatively,
the UA MAY cache enough information to calculate the Effective
Expiration Date.
o the value of the "report-uri" directive, if present.
If any other metadata from optional or future Expect-CT header
directives are present in the Expect-CT header, 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,
the UA MAY behave as if the max-age were effectively 60 days. (One
way to achieve this behavior is for the UA to simply store a value of
60 days instead of the 90-day value provided by the Expect-CT host.)
2.4. Evaluating Expect-CT Connections for CT Compliance
When a UA connects to a Known Expect-CT Host using a TLS connection,
if the TLS connection has errors, the UA MUST terminate the
connection without allowing the user to proceed anyway. (This
behavior is the same as that required by [RFC6797].)
If the 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, as soon as possible. It is acceptable to skip this CT
compliance check 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
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anchor, rather than a trust anchor built-in to the UA (or underlying
platform).
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.
If a connection to a Known 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 a non-recoverable error.
If a connection to a Known 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).
A UA that has previously noted a host as a Known Expect-CT Host MUST
evaluate CT compliance when setting up the TLS session, before
beginning an HTTP conversation over the TLS channel.
If the UA does not evaluate CT compliance, e.g. because the user has
elected to disable it, or because a presented certificate chain
chains up to a user-defined trust anchor, UAs SHOULD NOT send Expect-
CT reports.
3. Reporting Expect-CT Failure
When the UA attempts to connect to a Known Expect-CT Host and the
connection is not CT-qualified, the UA SHOULD report Expect-CT
failures to the "report-uri", if any, in the Known Expect-CT Host's
Expect-CT metadata.
When the UA receives an Expect-CT response header field over a
connection that is not CT-qualified, if the UA has not already sent
an Expect-CT report for this connection, then the UA SHOULD report
Expect-CT failures to the configured "report-uri", if any.
3.1. Generating a violation report
To generate a violation report object, the UA constructs a JSON
object with the following keys and values:
o "date-time": the value for this key indicates the time the UA
observed the CT compliance failure. The value is a string
formatted according to Section 5.6, "Internet Date/Time Format",
of [RFC3339].
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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
request that failed the CT compliance check. The value is
provided as an integer.
o "effective-expiration-date": the value indicates the Effective
Expiration Date (see Section 2.3.4) for the Expect-CT Host that
failed the CT compliance check. The value is provided as a string
formatted according to Section 5.6, "Internet Date/Time Format",
of [RFC3339].
o "served-certificate-chain": the value is the certificate chain as
served by the Expect-CT Host during TLS session setup. The value
is provided as an array of strings, which MUST appear in the order
that the certificates were served; each string in the array is the
Privacy-Enhanced Mail (PEM) representation of each X.509
certificate as described in [RFC7468].
o "validated-certificate-chain": the value is the certificate chain
as constructed by the UA during certificate chain verification.
(This may differ from the value of the "served-certificate-chain"
key.) The value is provided as an array of strings, which MUST
appear in the order matching the chain that the UA validated; each
string in the array is the Privacy-Enhanced Mail (PEM)
representation of each X.509 certificate as described in
[RFC7468].
o "scts": the value represents the SCTs (if any) that the UA
received for the Expect-CT host and their validation statuses.
The value is provided as an array of JSON objects. The SCTs may
appear in any order. Each JSON object in the array has the
following keys:
* A "version" key, with an integer value. The UA MUST set this
value to "1" if the SCT is in the format defined in Section 3.2
of [RFC6962] and "2" if it is in the format defined in
Section 4.6 of [I-D.ietf-trans-rfc6962-bis].
* The "status" key, with a string value that the UA MUST set to
one of the following values: "unknown" (indicating that the UA
does not have or does not trust the public key of the log from
which the SCT was issued), "valid" (indicating that the UA
successfully validated the SCT as described in Section 5.2 of
[RFC6962] or Section 8.2.3 of [I-D.ietf-trans-rfc6962-bis]), or
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"invalid" (indicating that the SCT validation failed because
of, e.g., a bad signature).
* The "source" key, with a string value that indicates from where
the UA obtained the SCT, as defined in Section 3 or [RFC6962]
and Section 6 of [I-D.ietf-trans-rfc6962-bis]. The UA MUST set
the value to one of "tls-extension", "ocsp", or "embedded".
* The "serialized_sct" key, with a string value. If the value of
the "version" key is "1", the UA MUST set this value to the
base64 encoded [RFC4648] serialized
"SignedCertificateTimestamp" structure from Section 3.2 of
[RFC6962]. 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 4.6 of [I-D.ietf-trans-rfc6962-bis].
3.2. Sending a violation report
The UA SHOULD report an Expect-CT failure 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 an Expect-CT failure when it receives an Expect-CT
header field which contains the "report-uri" directive over a
connection that does not comply with the UA's CT Policy.
The steps to report an Expect-CT failure are as follows.
1. Prepare a JSON object "report object" with the single key
"expect-ct-report", whose value is the result of generating a
violation report object as described in Section 3.1.
2. Let "report body" by the JSON stringification of "report object".
3. Let "report-uri" be the value of the "report-uri" directive in
the Expect-CT header field.
4. Send an HTTP POST request to "report-uri" with a "Content-Type"
header field of "application/expect-ct-report+json", and an
entity body consisting of "report body".
The UA MAY perform other operations as part of sending the HTTP POST
request, for example sending a CORS preflight as part of [FETCH].
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4. Security Considerations
When UAs support the Expect-CT header, it becomes a potential vector
for hostile header attacks against site owners. If a site owner uses
a certificate issued by a certificate authority which does not embed
SCTs nor serve SCTs via 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 in enforcing
mode with a long "max-age". Implementing user agents would note this
as an Expect-CT Host (see Section 2.3.3). After having done this,
the configuration could then be reverted to not comply with the CT
policy, prompting failures. Note 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 could themselves only cure this situation by one of:
reconfiguring their web server to transmit SCTs using the TLS
extension defined in Section 6.5 of [I-D.ietf-trans-rfc6962-bis],
obtaining a certificate from an alternative certificate authority
which provides SCTs by one of the other methods, or by waiting for
the user agents' 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 7.
4.1. Maximum max-age
There is a security trade-off in that low maximum values provide a
narrow window of protection for users that visit the Known Expect-CT
Host only infrequently, while high maximum values might result in a
denial of service to a UA in the event of a hostile header attack, or
simply an error on the part of the site-owner.
There is probably no ideal maximum for the "max-age" directive.
Since Expect-CT is primarily a policy-expansion and investigation
technology rather than an end-user protection, a value on the order
of 30 days (2,592,000 seconds) may be considered a balance between
these competing security concerns.
4.2. Avoiding amplification attacks
Another kind of hostile header attack uses the "report-uri" mechanism
on many hosts not currently exposing SCTs as a method to cause a
denial-of-service to the host receiving the reports. If some highly-
trafficked websites emitted a non-enforcing Expect-CT header 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
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header's fields to induce UAs to submit different reports to
different URIs to still cause the same effect.
5. Privacy Considerations
Expect-CT can be used to infer what Certificate Transparency policy
is in use, by attempting to retrieve specially-configured websites
which pass one user agents' policies but not another's. Note that
this consideration is true of UAs which enforce CT policies without
Expect-CT as well.
Additionally, reports submitted to the "report-uri" could reveal
information to a third party about which webpage 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 must store state about Known Expect-CT Hosts, and
hence which domains the UA has contacted.
Violation reports, as noted in Section 3, contain information about
the certificate chain that has violated the CT policy. In some
cases, such as 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 users to clear currently
noted Expect-CT hosts, and allow users to query the current state of
Known Expect-CT Hosts.
6. IANA Considerations
TBD
7. Usability Considerations
When the UA detects a Known Expect-CT Host in violation of the UA's
CT Policy, users will experience denials of service. It is advisable
for UAs to explain the reason why.
8. Authoring Considerations
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8.1. HTTP Header
Expect-CT could be specified as a TLS extension or X.509 certificate
extension instead of an HTTP response header. Using an HTTP header
as the mechanism for Expect-CT introduces a layering mismatch: for
example, the software that terminates TLS and validates Certificate
Transparency information might know nothing about HTTP.
Nevertheless, an HTTP header was chosen primarily for ease of
deployment. In practice, deploying new certificate extensions
requires certificate authorities to support them, and new TLS
extensions require server software updates, including possibly to
servers outside of the site owner's direct control (such as in the
case of a third-party CDN). Ease of deployment is a high priority
for Expect-CT because it is intended as a temporary transition
mechanism for user agents that are transitioning to universal
Certificate Transparency requirements.
9. Changes
9.1. Since -01
o Change SCT reporting format to support both RFC 6962 and 6962-bis
SCTs.
9.2. 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
10. References
10.1. Normative References
[FETCH] van Kesteren, A., "Fetch", n.d.,
.
[HTML] Hickson, I., Pieters, S., van Kesteren, A., Jaegenstedt,
P., and D. Denicola, "HTML", n.d.,
.
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[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-26 (work in progress), July 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
.
[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,
.
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[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,
.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, .
[W3C.REC-html401-19991224]
Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium Recommendation
REC-html401-19991224, December 1999,
.
10.2. 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
Author's Address
Emily Stark
Google
Email: estark@google.com
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