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2 HTTP Working Group E. Stark
3 Internet-Draft Google
4 Intended status: Experimental May 24, 2017
5 Expires: November 25, 2017
7 Expect-CT Extension for HTTP
8 draft-ietf-httpbis-expect-ct-01
10 Abstract
12 This document defines a new HTTP header, named Expect-CT, that allows
13 web host operators to instruct user agents to expect valid Signed
14 Certificate Timestamps (SCTs) to be served on connections to these
15 hosts. When configured in enforcement mode, user agents (UAs) will
16 remember that hosts expect SCTs and will refuse connections that do
17 not conform to the UA's Certificate Transparency policy. When
18 configured in report-only mode, UAs will report the lack of valid
19 SCTs to a URI configured by the host, but will allow the connection.
20 By turning on Expect-CT, web host operators can discover
21 misconfigurations in their Certificate Transparency deployments and
22 ensure that misissued certificates accepted by UAs are discoverable
23 in Certificate Transparency logs.
25 Note to Readers
27 Discussion of this draft takes place on the HTTP working group
28 mailing list (ietf-http-wg@w3.org), which is archived at
29 https://lists.w3.org/Archives/Public/ietf-http-wg/ .
31 Working Group information can be found at http://httpwg.github.io/ ;
32 source code and issues list for this draft can be found at
33 https://github.com/httpwg/http-extensions/labels/expect-ct .
35 Status of This Memo
37 This Internet-Draft is submitted in full conformance with the
38 provisions of BCP 78 and BCP 79.
40 Internet-Drafts are working documents of the Internet Engineering
41 Task Force (IETF). Note that other groups may also distribute
42 working documents as Internet-Drafts. The list of current Internet-
43 Drafts is at http://datatracker.ietf.org/drafts/current/.
45 Internet-Drafts are draft documents valid for a maximum of six months
46 and may be updated, replaced, or obsoleted by other documents at any
47 time. It is inappropriate to use Internet-Drafts as reference
48 material or to cite them other than as "work in progress."
49 This Internet-Draft will expire on November 25, 2017.
51 Copyright Notice
53 Copyright (c) 2017 IETF Trust and the persons identified as the
54 document authors. All rights reserved.
56 This document is subject to BCP 78 and the IETF Trust's Legal
57 Provisions Relating to IETF Documents
58 (http://trustee.ietf.org/license-info) in effect on the date of
59 publication of this document. Please review these documents
60 carefully, as they describe your rights and restrictions with respect
61 to this document. Code Components extracted from this document must
62 include Simplified BSD License text as described in Section 4.e of
63 the Trust Legal Provisions and are provided without warranty as
64 described in the Simplified BSD License.
66 Table of Contents
68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
69 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
70 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
71 2. Server and Client Behavior . . . . . . . . . . . . . . . . . 4
72 2.1. Response Header Field Syntax . . . . . . . . . . . . . . 4
73 2.1.1. The report-uri Directive . . . . . . . . . . . . . . 5
74 2.1.2. The enforce Directive . . . . . . . . . . . . . . . . 6
75 2.1.3. The max-age Directive . . . . . . . . . . . . . . . . 7
76 2.1.4. Examples . . . . . . . . . . . . . . . . . . . . . . 7
77 2.2. Server Processing Model . . . . . . . . . . . . . . . . . 7
78 2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 7
79 2.2.2. HTTP Request Type . . . . . . . . . . . . . . . . . . 8
80 2.3. User Agent Processing Model . . . . . . . . . . . . . . . 8
81 2.3.1. Expect-CT Header Field Processing . . . . . . . . . . 8
82 2.3.2. HTTP-Equiv Element Attribute . . . . . . . . . 9
83 2.3.3. Noting Expect-CT . . . . . . . . . . . . . . . . . . 9
84 2.3.4. Storage Model . . . . . . . . . . . . . . . . . . . . 9
85 2.4. Evaluating Expect-CT Connections for CT Compliance . . . 10
86 3. Reporting Expect-CT Failure . . . . . . . . . . . . . . . . . 11
87 3.1. Generating a violation report . . . . . . . . . . . . . . 11
88 3.2. Sending a violation report . . . . . . . . . . . . . . . 13
89 4. Security Considerations . . . . . . . . . . . . . . . . . . . 13
90 4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 14
91 4.2. Avoiding amplification attacks . . . . . . . . . . . . . 14
92 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 14
93 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
94 7. Usability Considerations . . . . . . . . . . . . . . . . . . 15
95 8. Authoring Considerations . . . . . . . . . . . . . . . . . . 15
96 8.1. HTTP Header . . . . . . . . . . . . . . . . . . . . . . . 15
98 9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
99 9.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 15
100 10. Normative References . . . . . . . . . . . . . . . . . . . . 16
101 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
103 1. Introduction
105 This document defines a new HTTP header that enables UAs to identify
106 web hosts that expect the presence of Signed Certificate Timestamps
107 (SCTs) [I-D.ietf-trans-rfc6962-bis] in future Transport Layer
108 Security (TLS) [RFC5246] connections.
110 Web hosts that serve the Expect-CT HTTP header are noted by the UA as
111 Known Expect-CT Hosts. The UA evaluates each connection to a Known
112 Expect-CT Host for compliance with the UA's Certificate Transparency
113 (CT) Policy. If the connection violates the CT Policy, the UA sends
114 a report to a URI configured by the Expect-CT Host and/or fails the
115 connection, depending on the configuration that the Expect-CT Host
116 has chosen.
118 If misconfigured, Expect-CT can cause unwanted connection failures
119 (for example, if a host deploys Expect-CT but then switches to a
120 legitimate certificate that is not logged in Certificate Transparency
121 logs, or if a web host operator believes their certificate to conform
122 to all UAs' CT policies but is mistaken). Web host operators are
123 advised to deploy Expect-CT with caution, by using the reporting
124 feature and gradually increasing the interval where the UA remembers
125 the host as a Known Expect-CT Host. These precautions can help web
126 host operators gain confidence that their Expect-CT deployment is not
127 causing unwanted connection failures.
129 Expect-CT is a trust-on-first-use (TOFU) mechanism. The first time a
130 UA connects to a host, it lacks the information necessary to require
131 SCTs for the connection. Thus, the UA will not be able to detect and
132 thwart an attack on the UA's first connection to the host. Still,
133 Expect-CT provides value by 1) allowing UAs to detect the use of
134 unlogged certificates after the initial communication, and 2)
135 allowing web hosts to be confident that UAs are only trusting
136 publicly-auditable certificates.
138 1.1. Requirements Language
140 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
141 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
142 "OPTIONAL" in this document are to be interpreted as described in RFC
143 2119 [RFC2119].
145 1.2. Terminology
147 Terminology is defined in this section.
149 Certificate Transparency Policy is a policy defined by the UA
150 concerning the number, sources, and delivery mechanisms of Signed
151 Certificate Timestamps that are served on TLS connections. The
152 policy defines the properties of a connection that must be met in
153 order for the UA to consider it CT-qualified.
155 Certificate Transparency Qualified describes a TLS connection for
156 which the UA has determined that a sufficient quantity and quality
157 of Signed Certificate Timestamps have been provided.
159 CT-qualified See Certificate Transparency Qualified.
161 CT Policy See Certificate Transparency Policy.
163 Effective Expect-CT Date is the time at which a UA observed a valid
164 Expect-CT header for a given host.
166 Expect-CT Host See HTTP Expect-CT Host.
168 HTTP Expect-CT is the overall name for the combined UA- and server-
169 side security policy defined by this specification.
171 HTTP Expect-CT Host is a conformant host implementing the HTTP
172 server aspects of HTTP Expect-CT. This means that an Expect-CT
173 Host returns the "Expect-CT" HTTP response header field in its
174 HTTP response messages sent over secure transport.
176 Known Expect-CT Host is an Expect-CT Host that the UA has noted as
177 such. See Section 2.3.3 for particulars.
179 UA is an acronym for "user agent". For the purposes of this
180 specification, a UA is an HTTP client application typically
181 actively manipulated by a user [RFC7230].
183 Unknown Expect-CT Host is an Expect-CT Host that the UA has not
184 noted.
186 2. Server and Client Behavior
188 2.1. Response Header Field Syntax
190 The "Expect-CT" header field is a new response header defined in this
191 specification. It is used by a server to indicate that UAs should
192 evaluate connections to the host emitting the header for CT
193 compliance (Section 2.4).
195 Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
196 header field, using the grammar defined in RFC 5234 [RFC5234] and the
197 rules defined in Section 3.2 of RFC 7230 [RFC7230].
199 Expect-CT = #expect-ct-directive
200 expect-ct-directive = directive-name [ "=" directive-value ]
201 directive-name = token
202 directive-value = token / quoted-string
204 Figure 1: Syntax of the Expect-CT header field
206 Optional white space ("OWS") is used as defined in Section 3.2.3 of
207 RFC 7230 [RFC7230]. "token" and "quoted-string" are used as defined
208 in Section 3.2.6 of RFC 7230 [RFC7230].
210 The directives defined in this specification are described below.
211 The overall requirements for directives are:
213 1. The order of appearance of directives is not significant.
215 2. A given directive MUST NOT appear more than once in a given
216 header field. Directives are either optional or required, as
217 stipulated in their definitions.
219 3. Directive names are case insensitive.
221 4. UAs MUST ignore any header fields containing directives, or other
222 header field value data, that do not conform to the syntax
223 defined in this specification. In particular, UAs must not
224 attempt to fix malformed header fields.
226 5. If a header field contains any directive(s) the UA does not
227 recognize, the UA MUST ignore those directives.
229 6. If the Expect-CT header field otherwise satisfies the above
230 requirements (1 through 5), the UA MUST process the directives it
231 recognizes.
233 2.1.1. The report-uri Directive
235 The OPTIONAL "report-uri" directive indicates the URI to which the UA
236 SHOULD report Expect-CT failures (Section 2.4). The UA POSTs the
237 reports to the given URI as described in Section 3.
239 The "report-uri" directive is REQUIRED to have a directive value, for
240 which the syntax is defined in Figure 2.
242 report-uri-value = absolute-URI
244 Figure 2: Syntax of the report-uri directive value
246 "absolute-URI" is defined in Section 4.3 of RFC 3986 [RFC3986].
248 Hosts may set "report-uri"s that use HTTP or HTTPS. If the scheme in
249 the "report-uri" is one that uses TLS (e.g., HTTPS), UAs MUST check
250 Expect-CT compliance when the host in the "report-uri" is a Known
251 Expect-CT Host; similarly, UAs MUST apply HSTS if the host in the
252 "report-uri" is a Known HSTS Host.
254 Note that the report-uri need not necessarily be in the same Internet
255 domain or web origin as the host being reported about.
257 UAs SHOULD make their best effort to report Expect-CT failures to the
258 "report-uri", but they may fail to report in exceptional conditions.
259 For example, if connecting the "report-uri" itself incurs an Expect-
260 CT failure or other certificate validation failure, the UA MUST
261 cancel the connection. Similarly, if Expect-CT Host A sets a
262 "report-uri" referring to Expect-CT Host B, and if B sets a "report-
263 uri" referring to A, and if both hosts fail to comply to the UA's CT
264 Policy, the UA SHOULD detect and break the loop by failing to send
265 reports to and about those hosts.
267 UAs SHOULD limit the rate at which they send reports. For example,
268 it is unnecessary to send the same report to the same "report-uri"
269 more than once.
271 2.1.2. The enforce Directive
273 The OPTIONAL "enforce" directive is a valueless directive that, if
274 present (i.e., it is "asserted"), signals to the UA that compliance
275 to the CT Policy should be enforced (rather than report-only) and
276 that the UA should refuse future connections that violate its CT
277 Policy. When both the "enforce" directive and "report-uri" directive
278 (as defined in Figure 2) are present, the configuration is referred
279 to as an "enforce-and-report" configuration, signalling to the UA
280 both that compliance to the CT Policy should be enforced and that
281 violations should be reported.
283 2.1.3. The max-age Directive
285 The "max-age" directive specifies the number of seconds after the
286 reception of the Expect-CT header field during which the UA SHOULD
287 regard the host from whom the message was received as a Known Expect-
288 CT Host.
290 The "max-age" directive is REQUIRED to be present within an "Expect-
291 CT" header field. The "max-age" directive is REQUIRED to have a
292 directive value, for which the syntax (after quoted-string
293 unescaping, if necessary) is defined in Figure 3.
295 max-age-value = delta-seconds
296 delta-seconds = 1*DIGIT
298 Figure 3: Syntax of the max-age directive value
300 "delta-seconds" is used as defined in Section 1.2.1 of RFC 7234
301 [RFC7234].
303 2.1.4. Examples
305 The following examples demonstrate valid Expect-CT response header
306 fields:
308 Expect-CT: max-age=86400,enforce
310 Expect-CT: max-age=86400, enforce, report-uri="https://foo.example/report"
312 Expect-CT: max-age=86400,report-uri="https://foo.example/report"
314 Figure 4: Examples of valid Expect-CT response header fields
316 2.2. Server Processing Model
318 This section describes the processing model that Expect-CT Hosts
319 implement. The model has 2 parts: (1) the processing rules for HTTP
320 request messages received over a secure transport (e.g.,
321 authenticated, non-anonymous TLS); and (2) the processing rules for
322 HTTP request messages received over non-secure transports, such as
323 TCP.
325 2.2.1. HTTP-over-Secure-Transport Request Type
327 When replying to an HTTP request that was conveyed over a secure
328 transport, an Expect-CT Host SHOULD include in its response exactly
329 one Expect-CT header field. The header field MUST satisfy the
330 grammar specified in Section 2.1.
332 Establishing a given host as an Expect-CT Host, in the context of a
333 given UA, is accomplished as follows:
335 1. Over the HTTP protocol running over secure transport, by
336 correctly returning (per this specification) at least one valid
337 Expect-CT header field to the UA.
339 2. Through other mechanisms, such as a client-side preloaded Expect-
340 CT Host list.
342 2.2.2. HTTP Request Type
344 Expect-CT Hosts SHOULD NOT include the Expect-CT header field in HTTP
345 responses conveyed over non-secure transport. UAs MUST ignore any
346 Expect-CT header received in an HTTP response conveyed over non-
347 secure transport.
349 2.3. User Agent Processing Model
351 The UA processing model relies on parsing domain names. Note that
352 internationalized domain names SHALL be canonicalized according to
353 the scheme in Section 10 of [RFC6797].
355 2.3.1. Expect-CT Header Field Processing
357 If the UA receives, over a secure transport, an HTTP response that
358 includes an Expect-CT header field conforming to the grammar
359 specified in Section 2.1, the UA MUST evaluate the connection on
360 which the header was received for compliance with the UA's CT Policy,
361 and then process the Expect-CT header field as follows.
363 If the connection complies with the UA's CT Policy (i.e. the
364 connection is CT-qualified), then the UA MUST either:
366 o Note the host as a Known Expect-CT Host if it is not already so
367 noted (see Section 2.3.3), or
369 o Update the UA's cached information for the Known Expect-CT Host if
370 the "enforce", "max-age", or "report-uri" header field value
371 directives convey information different from that already
372 maintained by the UA. If the "max-age" directive has a value of
373 0, the UA MUST remove its cached Expect-CT information if the host
374 was previously noted as a Known Expect-CT Host, and MUST NOT note
375 this host as a Known Expect-CT Host if it is not already noted.
377 If the connection does not comply with the UA's CT Policy (i.e. is
378 not CT-qualified), then the UA MUST NOT note this host as a Known
379 Expect-CT Host.
381 If the header field includes a "report-uri" directive, and the
382 connection does not comply with the UA's CT Policy (i.e. the
383 connection is not CT-qualified), and the UA has not already sent an
384 Expect-CT report for this connection, then the UA SHOULD send a
385 report to the specified "report-uri" as specified in Section 3.
387 The UA MUST ignore any Expect-CT header field not conforming to the
388 grammar specified in Section 2.1.
390 2.3.2. HTTP-Equiv Element Attribute
392 UAs MUST NOT heed "http-equiv="Expect-CT"" attribute settings on
393 "" elements [W3C.REC-html401-19991224] in received content.
395 2.3.3. Noting Expect-CT
397 Upon receipt of the Expect-CT response header field over an error-
398 free TLS connection (including the validation adding in Section 2.4),
399 the UA MUST note the host as a Known Expect-CT Host, storing the
400 host's domain name and its associated Expect-CT directives in non-
401 volatile storage. The domain name and associated Expect-CT
402 directives are collectively known as "Expect-CT metadata".
404 To note a host as a Known Expect-CT Host, the UA MUST set its Expect-
405 CT metadata given in the most recently received valid Expect-CT
406 header, as specified in Section 2.3.4.
408 For forward compatibility, the UA MUST ignore any unrecognized
409 Expect-CT header directives, while still processing those directives
410 it does recognize. Section 2.1 specifies the directives "enforce",
411 "max-age", and "report-uri", but future specifications and
412 implementations might use additional directives.
414 2.3.4. Storage Model
416 Known Expect-CT Hosts are identified only by domain names, and never
417 IP addresses. If the substring matching the host production from the
418 Request-URI (of the message to which the host responded)
419 syntactically matches the IP-literal or IPv4address productions from
420 Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a
421 Known Expect-CT Host.
423 Otherwise, if the substring does not congruently match an existing
424 Known Expect-CT Host's domain name, per the matching procedure
425 specified in Section 8.2 of [RFC6797], then the UA MUST add this host
426 to the Known Expect-CT Host cache. The UA caches:
428 o the Expect-CT Host's domain name,
429 o whether the "enforce" directive is present
431 o the Effective Expiration Date, which is the Effective Expect-CT
432 Date plus the value of the "max-age" directive. Alternatively,
433 the UA MAY cache enough information to calculate the Effective
434 Expiration Date.
436 o the value of the "report-uri" directive, if present.
438 If any other metadata from optional or future Expect-CT header
439 directives are present in the Expect-CT header, and the UA
440 understands them, the UA MAY note them as well.
442 UAs MAY set an upper limit on the value of max-age, so that UAs that
443 have noted erroneous Expect-CT hosts (whether by accident or due to
444 attack) have some chance of recovering over time. If the server sets
445 a max-age greater than the UA's upper limit, the UA MAY behave as if
446 the server set the max-age to the UA's upper limit. For example, if
447 the UA caps max-age at 5,184,000 seconds (60 days), and an Expect-CT
448 Host sets a max- age directive of 90 days in its Expect-CT header,
449 the UA MAY behave as if the max-age were effectively 60 days. (One
450 way to achieve this behavior is for the UA to simply store a value of
451 60 days instead of the 90-day value provided by the Expect-CT host.)
453 2.4. Evaluating Expect-CT Connections for CT Compliance
455 When a UA connects to a Known Expect-CT Host using a TLS connection,
456 if the TLS connection has errors, the UA MUST terminate the
457 connection without allowing the user to proceed anyway. (This
458 behavior is the same as that required by [RFC6797].)
460 If the connection has no errors, then the UA will apply an additional
461 correctness check: compliance with a CT Policy. A UA should evaluate
462 compliance with its CT Policy whenever connecting to a Known Expect-
463 CT Host, as soon as possible. It is acceptable to skip this CT
464 compliance check for some hosts according to local policy. For
465 example, a UA may disable CT compliance checks for hosts whose
466 validated certificate chain terminates at a user-defined trust
467 anchor, rather than a trust anchor built-in to the UA (or underlying
468 platform).
470 An Expect-CT Host is "expired" if the effective expiration date
471 refers to a date in the past. The UA MUST ignore any expired Expect-
472 CT Hosts in its cache and not treat such hosts as Known Expect-CT
473 hosts.
475 If a connection to a Known CT Host violates the UA's CT policy (i.e.
476 the connection is not CT-qualified), and if the Known Expect-CT
477 Host's Expect-CT metadata indicates an "enforce" configuration, the
478 UA MUST treat the CT compliance failure as a non-recoverable error.
480 If a connection to a Known CT Host violates the UA's CT policy, and
481 if the Known Expect-CT Host's Expect-CT metadata includes a "report-
482 uri", the UA SHOULD send an Expect-CT report to that "report-uri"
483 (Section 3).
485 A UA that has previously noted a host as a Known Expect-CT Host MUST
486 evaluate CT compliance when setting up the TLS session, before
487 beginning an HTTP conversation over the TLS channel.
489 If the UA does not evaluate CT compliance, e.g. because the user has
490 elected to disable it, or because a presented certificate chain
491 chains up to a user-defined trust anchor, UAs SHOULD NOT send Expect-
492 CT reports.
494 3. Reporting Expect-CT Failure
496 When the UA attempts to connect to a Known Expect-CT Host and the
497 connection is not CT-qualified, the UA SHOULD report Expect-CT
498 failures to the "report-uri", if any, in the Known Expect-CT Host's
499 Expect-CT metadata.
501 When the UA receives an Expect-CT response header field over a
502 connection that is not CT-qualified, if the UA has not already sent
503 an Expect-CT report for this connection, then the UA SHOULD report
504 Expect-CT failures to the configured "report-uri", if any.
506 3.1. Generating a violation report
508 To generate a violation report object, the UA constructs a JSON
509 object with the following keys and values:
511 o "date-time": the value for this key indicates the time the UA
512 observed the CT compliance failure. The value is a string
513 formatted according to Section 5.6, "Internet Date/Time Format",
514 of [RFC3339].
516 o "hostname": the value is the hostname to which the UA made the
517 original request that failed the CT compliance check. The value
518 is provided as a string.
520 o "port": the value is the port to which the UA made the original
521 request that failed the CT compliance check. The value is
522 provided as an integer.
524 o "effective-expiration-date": the value indicates the Effective
525 Expiration Date (see Section 2.3.4) for the Expect-CT Host that
526 failed the CT compliance check. The value is provided as a string
527 formatted according to Section 5.6, "Internet Date/Time Format",
528 of [RFC3339].
530 o "served-certificate-chain": the value is the certificate chain as
531 served by the Expect-CT Host during TLS session setup. The value
532 is provided as an array of strings, which MUST appear in the order
533 that the certificates were served; each string in the array is the
534 Privacy-Enhanced Mail (PEM) representation of each X.509
535 certificate as described in [RFC7468].
537 o "validated-certificate-chain": the value is the certificate chain
538 as constructed by the UA during certificate chain verification.
539 (This may differ from the value of the "served-certificate-chain"
540 key.) The value is provided as an array of strings, which MUST
541 appear in the order matching the chain that the UA validated; each
542 string in the array is the Privacy-Enhanced Mail (PEM)
543 representation of each X.509 certificate as described in
544 [RFC7468].
546 o "scts": the value represents the SCTs (if any) that the UA
547 received for the Expect-CT host and their validation statuses.
548 The value is provided as an array of JSON objects. The SCTs may
549 appear in any order. Each JSON object in the array has the
550 following keys:
552 * The "sct" key, with a value as defined in Section 4.6 of
553 [I-D.ietf-trans-rfc6962-bis].
555 * The "status" key, with a string value that the UA MUST set to
556 one of the following values: "unknown" (indicating that the UA
557 does not have or does not trust the public key of the log from
558 which the SCT was issued), "valid" (indicating that the UA
559 successfully validated the SCT as described in Section 8.2.3 of
560 [I-D.ietf-trans-rfc6962-bis]), or "invalid" (indicating that
561 the SCT validation failed because of, e.g., a bad signature).
563 * The "source" key, with a string value that indicates from where
564 the UA obtained the SCT, as defined in Section 6 of
565 [I-D.ietf-trans-rfc6962-bis]. The UA MUST set the value to one
566 of "tls-extension", "ocsp", or "embedded".
568 3.2. Sending a violation report
570 The UA SHOULD report an Expect-CT failure when a connection to a
571 Known Expect-CT Host does not comply with the UA's CT Policy and the
572 host's Expect-CT metadata contains a "report-uri". Additionally, the
573 UA SHOULD report an Expect-CT failure when it receives an Expect-CT
574 header field which contains the "report-uri" directive over a
575 connection that does not comply with the UA's CT Policy.
577 The steps to report an Expect-CT failure are as follows.
579 1. Prepare a JSON object "report object" with the single key
580 "expect-ct-report", whose value is the result of generating a
581 violation report object as described in Section 3.1.
583 2. Let "report body" by the JSON stringification of "report object".
585 3. Let "report-uri" be the value of the "report-uri" directive in
586 the Expect-CT header field.
588 4. Send an HTTP POST request to "report-uri" with a "Content-Type"
589 header field of "application/expect-ct-report+json", and an
590 entity body consisting of "report body".
592 The UA MAY perform other operations as part of sending the HTTP POST
593 request, for example sending a CORS preflight as part of [FETCH].
595 4. Security Considerations
597 When UAs support the Expect-CT header, it becomes a potential vector
598 for hostile header attacks against site owners. If a site owner uses
599 a certificate issued by a certificate authority which does not embed
600 SCTs nor serve SCTs via OCSP or TLS extension, a malicious server
601 operator or attacker could temporarily reconfigure the host to comply
602 with the UA's CT policy, and add the Expect-CT header in enforcing
603 mode with a long "max-age". Implementing user agents would note this
604 as an Expect-CT Host (see Section 2.3.3). After having done this,
605 the configuration could then be reverted to not comply with the CT
606 policy, prompting failures. Note this scenario would require the
607 attacker to have substantial control over the infrastructure in
608 question, being able to obtain different certificates, change server
609 software, or act as a man-in-the-middle in connections.
611 Site operators could themselves only cure this situation by one of:
612 reconfiguring their web server to transmit SCTs using the TLS
613 extension defined in Section 6.5 of [I-D.ietf-trans-rfc6962-bis],
614 obtaining a certificate from an alternative certificate authority
615 which provides SCTs by one of the other methods, or by waiting for
616 the user agents' persisted notation of this as an Expect-CT host to
617 reach its "max-age". User agents may choose to implement mechanisms
618 for users to cure this situation, as noted in Section 7.
620 4.1. Maximum max-age
622 There is a security trade-off in that low maximum values provide a
623 narrow window of protection for users that visit the Known Expect-CT
624 Host only infrequently, while high maximum values might result in a
625 denial of service to a UA in the event of a hostile header attack, or
626 simply an error on the part of the site-owner.
628 There is probably no ideal maximum for the "max-age" directive.
629 Since Expect-CT is primarily a policy-expansion and investigation
630 technology rather than an end-user protection, a value on the order
631 of 30 days (2,592,000 seconds) may be considered a balance between
632 these competing security concerns.
634 4.2. Avoiding amplification attacks
636 Another kind of hostile header attack uses the "report-uri" mechanism
637 on many hosts not currently exposing SCTs as a method to cause a
638 denial-of-service to the host receiving the reports. If some highly-
639 trafficked websites emitted a non-enforcing Expect-CT header with a
640 "report-uri", implementing UAs' reports could flood the reporting
641 host. It is noted in Section 2.1.1 that UAs should limit the rate at
642 which they emit reports, but an attacker may alter the Expect-CT
643 header's fields to induce UAs to submit different reports to
644 different URIs to still cause the same effect.
646 5. Privacy Considerations
648 Expect-CT can be used to infer what Certificate Transparency policy
649 is in use, by attempting to retrieve specially-configured websites
650 which pass one user agents' policies but not another's. Note that
651 this consideration is true of UAs which enforce CT policies without
652 Expect-CT as well.
654 Additionally, reports submitted to the "report-uri" could reveal
655 information to a third party about which webpage is being accessed
656 and by which IP address, by using individual "report-uri" values for
657 individually-tracked pages. This information could be leaked even if
658 client-side scripting were disabled.
660 Implementations must store state about Known Expect-CT Hosts, and
661 hence which domains the UA has contacted.
663 Violation reports, as noted in Section 3, contain information about
664 the certificate chain that has violated the CT policy. In some
665 cases, such as organization-wide compromise of the end-to-end
666 security of TLS, this may include information about the interception
667 tools and design used by the organization that the organization would
668 otherwise prefer not be disclosed.
670 Because Expect-CT causes remotely-detectable behavior, it's advisable
671 that UAs offer a way for privacy-sensitive users to clear currently
672 noted Expect-CT hosts, and allow users to query the current state of
673 Known Expect-CT Hosts.
675 6. IANA Considerations
677 TBD
679 7. Usability Considerations
681 When the UA detects a Known Expect-CT Host in violation of the UA's
682 CT Policy, users will experience denials of service. It is advisable
683 for UAs to explain the reason why.
685 8. Authoring Considerations
687 8.1. HTTP Header
689 Expect-CT could be specified as a TLS extension or X.509 certificate
690 extension instead of an HTTP response header. Using an HTTP header
691 as the mechanism for Expect-CT introduces a layering mismatch: for
692 example, the software that terminates TLS and validates Certificate
693 Transparency information might know nothing about HTTP.
694 Nevertheless, an HTTP header was chosen primarily for ease of
695 deployment. In practice, deploying new certificate extensions
696 requires certificate authorities to support them, and new TLS
697 extensions require server software updates, including possibly to
698 servers outside of the site owner's direct control (such as in the
699 case of a third-party CDN). Ease of deployment is a high priority
700 for Expect-CT because it is intended as a temporary transition
701 mechanism for user agents that are transitioning to universal
702 Certificate Transparency requirements.
704 9. Changes
706 9.1. Since -00
708 o Editorial changes
709 o Change Content-Type header of reports to 'application/expect-ct-
710 report+json'
712 o Update header field syntax to match convention (issue #327)
714 o Reference RFC 6962-bis instead of RFC 6962
716 10. Normative References
718 [FETCH] van Kesteren, A., "Fetch", n.d.,
719 .
721 [HTML] Hickson, I., Pieters, S., van Kesteren, A., Jaegenstedt,
722 P., and D. Denicola, "HTML", n.d.,
723 .
725 [I-D.ietf-trans-rfc6962-bis]
726 Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
727 Stradling, "Certificate Transparency Version 2.0", draft-
728 ietf-trans-rfc6962-bis-24 (work in progress), December
729 2016.
731 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
732 Requirement Levels", BCP 14, RFC 2119,
733 DOI 10.17487/RFC2119, March 1997,
734 .
736 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
737 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
738 .
740 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
741 Resource Identifier (URI): Generic Syntax", STD 66,
742 RFC 3986, DOI 10.17487/RFC3986, January 2005,
743 .
745 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
746 Specifications: ABNF", STD 68, RFC 5234,
747 DOI 10.17487/RFC5234, January 2008,
748 .
750 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
751 (TLS) Protocol Version 1.2", RFC 5246,
752 DOI 10.17487/RFC5246, August 2008,
753 .
755 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
756 Transport Security (HSTS)", RFC 6797,
757 DOI 10.17487/RFC6797, November 2012,
758 .
760 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
761 Protocol (HTTP/1.1): Message Syntax and Routing",
762 RFC 7230, DOI 10.17487/RFC7230, June 2014,
763 .
765 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
766 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
767 RFC 7234, DOI 10.17487/RFC7234, June 2014,
768 .
770 [RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
771 PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
772 April 2015, .
774 [W3C.REC-html401-19991224]
775 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
776 Specification", World Wide Web Consortium Recommendation
777 REC-html401-19991224, December 1999,
778 .
780 Author's Address
782 Emily Stark
783 Google
785 Email: estark@google.com