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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Using TLS in Applications D. Margolis 3 Internet-Draft M. Risher 4 Intended status: Standards Track Google, Inc 5 Expires: June 7, 2018 B. Ramakrishnan 6 Yahoo!, Inc 7 A. Brotman 8 Comcast, Inc 9 J. Jones 10 Microsoft, Inc 11 December 4, 2017 13 SMTP MTA Strict Transport Security (MTA-STS) 14 draft-ietf-uta-mta-sts-12 16 Abstract 18 SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a 19 mechanism enabling mail service providers to declare their ability to 20 receive Transport Layer Security (TLS) secure SMTP connections, and 21 to specify whether sending SMTP servers should refuse to deliver to 22 MX hosts that do not offer TLS with a trusted server certificate. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at https://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on June 7, 2018. 41 Copyright Notice 43 Copyright (c) 2017 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (https://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 59 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 60 2. Related Technologies . . . . . . . . . . . . . . . . . . . . 3 61 3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4 62 3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4 63 3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 5 64 3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 8 65 3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 9 66 4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 9 67 4.1. MX Certificate Validation . . . . . . . . . . . . . . . . 10 68 5. Policy Application . . . . . . . . . . . . . . . . . . . . . 10 69 5.1. Policy Application Control Flow . . . . . . . . . . . . . 11 70 6. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 11 71 7. Interoperability Considerations . . . . . . . . . . . . . . . 12 72 7.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 12 73 7.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 12 74 8. Operational Considerations . . . . . . . . . . . . . . . . . 13 75 8.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 13 76 8.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 13 77 8.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 14 78 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 79 9.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 14 80 9.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 15 81 9.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 15 82 10. Security Considerations . . . . . . . . . . . . . . . . . . . 15 83 10.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 16 84 10.2. Preventing Policy Discovery . . . . . . . . . . . . . . 16 85 10.3. Denial of Service . . . . . . . . . . . . . . . . . . . 17 86 10.4. Weak Policy Constraints . . . . . . . . . . . . . . . . 18 87 10.5. Compromise of the Web PKI System . . . . . . . . . . . . 18 88 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18 89 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 90 12.1. Normative References . . . . . . . . . . . . . . . . . . 19 91 12.2. Informative References . . . . . . . . . . . . . . . . . 20 92 Appendix A. MTA-STS example record & policy . . . . . . . . . . 21 93 Appendix B. Message delivery pseudocode . . . . . . . . . . . . 21 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 96 1. Introduction 98 The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and 99 hosts to negotiate the use of a TLS channel for encrypted mail 100 transmission. 102 While this opportunistic encryption protocol by itself provides a 103 high barrier against passive man-in-the-middle traffic interception, 104 any attacker who can delete parts of the SMTP session (such as the 105 "250 STARTTLS" response) or who can redirect the entire SMTP session 106 (perhaps by overwriting the resolved MX record of the delivery 107 domain) can perform downgrade or interception attacks. 109 This document defines a mechanism for recipient domains to publish 110 policies specifying: 112 o whether MTAs sending mail to this domain can expect PKIX- 113 authenticated TLS support 115 o what a conforming client should do with messages when TLS cannot 116 be successfully negotiated 118 1.1. Terminology 120 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 121 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 122 document are to be interpreted as described in [RFC2119]. 124 We also define the following terms for further use in this document: 126 o MTA-STS Policy: A commitment by the Policy Domain to support PKIX 127 authenticated TLS for the specified MX hosts. 129 o Policy Domain: The domain for which an MTA-STS Policy is defined. 130 This is the next-hop domain; when sending mail to 131 "alice@example.com" this would ordinarily be "example.com", but 132 this may be overridden by explicit routing rules (as described in 133 Section 3.4, "Policy Selection for Smart Hosts and Subdomains"). 135 2. Related Technologies 137 The DANE TLSA record [RFC7672] is similar, in that DANE is also 138 designed to upgrade unauthenticated encryption or plaintext 139 transmission into authenticated, downgrade-resistant encrypted 140 transmission. DANE requires DNSSEC [RFC4033] for authentication; the 141 mechanism described here instead relies on certificate authorities 142 (CAs) and does not require DNSSEC, at a cost of risking malicious 143 downgrades. For a thorough discussion of this trade-off, see 144 Section 10, "Security Considerations". 146 In addition, MTA-STS provides an optional report-only mode, enabling 147 soft deployments to detect policy failures; partial deployments can 148 be achieved in DANE by deploying TLSA records only for some of a 149 domain's MXs, but such a mechanism is not possible for the per-domain 150 policies used by MTA-STS. 152 The primary motivation of MTA-STS is to provide a mechanism for 153 domains to ensure transport security even when deploying DNSSEC is 154 undesirable or impractical. However, MTA-STS is designed not to 155 interfere with DANE deployments when the two overlap; in particular, 156 senders who implement MTA-STS validation MUST NOT allow a "valid" or 157 "report-only" MTA-STS validation to override a failing DANE 158 validation. 160 3. Policy Discovery 162 MTA-STS policies are distributed via HTTPS from a "well-known" 163 [RFC5785] path served within the Policy Domain, and their presence 164 and current version are indicated by a TXT record at the Policy 165 Domain. These TXT records additionally contain a policy "id" field, 166 allowing sending MTAs to check the currency of a cached policy 167 without performing an HTTPS request. 169 To discover if a recipient domain implements MTA-STS, a sender need 170 only resolve a single TXT record. To see if an updated policy is 171 available for a domain for which the sender has a previously cached 172 policy, the sender need only check the TXT record's version "id" 173 against the cached value. 175 3.1. MTA-STS TXT Records 177 The MTA-STS TXT record is a TXT record with the name "_mta-sts" at 178 the Policy Domain. For the domain "example.com", this record would 179 be "_mta-sts.example.com". MTA-STS TXT records MUST be US-ASCII, 180 semicolon-separated key/value pairs containing the following fields: 182 o "v": (plain-text, required). Currently only "STSv1" is supported. 184 o "id": (plain-text, required). A short string used to track policy 185 updates. This string MUST uniquely identify a given instance of a 186 policy, such that senders can determine when the policy has been 187 updated by comparing to the "id" of a previously seen policy. 188 There is no implied ordering of "id" fields between revisions. 190 An example TXT record is as below: 192 "_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"" 194 The formal definition of the "_mta-sts" TXT record, defined using 195 [RFC7405], is as follows: 197 sts-text-record = sts-version 1*(field-delim sts-field) [field-delim] 199 sts-field = sts-id / ; Note that sts-id record 200 sts-extension ; is required. 202 field-delim = *WSP ";" *WSP 204 sts-version = %s"v=STSv1" 206 sts-id = %s"id=" 1*32(ALPHA / DIGIT) ; id=... 208 sts-extension = sts-ext-name "=" sts-ext-value ; name=value 210 sts-ext-name = (ALPHA / DIGIT) 211 *31(ALPHA / DIGIT / "_" / "-" / ".") 213 sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) 214 ; chars excluding "=", ";", SP, and control chars 216 If multiple TXT records for "_mta-sts" are returned by the resolver, 217 records which do not begin with "v=STSv1;" are discarded. If the 218 number of resulting records is not one, senders MUST assume the 219 recipient domain does not implement MTA-STS and skip the remaining 220 steps of policy discovery. If the resulting TXT record contains 221 multiple strings, then the record MUST be treated as if those strings 222 are concatenated together without adding spaces. 224 3.2. MTA-STS Policies 226 The policy itself is a set of key/value pairs (similar to [RFC5322] 227 header fields) served via the HTTPS GET method from the fixed 228 [RFC5785] "well-known" path of ".well-known/mta-sts.txt" served by 229 the "mta-sts" host at the Policy Domain. Thus for "example.com" the 230 path is "https://mta-sts.example.com/.well-known/mta-sts.txt". 232 The [RFC7231] "Content-Type" media type for this resource MUST be 233 "text/plain". When fetching a policy, senders SHOULD validate that 234 the media type is "text/plain" to guard against cases where 235 webservers allow untrusted users to host non-text content (typically, 236 HTML or images) at a user-defined path. Additional "Content-Type" 237 parameters are ignored. 239 This resource contains the following newline-separated key/value 240 pairs: 242 o "version": (plain-text). Currently only "STSv1" is supported. 244 o "mode": (plain-text). One of "enforce", "testing", or "none", 245 indicating the expected behavior of a sending MTA in the case of a 246 policy validation failure. 248 o "max_age": Max lifetime of the policy (plain-text non-negative 249 integer seconds, maximum value of 31557600). Well-behaved clients 250 SHOULD cache a policy for up to this value from last policy fetch 251 time. To mitigate the risks of attacks at policy refresh time, it 252 is expected that this value typically be in the range of weeks or 253 greater. 255 o "mx": MX identity patterns (list of plain-text strings). One or 256 more patterns matching a Common Name ([RFC6125]) or Subject 257 Alternative Name ([RFC5280]) DNS-ID present in the X.509 258 certificate presented by any MX receiving mail for this domain. 259 For example: "mx: mail.example.com mx: .example.net" indicates 260 that mail for this domain might be handled by any MX with a 261 certificate valid for a host at "mail.example.com" or 262 "example.net". Valid patterns can be either fully specified names 263 ("example.com") or suffixes (".example.net") matching the right- 264 hand parts of a server's identity; the latter case are 265 distinguished by a leading period. If there are more than one MX 266 specified by the policy, they MUST be on separate lines within the 267 policy file. In the case of Internationalized Domain Names 268 ([RFC5891]), the MX MUST specify the Punycode-encoded A-label 269 [RFC3492] and not the Unicode-encoded U-label. The full semantics 270 of certificate validation are described in Section 4.1, "MX 271 Certificate Validation." 273 An example policy is as below: 275 version: STSv1 276 mode: enforce 277 mx: mail.example.com 278 mx: .example.net 279 mx: backupmx.example.com 280 max_age: 123456 282 The formal definition of the policy resource, defined using 283 [RFC7405], is as follows: 285 sts-policy-record = *WSP sts-policy-field *WSP 286 *(CRLF *WSP sts-policy-field *WSP) 288 [CRLF] 290 sts-policy-field = sts-policy-version / ; required once 291 sts-policy-mode / ; required once 292 sts-policy-max-age / ; required once 294 0*(sts-policy-mx *WSP CRLF) / 295 ; required at least once, except when 296 ; mode is "none" 298 sts-policy-extension ; other fields 300 field-delim = ":" *WSP 302 sts-policy-version = sts-policy-version-field field-delim 303 sts-policy-version-value 305 sts-policy-version-field = %s"version" 307 sts-policy-version-value = %s"STSv1" 309 sts-policy-mode = sts-policy-mode-field field-delim 310 sts-policy-mode-value 312 sts-policy-mode-field = %s"mode" 314 sts-policy-model-value = %s"testing" / %s"enforce" / %s"none" 316 sts-policy-mx = sts-policy-mx-field field-delim 317 sts-policy-mx-value 319 sts-policy-mx-field = %s"mx" 321 sts-policy-mx-value = 1*(ALPHA / DIGIT / "_" / "-" / ".") 323 sts-policy-max-age = sts-policy-max-age-field field-delim 324 sts-policy-max-age-value 326 sts-policy-max-age-field = %s"max_age" 328 sts-policy-max-age-value = 1*10(DIGIT) 330 sts-policy-extension = sts-policy-ext-name ; additional 331 field-delim ; extension 332 sts-policy-ext-value ; fields 334 sts-policy-ext-name = (ALPHA / DIGIT) 335 *31(ALPHA / DIGIT / "_" / "-" / ".") 337 sts-policy-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) 338 ; chars, excluding "=", ";", SP, and 339 ; control chars 341 Parsers MUST accept TXT records and policy files which are 342 syntactically valid (i.e. valid key/value pairs separated by semi- 343 colons for TXT records) and but containing additional key/value pairs 344 not specified in this document, in which case unknown fields SHALL be 345 ignored. If any non-repeated field--i.e. all fields excepting "mx"-- 346 is duplicated, all entries except for the first SHALL be ignored. If 347 any field is not specified, the policy SHALL be treated as invalid. 349 3.3. HTTPS Policy Fetching 351 When fetching a new policy or updating a policy, the HTTPS endpoint 352 MUST present a X.509 certificate which is valid for the "mta-sts" 353 host (e.g. "mta-sts.example.com") as described below, chain to a 354 root CA that is trusted by the sending MTA, and be non-expired. It 355 is expected that sending MTAs use a set of trusted CAs similar to 356 those in widely deployed Web browsers and operating systems. 358 The certificate is valid for the "mta-sts" host with respect to the 359 rules described in [RFC6125], with the following application-specific 360 considerations: 362 o Matching is performed only against the DNS-ID and CN-ID 363 identifiers. 365 o DNS domain names in server certificates MAY contain the wildcard 366 character '*' as the complete left-most label within the 367 identifier. 369 The certificate MAY be checked for revocation via the Online 370 Certificate Status Protocol (OCSP) [RFC6960], certificate revocation 371 lists (CRLs), or some other mechanism. 373 Policies fetched via HTTPS are only valid if the HTTP response code 374 is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP 375 caching (as specified in [RFC7234]) MUST NOT be used. 377 Senders may wish to rate-limit the frequency of attempts to fetch the 378 HTTPS endpoint even if a valid TXT record for the recipient domain 379 exists. In the case that the HTTPS GET fails, we suggest 380 implementions may limit further attempts to a period of five minutes 381 or longer per version ID, to avoid overwhelming resource-constrained 382 recipients with cascading failures. 384 Senders MAY impose a timeout on the HTTPS GET and/or a limit on the 385 maximum size of the response body to avoid long delays or resource 386 exhaustion during attempted policy updates. A suggested timeout is 387 one minute, and a suggested maximum policy size 64 kilobytes; policy 388 hosts SHOULD respond to requests with a complete policy body within 389 that timeout and size limit. 391 If a valid TXT record is found but no policy can be fetched via HTTPS 392 (for any reason), and there is no valid (non-expired) previously- 393 cached policy, senders MUST continue with delivery as though the 394 domain has not implemented MTA-STS. 396 Conversely, if no "live" policy can be discovered via DNS or fetched 397 via HTTPS, but a valid (non-expired) policy exists in the sender's 398 cache, the sender MUST apply that cached policy. 400 Finally, to mitigate the risk of persistent interference with policy 401 refresh, as discussed in-depth in Section 10, MTAs SHOULD 402 proactivecly refresh cached policies before they expire; a suggested 403 refresh frequency is once per day. To enable administrators to 404 discover problems with policy refresh, MTAs SHOULD alert 405 administrators (through the use of logs or similar) when such 406 attempts fail, unless the cached policy mode is "none". 408 3.4. Policy Selection for Smart Hosts and Subdomains 410 When sending mail via a "smart host"--an intermediate SMTP relay 411 rather than the message recipient's server--compliant senders MUST 412 treat the smart host domain as the policy domain for the purposes of 413 policy discovery and application. 415 When sending mail to a mailbox at a subdomain, compliant senders MUST 416 NOT attempt to fetch a policy from the parent zone. Thus for mail 417 sent to "user@mail.example.com", the policy can be fetched only from 418 "mail.example.com", not "example.com". 420 4. Policy Validation 422 When sending to an MX at a domain for which the sender has a valid 423 and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST 424 validate: 426 1. That the recipient MX supports STARTTLS and offers a valid PKIX- 427 based TLS certificate. 429 2. That at least one of the policy's "mx" patterns matches at least 430 one of the identities presented in the MX's X.509 certificate, as 431 described in "MX Certificate Validation". 433 This section does not dictate the behavior of sending MTAs when 434 policies fail to validate; see Section 5, "Policy Application" for a 435 description of sending MTA behavior when policy validation fails. 437 4.1. MX Certificate Validation 439 The certificate presented by the receiving MX MUST chain to a root CA 440 that is trusted by the sending MTA and be non-expired. The 441 certificate MUST have a CN-ID ([RFC6125]) or subject alternative name 442 (SAN, [RFC5280]) with a DNS-ID matching the "mx" pattern. The MX's 443 certificate MAY also be checked for revocation via OCSP [RFC6960], 444 CRLs [RFC6818], or some other mechanism. 446 Because the "mx" patterns are not hostnames, however, matching is not 447 identical to other common cases of X.509 certificate authentication 448 (as described, for example, in [RFC6125]). Consider the example 449 policy given above, with an "mx" pattern containing ".example.com". 450 In this case, if the MX server's X.509 certificate contains a SAN 451 matching "*.example.com", we are required to implement "wildcard-to- 452 wildcard" matching. 454 To simplify this case, we impose the following constraints on 455 wildcard certificates, identical to those in [RFC7672] section 3.2.3 456 and [RFC6125] section 6.4.3: wildcards are valid in DNS-IDs or CN- 457 IDs, but must be the entire first label of the identifier (that is, 458 "*.example.com", not "mail*.example.com"). Senders who are comparing 459 a "suffix" MX pattern with a wildcard identifier should thus strip 460 the wildcard and ensure that the two sides match label-by-label, 461 until all labels of the shorter side (if unequal length) are 462 consumed. 464 Note that a wildcard must match a label; an "mx" pattern of 465 ".example.com" thus does not match a SAN of "example.com", nor does a 466 SAN of "*.example.com" match an "mx" of "example.com". 468 A simple pseudocode implementation of this algorithm is presented in 469 Appendix B. 471 5. Policy Application 473 When sending to an MX at a domain for which the sender has a valid, 474 non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies 475 the result of a policy validation failure one of two ways, depending 476 on the value of the policy "mode" field: 478 1. "enforce": In this mode, sending MTAs MUST NOT deliver the 479 message to hosts which fail MX matching or certificate 480 validation. 482 2. "report": In this mode, sending MTAs which also implement the 483 TLSRPT specification [I-D.ietf-uta-smtp-tlsrpt] merely send a 484 report indicating policy application failures (so long as TLSRPT 485 is also implemented by the recipient domain). 487 3. "none": In this mode, sending MTAs should treat the policy domain 488 as though it does not have any active policy; see Section 8.3, 489 "Removing MTA-STS", for use of this mode value. 491 When a message fails to deliver due to an "enforce" policy, a 492 compliant MTA MUST NOT permanently fail to deliver messages before 493 checking for the presence of an updated policy at the Policy Domain. 494 (In all cases, MTAs SHOULD treat such failures as transient errors 495 and retry delivery later.) This allows implementing domains to 496 update long-lived policies on the fly. 498 5.1. Policy Application Control Flow 500 An example control flow for a compliant sender consists of the 501 following steps: 503 1. Check for a cached policy whose time-since-fetch has not exceeded 504 its "max_age". If none exists, attempt to fetch a new policy 505 (perhaps asynchronously, so as not to block message delivery). 506 Optionally, sending MTAs may unconditionally check for a new 507 policy at this step. 509 2. For each candidate MX, in order of MX priority, attempt to 510 deliver the message, enforcing STARTTLS and, assuming a policy is 511 present, PKIX certificate validation as described in Section 4.1, 512 "MX Certificate Validation." 514 3. A message delivery MUST NOT be permanently failed until the 515 sender has first checked for the presence of a new policy (as 516 indicated by the "id" field in the "_mta-sts" TXT record). If a 517 new policy is not found, existing rules for the case of temporary 518 message delivery failures apply (as discussed in [RFC5321] 519 section 4.5.4.1). 521 6. Reporting Failures 523 MTA-STS is intended to be used along with TLSRPT 524 [I-D.ietf-uta-smtp-tlsrpt] in order to ensure implementing domains 525 can detect cases of both benign and malicious failures, and to ensure 526 that failures that indicate an active attack are discoverable. As 527 such, senders who also implement TLSRPT SHOULD treat the following 528 events as reportable failures: 530 o HTTPS policy fetch failures when a valid TXT record is present. 532 o Policy fetch failures of any kind when a valid policy exists in 533 the policy cache, except if that policy's mode is "none". 535 o Delivery attempts in which a contacted MX does not support 536 STARTTLS or does not present a certificate which validates 537 according to the applied policy, except if that policy's mode is 538 "none". 540 7. Interoperability Considerations 542 7.1. SNI Support 544 To ensure that the server sends the right certificate chain, the SMTP 545 client MUST have support for the TLS SNI extension [RFC6066]. When 546 connecting to a HTTP server to retrieve the MTA-STS policy, the SNI 547 extension MUST contain the name of the policy host (e.g. "mta- 548 sts.example.com"). When connecting to an SMTP server, the SNI 549 extension MUST contain the MX hostname. 551 HTTP servers used to deliver MTA-STS policies MUST have support for 552 the TLS SNI extension and MAY rely on SNI to determine which 553 certificate chain to present to the client. In either case, HTTP 554 servers MUST respond with a certificate chain that matches the policy 555 hostname or abort the TLS handshake if unable to do so. 557 SMTP servers MUST have support for the TLS SNI extension and MAY rely 558 on SNI to determine which certificate chain to present to the client. 559 If the client sends no SNI extension or sends an SNI extension for an 560 unsupported server name, the server MUST simply send a fallback 561 certificate chain of its choice. The reason for not enforcing strict 562 matching of the requested SNI hostname is that MTA-STS TLS clients 563 may be typically willing to accept multiple server names but can only 564 send one name in the SNI extension. The server's fallback 565 certificate may match a different name that is acceptable to the 566 client, e.g., the original next-hop domain. 568 7.2. Minimum TLS Version Support 570 MTAs supporting MTA-STS MUST have support for TLS version 1.2 571 [RFC5246] or higher. The general TLS usage guidance in [RFC7525] 572 SHOULD be followed. 574 8. Operational Considerations 576 8.1. Policy Updates 578 Updating the policy requires that the owner make changes in two 579 places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and 580 at the corresponding HTTPS endpoint. As a result, recipients should 581 expect a policy will continue to be used by senders until both the 582 HTTPS and TXT endpoints are updated and the TXT record's TTL has 583 passed. 585 In other words, a sender who is unable to successfully deliver a 586 message while applying a cache of the recipient's now-outdated policy 587 may be unable to discover that a new policy exists until the DNS TTL 588 has passed. Recipients should therefore ensure that old policies 589 continue to work for message delivery during this period of time, or 590 risk message delays. 592 Recipients should also prefer to update the HTTPS policy body before 593 updating the TXT record; this ordering avoids the risk that senders, 594 seeing a new TXT record, mistakenly cache the old policy from HTTPS. 596 8.2. Policy Delegation 598 Domain owners commonly delegate SMTP hosting to a different 599 organization, such as an ISP or a Web host. In such a case, they may 600 wish to also delegate the MTA-STS policy to the same organization 601 which can be accomplished with two changes. 603 First, the Policy Domain must point the "_mta-sts" record, via CNAME, 604 to the "_mta-sts" record maintained by the hosting organization. 605 This allows the hosting organization to control update signaling. 607 Second, the Policy Domain must point the "well-known" policy location 608 to the hosting organization. This can be done either by setting the 609 "mta-sts" record to an IP address or CNAME specified by the hosting 610 organization and by giving the hosting organization a TLS certificate 611 which is valid for that host, or by setting up a "reverse proxy" 612 (also known as a "gateway") server that serves as the Policy Domain's 613 policy the policy currently served by the hosting organization. 615 For example, given a user domain "user.example" hosted by a mail 616 provider "provider.example", the following configuration would allow 617 policy delegation: 619 DNS: 621 _mta-sts.user.example. IN CNAME _mta-sts.provider.example. 623 Policy: 625 > GET /.well-known/mta-sts.txt 626 > Host: mta-sts.user.example 627 < HTTP/1.1 200 OK # Response proxies content from 628 # https://mta-sts.provider.example 630 Note that while sending MTAs MUST NOT use HTTP caching when fetching 631 policies via HTTPS, such caching may nonetheless be useful to a 632 reverse proxy configured as described in this section. An HTTPS 633 policy endpoint expecting to be proxied for multiple hosted domains-- 634 as with a large mail hosting provider or similar--may wish to 635 indicate an HTTP Cache-Control "max-age" response directive (as 636 specified in [RFC7234]) of 60 seconds as a reasonable value to save 637 reverse proxies an unnecessarily high-rate of proxied policy 638 fetching. 640 8.3. Removing MTA-STS 642 In order to facilitate clean opt-out of MTA-STS by implementing 643 policy domains, and to distinguish clearly between failures which 644 indicate attacks and those which indicate such opt-outs, MTA-STS 645 implements the "none" mode, which allows validated policies to 646 indicate authoritatively that the policy domain wishes to no longer 647 implement MTA-STS and may, in the future, remove the MTA-STS TXT and 648 policy endpoints entirely. 650 A suggested workflow to implement such an opt out is as follows: 652 1. Publish a new policy with "mode" equal to "none" and a small 653 "max_age" (e.g. one day). 655 2. Publish a new TXT record to trigger fetching of the new policy. 657 3. When all previously served policies have expired--normally this 658 is the time the previously published policy was last served plus 659 that policy's "max_age", but note that older policies may have 660 been served with a greater "max_age", allowing overlapping policy 661 caches--safely remove the TXT record and HTTPS endpoint. 663 9. IANA Considerations 665 9.1. Well-Known URIs Registry 667 A new "well-known" URI as described in Section 3 will be registered 668 in the Well-Known URIs registry as described below: 670 URI Suffix: mta-sts.txt Change Controller: IETF 672 9.2. MTA-STS TXT Record Fields 674 IANA is requested to create a new registry titled "MTA-STS TXT Record 675 Fields". The initial entries in the registry are: 677 +------------+--------------------+------------------------+ 678 | Field Name | Description | Reference | 679 +------------+--------------------+------------------------+ 680 | v | Record version | Section 3.1 of RFC XXX | 681 | id | Policy instance ID | Section 3.1 of RFC XXX | 682 +------------+--------------------+------------------------+ 684 New fields are added to this registry using IANA's "Expert Review" 685 policy. 687 9.3. MTA-STS Policy Fields 689 IANA is requested to create a new registry titled "MTA-STS Policy 690 Fields". The initial entries in the registry are: 692 +------------+----------------------+------------------------+ 693 | Field Name | Description | Reference | 694 +------------+----------------------+------------------------+ 695 | version | Policy version | Section 3.2 of RFC XXX | 696 | mode | Enforcement behavior | Section 3.2 of RFC XXX | 697 | max_age | Policy lifetime | Section 3.2 of RFC XXX | 698 | mx | MX identities | Section 3.2 of RFC XXX | 699 +------------+----------------------+------------------------+ 701 New fields are added to this registry using IANA's "Expert Review" 702 policy. 704 10. Security Considerations 706 SMTP MTA Strict Transport Security attempts to protect against an 707 active attacker who wishes to intercept or tamper with mail between 708 hosts who support STARTTLS. There are two classes of attacks 709 considered: 711 o Foiling TLS negotiation, for example by deleting the "250 712 STARTTLS" response from a server or altering TLS session 713 negotiation. This would result in the SMTP session occurring over 714 plaintext, despite both parties supporting TLS. 716 o Impersonating the destination mail server, whereby the sender 717 might deliver the message to an impostor, who could then monitor 718 and/or modify messages despite opportunistic TLS. This 719 impersonation could be accomplished by spoofing the DNS MX record 720 for the recipient domain, or by redirecting client connections 721 intended for the legitimate recipient server (for example, by 722 altering BGP routing tables). 724 MTA-STS can thwart such attacks only if the sender is able to 725 previously obtain and cache a policy for the recipient domain, and 726 only if the attacker is unable to obtain a valid certificate that 727 complies with that policy. Below, we consider specific attacks on 728 this model. 730 10.1. Obtaining a Signed Certificate 732 SMTP MTA-STS relies on certificate validation via PKIX based TLS 733 identity checking [RFC6125]. Attackers who are able to obtain a 734 valid certificate for the targeted recipient mail service (e.g. by 735 compromising a certificate authority) are thus able to circumvent STS 736 authentication. 738 10.2. Preventing Policy Discovery 740 Since MTA-STS uses DNS TXT records for policy discovery, an attacker 741 who is able to block DNS responses can suppress the discovery of an 742 MTA-STS Policy, making the Policy Domain appear not to have an MTA- 743 STS Policy. The sender policy cache is designed to resist this 744 attack by decreasing the frequency of policy discovery and thus 745 reducing the window of vulnerability; it is nonetheless a risk that 746 attackers who can predict or induce policy discovery--for example, by 747 inducing a sending domain to send mail to a never-before-contacted 748 recipient while carrying out a man-in-the-middle attack--may be able 749 to foil policy discovery and effectively downgrade the security of 750 the message delivery. 752 Since this attack depends upon intercepting initial policy discovery, 753 we strongly recommend implementers to prefer policy "max_age" values 754 to be as long as is practical. 756 Because this attack is also possible upon refresh of a cached policy, 757 we suggest implementers do not wait until a cached policy has expired 758 before checking for an update; if senders attempt to refresh the 759 cache regularly (for instance, by checking their cached version 760 string against the TXT record on each successful send, or in a 761 background task that runs daily or weekly), an attacker would have to 762 foil policy discovery consistently over the lifetime of a cached 763 policy to prevent a successful refresh. 765 Additionally, MTAs should alert administrators to repeated policy 766 refresh failures long before cached policies expire (through warning 767 logs or similar applicable mechanisms), allowing administrators to 768 detect such a persistent attack on policy refresh. (However, they 769 should not implement such alerts if the cached policy has a "none" 770 mode, to allow clean MTA-STS removal, as described in Section 8.3.) 772 Resistance to downgrade attacks of this nature--due to the ability to 773 authoritatively determine "lack of a record" even for non- 774 participating recipients--is a feature of DANE, due to its use of 775 DNSSEC for policy discovery. 777 10.3. Denial of Service 779 We additionally consider the Denial of Service risk posed by an 780 attacker who can modify the DNS records for a recipient domain. 781 Absent MTA-STS, such an attacker can cause a sending MTA to cache 782 invalid MX records, but only for however long the sending resolver 783 caches those records. With MTA-STS, the attacker can additionally 784 advertise a new, long-"max_age" MTA-STS policy with "mx" constraints 785 that validate the malicious MX record, causing senders to cache the 786 policy and refuse to deliver messages once the victim has resecured 787 the MX records. 789 This attack is mitigated in part by the ability of a victim domain to 790 (at any time) publish a new policy updating the cached, malicious 791 policy, though this does require the victim domain to both obtain a 792 valid CA-signed certificate and to understand and properly configure 793 MTA-STS. 795 Similarly, we consider the possibility of domains that deliberately 796 allow untrusted users to serve untrusted content on user-specified 797 subdomains. In some cases (e.g. the service Tumblr.com) this takes 798 the form of providing HTTPS hosting of user-registered subdomains; in 799 other cases (e.g. dynamic DNS providers) this takes the form of 800 allowing untrusted users to register custom DNS records at the 801 provider's domain. 803 In these cases, there is a risk that untrusted users would be able to 804 serve custom content at the "mta-sts" host, including serving an 805 illegitimate MTA-STS policy. We believe this attack is rendered more 806 difficult by the need for the attacker to also serve the "_mta-sts" 807 TXT record on the same domain--something not, to our knowledge, 808 widely provided to untrusted users. This attack is additionally 809 mitigated by the aforementioned ability for a victim domain to update 810 an invalid policy at any future date. 812 10.4. Weak Policy Constraints 814 Even if an attacker cannot modify a served policy, the potential 815 exists for configurations that allow attackers on the same domain to 816 receive mail for that domain. For example, an easy configuration 817 option when authoring an MTA-STS Policy for "example.com" is to set 818 the "mx" equal to ".example.com"; recipient domains must consider in 819 this case the risk that any user possessing a valid hostname and CA- 820 signed certificate (for example, "dhcp-123.example.com") will, from 821 the perspective of MTA-STS Policy validation, be a valid MX host for 822 that domain. 824 10.5. Compromise of the Web PKI System 826 A host of risks apply to the PKI system used for certificate 827 authentication, both of the "mta-sts" HTTPS host's certificate and 828 the SMTP servers' certificates. These risks are broadly applicable 829 within the Web PKI ecosystem and are not specific to MTA-STS; 830 nonetheless, they deserve some consideration in this context. 832 Broadly speaking, attackers may compromise the system by obtaining 833 certificates under fraudulent circumstances (i.e. by impersonating 834 the legitimate owner of the victim domain), by compromising a 835 Certificate Authority or Delegate Authority's private keys, by 836 obtaining a legitimate certificate issued to the victim domain, and 837 similar. 839 One approach commonly employed by Web browsers to help mitigate 840 against some of these attacks is to allow for revocation of 841 compromised or fraudulent certificates via OCSP [RFC6960] or CRLs 842 [RFC6818]. Such mechanisms themselves represent tradeoffs and are 843 not universally implemented; we nonetheless recommend implementors of 844 MTA-STS to implement revocation mechanisms which are most applicable 845 to their implementations. 847 11. Contributors 849 Nicolas Lidzborski Google, Inc nlidz (at) google (dot com) 851 Wei Chuang Google, Inc weihaw (at) google (dot com) 853 Brandon Long Google, Inc blong (at) google (dot com) 855 Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com) 857 Klaus Umbach 1&1 Mail & Media Development & Technology GmbH 858 klaus.umbach (at) 1und1 (dot de) 859 Markus Laber 1&1 Mail & Media Development & Technology GmbH 860 markus.laber (at) 1und1 (dot de) 862 12. References 864 12.1. Normative References 866 [I-D.ietf-uta-smtp-tlsrpt] 867 Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J., 868 and M. Risher, "SMTP TLS Reporting", draft-ietf-uta-smtp- 869 tlsrpt-11 (work in progress), November 2017. 871 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 872 Requirement Levels", BCP 14, RFC 2119, 873 DOI 10.17487/RFC2119, March 1997, 874 . 876 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 877 for Internationalized Domain Names in Applications 878 (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, 879 . 881 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 882 (TLS) Protocol Version 1.2", RFC 5246, 883 DOI 10.17487/RFC5246, August 2008, 884 . 886 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 887 Housley, R., and W. Polk, "Internet X.509 Public Key 888 Infrastructure Certificate and Certificate Revocation List 889 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 890 . 892 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 893 DOI 10.17487/RFC5321, October 2008, 894 . 896 [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known 897 Uniform Resource Identifiers (URIs)", RFC 5785, 898 DOI 10.17487/RFC5785, April 2010, 899 . 901 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 902 Extensions: Extension Definitions", RFC 6066, 903 DOI 10.17487/RFC6066, January 2011, 904 . 906 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 907 Verification of Domain-Based Application Service Identity 908 within Internet Public Key Infrastructure Using X.509 909 (PKIX) Certificates in the Context of Transport Layer 910 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 911 2011, . 913 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 914 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 915 DOI 10.17487/RFC7231, June 2014, 916 . 918 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 919 RFC 7405, DOI 10.17487/RFC7405, December 2014, 920 . 922 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 923 "Recommendations for Secure Use of Transport Layer 924 Security (TLS) and Datagram Transport Layer Security 925 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 926 2015, . 928 12.2. Informative References 930 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 931 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 932 February 2002, . 934 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 935 Rose, "DNS Security Introduction and Requirements", 936 RFC 4033, DOI 10.17487/RFC4033, March 2005, 937 . 939 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 940 DOI 10.17487/RFC5322, October 2008, 941 . 943 [RFC5891] Klensin, J., "Internationalized Domain Names in 944 Applications (IDNA): Protocol", RFC 5891, 945 DOI 10.17487/RFC5891, August 2010, 946 . 948 [RFC6818] Yee, P., "Updates to the Internet X.509 Public Key 949 Infrastructure Certificate and Certificate Revocation List 950 (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January 951 2013, . 953 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 954 Galperin, S., and C. Adams, "X.509 Internet Public Key 955 Infrastructure Online Certificate Status Protocol - OCSP", 956 RFC 6960, DOI 10.17487/RFC6960, June 2013, 957 . 959 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 960 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 961 RFC 7234, DOI 10.17487/RFC7234, June 2014, 962 . 964 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 965 Opportunistic DNS-Based Authentication of Named Entities 966 (DANE) Transport Layer Security (TLS)", RFC 7672, 967 DOI 10.17487/RFC7672, October 2015, 968 . 970 Appendix A. MTA-STS example record & policy 972 The owner of "example.com" wishes to begin using MTA-STS with a 973 policy that will solicit reports from senders without affecting how 974 the messages are processed, in order to verify the identity of MXs 975 that handle mail for "example.com", confirm that TLS is correctly 976 used, and ensure that certificates presented by the recipient MX 977 validate. 979 MTA-STS policy indicator TXT RR: 981 _mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;" 983 MTA-STS Policy file served as the response body at "https://mta- 984 sts.example.com/.well-known/mta-sts.txt": 986 version: STSv1 987 mode: report 988 mx: mx1.example.com 989 mx: mx2.example.com 990 mx: mx.backup-example.com 991 max_age: 12345678 993 Appendix B. Message delivery pseudocode 995 Below is pseudocode demonstrating the logic of a compliant sending 996 MTA. 998 While this pseudocode implementation suggests synchronous policy 999 retrieval in the delivery path, in a working implementation that may 1000 be undesirable, and we expect some implementers to instead prefer a 1001 background fetch that does not block delivery if no cached policy is 1002 present. 1004 func isEnforce(policy) { 1005 // Return true if the policy mode is "enforce". 1006 } 1008 func isNonExpired(policy) { 1009 // Return true if the policy is not expired. 1010 } 1012 func tryStartTls(connection) { 1013 // Attempt to open an SMTP connection with STARTTLS with the MX. 1014 } 1016 func isWildcardMatch(pat, host) { 1017 // Literal matches are true. 1018 if pat == host { 1019 return true 1020 } 1021 // Leading '.' matches a wildcard against the first part, i.e. 1022 // .example.com matches x.example.com but not x.y.example.com. 1023 if pat[0] == '.' { 1024 parts = SplitN(host, '.', 2) // Split on the first '.'. 1025 if len(parts) > 1 && parts[1] == pat[1:] { 1026 return true 1027 } 1028 } 1029 return false 1030 } 1032 func certMatches(connection, policy) { 1033 // Assume a handy function to return CN and DNS-ID SANs. 1034 for san in getDnsIdSansAndCnFromCert(connection) { 1035 for mx in policy.mx { 1036 // Return if the server certificate from "connection" matches the 1037 // "mx" host. 1038 if san[0] == '*' { 1039 // Invalid wildcard! 1040 if san[1] != '.' continue 1041 san = san[1:] 1042 } 1043 if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) { 1044 return true 1045 } 1046 } 1047 } 1048 return false 1049 } 1051 func tryDeliverMail(connection, message) { 1052 // Attempt to deliver "message" via "connection". 1053 } 1055 func tryGetNewPolicy(domain) { 1056 // Check for an MTA-STS TXT record for "domain" in DNS, and return the 1057 // indicated policy. 1058 } 1060 func cachePolicy(domain, policy) { 1061 // Store "policy" as the cached policy for "domain". 1062 } 1064 func tryGetCachedPolicy(domain) { 1065 // Return a cached policy for "domain". 1066 } 1068 func reportError(error) { 1069 // Report an error via TLSRPT. 1070 } 1072 func tryMxAccordingTo(message, mx, policy) { 1073 connection := connect(mx) 1074 if !connection { 1075 return false // Can't connect to the MX so it's not an MTA-STS 1076 // error. 1077 } 1078 secure := true 1079 if !tryStartTls(connection) { 1080 secure = false 1081 reportError(E_NO_VALID_TLS) 1082 } else if !certMatches(connection, policy) { 1083 secure = false 1084 reportError(E_CERT_MISMATCH) 1085 } 1086 if secure || !isEnforce(policy) { 1087 return tryDeliverMail(connection, message) 1088 } 1089 return false 1090 } 1092 func tryWithPolicy(message, domain, policy) { 1093 mxes := getMxForDomain(domain) 1094 for mx in mxes { 1095 if tryMxAccordingTo(message, mx, policy) { 1096 return true 1097 } 1098 } 1099 return false 1100 } 1102 func handleMessage(message) { 1103 domain := ... // domain part after '@' from recipient 1104 policy := tryGetNewPolicy(domain) 1105 if policy { 1106 cachePolicy(domain, policy) 1107 } else { 1108 policy = tryGetCachedPolicy(domain) 1109 } 1110 if policy { 1111 return tryWithPolicy(message, domain, policy) 1112 } 1113 // Try to deliver the message normally (i.e. without MTA-STS). 1114 } 1116 Authors' Addresses 1118 Daniel Margolis 1119 Google, Inc 1121 Email: dmargolis (at) google (dot com) 1123 Mark Risher 1124 Google, Inc 1126 Email: risher (at) google (dot com) 1128 Binu Ramakrishnan 1129 Yahoo!, Inc 1131 Email: rbinu (at) yahoo-inc (dot com) 1133 Alexander Brotman 1134 Comcast, Inc 1136 Email: alex_brotman (at) comcast (dot com) 1137 Janet Jones 1138 Microsoft, Inc 1140 Email: janet.jones (at) microsoft (dot com)