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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: November 4, 2018 B. Ramakrishnan 6 Yahoo!, Inc 7 A. Brotman 8 Comcast, Inc 9 J. Jones 10 Microsoft, Inc 11 May 3, 2018 13 SMTP MTA Strict Transport Security (MTA-STS) 14 draft-ietf-uta-mta-sts-17 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 http://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 November 3, 2018. 41 Copyright Notice 43 Copyright (c) 2018 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 (http://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 . . . . . . . . . . . . . . . . . . . . . . 10 67 4.1. MX Certificate Validation . . . . . . . . . . . . . . . . 10 68 5. Policy Application . . . . . . . . . . . . . . . . . . . . . 11 69 5.1. Policy Application Control Flow . . . . . . . . . . . . . 11 70 6. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 12 71 7. Interoperability Considerations . . . . . . . . . . . . . . . 12 72 7.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 12 73 7.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 13 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 . . . . . . . . . . . . . . . . . . . . . 15 79 9.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 15 80 9.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 15 81 9.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 15 82 10. Security Considerations . . . . . . . . . . . . . . . . . . . 16 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 . . . . . . . . . . . . . . . . . . . . . . . . 19 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 . . . . . . . . . . . . 22 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", "NOT RECOMMENDED", "MAY", and 122 "OPTIONAL" in this document are to be interpreted as described in 123 [BCP 14] [RFC2119] [RFC8174] when, and only when, they appear in all 124 capitals, as shown here. 126 We also define the following terms for further use in this document: 128 o MTA-STS Policy: A commitment by the Policy Domain to support PKIX 129 [RFC5280] authenticated TLS for the specified MX hosts. 131 o Policy Domain: The domain for which an MTA-STS Policy is defined. 132 This is the next-hop domain; when sending mail to 133 "alice@example.com" this would ordinarily be "example.com", but 134 this may be overridden by explicit routing rules (as described in 135 Section 3.4, "Policy Selection for Smart Hosts and Subdomains"). 137 2. Related Technologies 139 The DANE TLSA record [RFC7672] is similar, in that DANE is also 140 designed to upgrade unauthenticated encryption or plaintext 141 transmission into authenticated, downgrade-resistant encrypted 142 transmission. DANE requires DNSSEC [RFC4033] for authentication; the 143 mechanism described here instead relies on certificate authorities 144 (CAs) and does not require DNSSEC, at a cost of risking malicious 145 downgrades. For a thorough discussion of this trade-off, see 146 Section 10, "Security Considerations". 148 In addition, MTA-STS provides an optional testing-only mode, enabling 149 soft deployments to detect policy failures; partial deployments can 150 be achieved in DANE by deploying TLSA records only for some of a 151 domain's MXs, but such a mechanism is not possible for the per-domain 152 policies used by MTA-STS. 154 The primary motivation of MTA-STS is to provide a mechanism for 155 domains to ensure transport security even when deploying DNSSEC is 156 undesirable or impractical. However, MTA-STS is designed not to 157 interfere with DANE deployments when the two overlap; in particular, 158 senders who implement MTA-STS validation MUST NOT allow a "valid" or 159 "testing"-only MTA-STS validation to override a failing DANE 160 validation. 162 3. Policy Discovery 164 MTA-STS policies are distributed via HTTPS from a "well-known" 165 [RFC5785] path served within the Policy Domain, and their presence 166 and current version are indicated by a TXT record at the Policy 167 Domain. These TXT records additionally contain a policy "id" field, 168 allowing sending MTAs to check the currency of a cached policy 169 without performing an HTTPS request. 171 To discover if a recipient domain implements MTA-STS, a sender need 172 only resolve a single TXT record. To see if an updated policy is 173 available for a domain for which the sender has a previously cached 174 policy, the sender need only check the TXT record's version "id" 175 against the cached value. 177 3.1. MTA-STS TXT Records 179 The MTA-STS TXT record is a TXT record with the name "_mta-sts" at 180 the Policy Domain. For the domain "example.com", this record would 181 be "_mta-sts.example.com". MTA-STS TXT records MUST be US-ASCII, 182 semicolon-separated key/value pairs containing the following fields: 184 o "v": (plain-text, required). Currently only "STSv1" is supported. 186 o "id": (plain-text, required). A short string used to track policy 187 updates. This string MUST uniquely identify a given instance of a 188 policy, such that senders can determine when the policy has been 189 updated by comparing to the "id" of a previously seen policy. 190 There is no implied ordering of "id" fields between revisions. 192 An example TXT record is as below: 194 "_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"" 196 The formal definition of the "_mta-sts" TXT record, defined using 197 [RFC7405], is as follows: 199 sts-text-record = sts-version 1*(field-delim sts-field) [field-delim] 201 sts-field = sts-id / ; Note that sts-id record 202 sts-extension ; is required. 204 field-delim = *WSP ";" *WSP 206 sts-version = %s"v=STSv1" 208 sts-id = %s"id=" 1*32(ALPHA / DIGIT) ; id=... 210 sts-extension = sts-ext-name "=" sts-ext-value ; name=value 212 sts-ext-name = (ALPHA / DIGIT) 213 *31(ALPHA / DIGIT / "_" / "-" / ".") 215 sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) 216 ; chars excluding "=", ";", and control chars 218 The TXT record MUST begin with sts-version field, and the order of 219 other fields is not significant. If multiple TXT records for "_mta- 220 sts" are returned by the resolver, records which do not begin with 221 "v=STSv1;" are discarded. If the number of resulting records is not 222 one, senders MUST assume the recipient domain does not implement MTA- 223 STS and skip the remaining steps of policy discovery. If the 224 resulting TXT record contains multiple strings, then the record MUST 225 be treated as if those strings are concatenated together without 226 adding spaces. 228 3.2. MTA-STS Policies 230 The policy itself is a set of key/value pairs (similar to [RFC5322] 231 header fields) served via the HTTPS GET method from the fixed 232 [RFC5785] "well-known" path of ".well-known/mta-sts.txt" served by 233 the "mta-sts" host at the Policy Domain. Thus for "example.com" the 234 path is "https://mta-sts.example.com/.well-known/mta-sts.txt". 236 When fetching a policy, senders SHOULD validate that the media type 237 is "text/plain" to guard against cases where webservers allow 238 untrusted users to host non-text content (typically, HTML or images) 239 at a user-defined path. All parameters other charset=utf-8 or 240 charset=us-ascii are ignored. Additional "Content-Type" parameters 241 are also ignored. 243 This resource contains the following CRLF-separated key/value pairs: 245 o "version": (plain-text). Currently only "STSv1" is supported. 247 o "mode": (plain-text). One of "enforce", "testing", or "none", 248 indicating the expected behavior of a sending MTA in the case of a 249 policy validation failure. See Section 5, "Policy Application." 250 for more details about the three modes. 252 o "max_age": Max lifetime of the policy (plain-text non-negative 253 integer seconds, maximum value of 31557600). Well-behaved clients 254 SHOULD cache a policy for up to this value from last policy fetch 255 time. To mitigate the risks of attacks at policy refresh time, it 256 is expected that this value typically be in the range of weeks or 257 greater. 259 o "mx": MX identity patterns (list of plain-text strings). One or 260 more patterns matching a Common Name or Subject Alternative Name 261 ([RFC5280]) DNS-ID ([RFC6125]) present in the X.509 certificate 262 presented by any MX receiving mail for this domain. For example: 264 mx: mail.example.com 265 mx: .example.net 267 indicates that mail for this domain might be handled by any MX with a 268 certificate valid for a host at "mail.example.com" or "example.net". 269 Valid patterns can be either fully specified names ("example.com") or 270 suffixes (".example.net") matching the right-hand parts of a server's 271 identity; the latter case are distinguished by a leading period. If 272 there are more than one MX specified by the policy, they MUST be on 273 separate lines within the policy file. In the case of 274 Internationalized Domain Names ([RFC5891]), the MX MUST specify the 275 Punycode-encoded A-label [RFC3492] and not the Unicode-encoded 276 U-label. The full semantics of certificate validation are described 277 in Section 4.1, "MX Certificate Validation." 279 An example policy is as below: 281 version: STSv1 282 mode: enforce 283 mx: mail.example.com 284 mx: .example.net 285 mx: backupmx.example.com 286 max_age: 123456 288 The formal definition of the policy resource, defined using 289 [RFC7405], is as follows: 291 sts-policy-record = sts-policy-field *WSP 292 *(sts-policy-term sts-policy-field *WSP) 293 [sts-policy-term] 295 sts-policy-field = sts-policy-version / ; required once 296 sts-policy-mode / ; required once 297 sts-policy-max-age / ; required once 299 sts-policy-mx / 300 ; required at least once, except when 301 ; mode is "none" 303 sts-policy-extension ; other fields 305 field-delim = ":" *WSP 307 sts-policy-version = sts-policy-version-field field-delim 308 sts-policy-version-value 310 sts-policy-version-field = %s"version" 312 sts-policy-version-value = %s"STSv1" 314 sts-policy-mode = sts-policy-mode-field field-delim 315 sts-policy-mode-value 317 sts-policy-mode-field = %s"mode" 319 sts-policy-mode-value = %s"testing" / %s"enforce" / %s"none" 321 sts-policy-mx = sts-policy-mx-field field-delim 322 sts-policy-mx-value 324 sts-policy-mx-field = %s"mx" 326 sts-policy-mx-value = 1*(ALPHA / DIGIT / "_" / "-" / ".") 328 sts-policy-max-age = sts-policy-max-age-field field-delim 329 sts-policy-max-age-value 331 sts-policy-max-age-field = %s"max_age" 333 sts-policy-max-age-value = 1*10(DIGIT) 335 sts-policy-extension = sts-policy-ext-name ; additional 336 field-delim ; extension 337 sts-policy-ext-value ; fields 339 sts-policy-ext-name = (ALPHA / DIGIT) 340 *31(ALPHA / DIGIT / "_" / "-" / ".") 342 sts-policy-term = CRLF / LF 344 sts-policy-ext-value = sts-policy-vchar 345 [*(%x20 / sts-policy-vchar) 346 sts-policy-vchar] 347 ; chars, including UTF-8 [RFC3629], 348 ; excluding CTLs and no 349 ; leading/trailing spaces 351 sts-policy-vchar = %x21-7E / UTF8-2 / UTF8-3 / UTF8-4 353 Parsers MUST accept TXT records and policy files which are 354 syntactically valid (i.e. valid key/value pairs separated by semi- 355 colons for TXT records) and but containing additional key/value pairs 356 not specified in this document, in which case unknown fields SHALL be 357 ignored. If any non-repeated field--i.e. all fields excepting "mx"-- 358 is duplicated, all entries except for the first SHALL be ignored. If 359 any field is not specified, the policy SHALL be treated as invalid. 361 3.3. HTTPS Policy Fetching 363 When fetching a new policy or updating a policy, the HTTPS endpoint 364 MUST present a X.509 certificate which is valid for the "mta-sts" 365 host (e.g. "mta-sts.example.com") as described below, chain to a 366 root CA that is trusted by the sending MTA, and be non-expired. It 367 is expected that sending MTAs use a set of trusted CAs similar to 368 those in widely deployed Web browsers and operating systems. See 369 [RFC5280] for more details about certificate verification. 371 The certificate is valid for the "mta-sts" host with respect to the 372 rules described in [RFC6125], with the following application-specific 373 considerations: 375 o Matching is performed only against the DNS-ID identifiers. 377 o DNS domain names in server certificates MAY contain the wildcard 378 character '*' as the complete left-most label within the 379 identifier. 381 The certificate MAY be checked for revocation via the Online 382 Certificate Status Protocol (OCSP) [RFC6960], certificate revocation 383 lists (CRLs), or some other mechanism. 385 Policies fetched via HTTPS are only valid if the HTTP response code 386 is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP 387 caching (as specified in [RFC7234]) MUST NOT be used. 389 Senders may wish to rate-limit the frequency of attempts to fetch the 390 HTTPS endpoint even if a valid TXT record for the recipient domain 391 exists. In the case that the HTTPS GET fails, we suggest 392 implementions may limit further attempts to a period of five minutes 393 or longer per version ID, to avoid overwhelming resource-constrained 394 recipients with cascading failures. 396 Senders MAY impose a timeout on the HTTPS GET and/or a limit on the 397 maximum size of the response body to avoid long delays or resource 398 exhaustion during attempted policy updates. A suggested timeout is 399 one minute, and a suggested maximum policy size 64 kilobytes; policy 400 hosts SHOULD respond to requests with a complete policy body within 401 that timeout and size limit. 403 If a valid TXT record is found but no policy can be fetched via HTTPS 404 (for any reason), and there is no valid (non-expired) previously- 405 cached policy, senders MUST continue with delivery as though the 406 domain has not implemented MTA-STS. 408 Conversely, if no "live" policy can be discovered via DNS or fetched 409 via HTTPS, but a valid (non-expired) policy exists in the sender's 410 cache, the sender MUST apply that cached policy. 412 Finally, to mitigate the risk of persistent interference with policy 413 refresh, as discussed in-depth in Section 10, MTAs SHOULD proactively 414 refresh cached policies before they expire; a suggested refresh 415 frequency is once per day. To enable administrators to discover 416 problems with policy refresh, MTAs SHOULD alert administrators 417 (through the use of logs or similar) when such attempts fail, unless 418 the cached policy mode is "none". 420 3.4. Policy Selection for Smart Hosts and Subdomains 422 When sending mail via a "smart host"--an administratively configured 423 intermediate SMTP relay, which is different from the message 424 recipient's server as determined from DNS --compliant senders MUST 425 treat the smart host domain as the policy domain for the purposes of 426 policy discovery and application. 428 When sending mail to a mailbox at a subdomain, compliant senders MUST 429 NOT attempt to fetch a policy from the parent zone. Thus for mail 430 sent to "user@mail.example.com", the policy can be fetched only from 431 "mail.example.com", not "example.com". 433 4. Policy Validation 435 When sending to an MX at a domain for which the sender has a valid 436 and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST 437 validate: 439 1. That the recipient MX supports STARTTLS and offers a valid PKIX- 440 based TLS certificate. 442 2. That at least one of the policy's "mx" patterns matches at least 443 one of the identities presented in the MX's X.509 certificate, as 444 described in "MX Certificate Validation". 446 This section does not dictate the behavior of sending MTAs when 447 policies fail to validate; see Section 5, "Policy Application" for a 448 description of sending MTA behavior when policy validation fails. 450 4.1. MX Certificate Validation 452 The certificate presented by the receiving MX MUST chain to a root CA 453 that is trusted by the sending MTA and be non-expired. The 454 certificate MUST have a subject alternative name (SAN, [RFC5280]) 455 with a DNS-ID ([RFC6125]) matching the "mx" pattern. The MX's 456 certificate MAY also be checked for revocation via OCSP [RFC6960], 457 CRLs [RFC6818], or some other mechanism. 459 Because the "mx" patterns are not hostnames, however, matching is not 460 identical to other common cases of X.509 certificate authentication 461 (as described, for example, in [RFC6125]). Consider the example 462 policy given above, with an "mx" pattern containing ".example.com". 463 In this case, if the MX server's X.509 certificate contains a SAN 464 matching "*.example.com", we are required to implement "wildcard-to- 465 wildcard" matching. 467 To simplify this case, we impose the following constraints on 468 wildcard certificates, identical to those in [RFC7672] section 3.2.3 469 and [RFC6125] section 6.4.3: wildcards are valid in DNS-IDs, but must 470 be the entire first label of the identifier (that is, 471 "*.example.com", not "mail*.example.com"). Senders who are comparing 472 a "suffix" MX pattern with a wildcard identifier should thus strip 473 the wildcard and ensure that the two sides match label-by-label, 474 until all labels of the shorter side (if unequal length) are 475 consumed. 477 Note that a wildcard must match a label; an "mx" pattern of 478 ".example.com" thus does not match a SAN of "example.com", nor does a 479 SAN of "*.example.com" match an "mx" of "example.com". 481 A simple pseudocode implementation of this algorithm is presented in 482 Appendix B. 484 5. Policy Application 486 When sending to an MX at a domain for which the sender has a valid, 487 non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies 488 the result of a policy validation failure one of two ways, depending 489 on the value of the policy "mode" field: 491 1. "enforce": In this mode, sending MTAs MUST NOT deliver the 492 message to hosts which fail MX matching or certificate 493 validation. 495 2. "testing": In this mode, sending MTAs which also implement the 496 TLSRPT specification [I-D.ietf-uta-smtp-tlsrpt] merely send a 497 report indicating policy application failures (so long as TLSRPT 498 is also implemented by the recipient domain). 500 3. "none": In this mode, sending MTAs should treat the policy domain 501 as though it does not have any active policy; see Section 8.3, 502 "Removing MTA-STS", for use of this mode value. 504 When a message fails to deliver due to an "enforce" policy, a 505 compliant MTA MUST NOT permanently fail to deliver messages before 506 checking for the presence of an updated policy at the Policy Domain. 507 (In all cases, MTAs SHOULD treat such failures as transient errors 508 and retry delivery later.) This allows implementing domains to 509 update long-lived policies on the fly. 511 5.1. Policy Application Control Flow 513 An example control flow for a compliant sender consists of the 514 following steps: 516 1. Check for a cached policy whose time-since-fetch has not exceeded 517 its "max_age". If none exists, attempt to fetch a new policy 518 (perhaps asynchronously, so as not to block message delivery). 519 Optionally, sending MTAs may unconditionally check for a new 520 policy at this step. 522 2. For each candidate MX, in order of MX priority, attempt to 523 deliver the message, enforcing STARTTLS and, assuming a policy is 524 present, PKIX certificate validation as described in Section 4.1, 525 "MX Certificate Validation." 527 3. A message delivery MUST NOT be permanently failed until the 528 sender has first checked for the presence of a new policy (as 529 indicated by the "id" field in the "_mta-sts" TXT record). If a 530 new policy is not found, existing rules for the case of temporary 531 message delivery failures apply (as discussed in [RFC5321] 532 section 4.5.4.1). 534 6. Reporting Failures 536 MTA-STS is intended to be used along with TLSRPT 537 [I-D.ietf-uta-smtp-tlsrpt] in order to ensure implementing domains 538 can detect cases of both benign and malicious failures, and to ensure 539 that failures that indicate an active attack are discoverable. As 540 such, senders who also implement TLSRPT SHOULD treat the following 541 events as reportable failures: 543 o HTTPS policy fetch failures when a valid TXT record is present. 545 o Policy fetch failures of any kind when a valid policy exists in 546 the policy cache, except if that policy's mode is "none". 548 o Delivery attempts in which a contacted MX does not support 549 STARTTLS or does not present a certificate which validates 550 according to the applied policy, except if that policy's mode is 551 "none". 553 7. Interoperability Considerations 555 7.1. SNI Support 557 To ensure that the server sends the right certificate chain, the SMTP 558 client MUST have support for the TLS SNI extension [RFC6066]. When 559 connecting to a HTTP server to retrieve the MTA-STS policy, the SNI 560 extension MUST contain the name of the policy host (e.g. "mta- 561 sts.example.com"). When connecting to an SMTP server, the SNI 562 extension MUST contain the MX hostname. 564 HTTP servers used to deliver MTA-STS policies MAY rely on SNI to 565 determine which certificate chain to present to the client. HTTP 566 servers MUST respond with a certificate chain that matches the policy 567 hostname or abort the TLS handshake if unable to do so. Clients that 568 do not send SNI information may not see the expected certificate 569 chain. 571 SMTP servers MAY rely on SNI to determine which certificate chain to 572 present to the client. However servers that have one identity and a 573 single matching certificate do not require SNI support. Servers MUST 574 NOT enforce the use of SNI by clients, as the client may be using 575 unauthenticated opportunistic TLS and may not expect any particular 576 certificate from the server. If the client sends no SNI extension or 577 sends an SNI extension for an unsupported server name, the server 578 MUST simply send a fallback certificate chain of its choice. The 579 reason for not enforcing strict matching of the requested SNI 580 hostname is that MTA-STS TLS clients may be typically willing to 581 accept multiple server names but can only send one name in the SNI 582 extension. The server's fallback certificate may match a different 583 name that is acceptable to the client, e.g., the original next-hop 584 domain. 586 7.2. Minimum TLS Version Support 588 MTAs supporting MTA-STS MUST have support for TLS version 1.2 589 [RFC5246] or higher. The general TLS usage guidance in [RFC7525] 590 SHOULD be followed. 592 8. Operational Considerations 594 8.1. Policy Updates 596 Updating the policy requires that the owner make changes in two 597 places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and 598 at the corresponding HTTPS endpoint. As a result, recipients should 599 expect a policy will continue to be used by senders until both the 600 HTTPS and TXT endpoints are updated and the TXT record's TTL has 601 passed. 603 In other words, a sender who is unable to successfully deliver a 604 message while applying a cache of the recipient's now-outdated policy 605 may be unable to discover that a new policy exists until the DNS TTL 606 has passed. Recipients should therefore ensure that old policies 607 continue to work for message delivery during this period of time, or 608 risk message delays. 610 Recipients should also prefer to update the HTTPS policy body before 611 updating the TXT record; this ordering avoids the risk that senders, 612 seeing a new TXT record, mistakenly cache the old policy from HTTPS. 614 8.2. Policy Delegation 616 Domain owners commonly delegate SMTP hosting to a different 617 organization, such as an ISP or a Web host. In such a case, they may 618 wish to also delegate the MTA-STS policy to the same organization 619 which can be accomplished with two changes. 621 First, the Policy Domain must point the "_mta-sts" record, via CNAME, 622 to the "_mta-sts" record maintained by the hosting organization. 623 This allows the hosting organization to control update signaling. 625 Second, the Policy Domain must point the "well-known" policy location 626 to the hosting organization. This can be done either by setting the 627 "mta-sts" record to an IP address or CNAME specified by the hosting 628 organization and by giving the hosting organization a TLS certificate 629 which is valid for that host, or by setting up a "reverse proxy" 630 (also known as a "gateway") server that serves as the Policy Domain's 631 policy the policy currently served by the hosting organization. 633 For example, given a user domain "user.example" hosted by a mail 634 provider "provider.example", the following configuration would allow 635 policy delegation: 637 DNS: 639 _mta-sts.user.example. IN CNAME _mta-sts.provider.example. 641 Policy: 643 > GET /.well-known/mta-sts.txt 644 > Host: mta-sts.user.example 645 < HTTP/1.1 200 OK # Response proxies content from 646 # https://mta-sts.provider.example 648 Note that while sending MTAs MUST NOT use HTTP caching when fetching 649 policies via HTTPS, such caching may nonetheless be useful to a 650 reverse proxy configured as described in this section. An HTTPS 651 policy endpoint expecting to be proxied for multiple hosted domains-- 652 as with a large mail hosting provider or similar--may wish to 653 indicate an HTTP Cache-Control "max-age" response directive (as 654 specified in [RFC7234]) of 60 seconds as a reasonable value to save 655 reverse proxies an unnecessarily high-rate of proxied policy 656 fetching. 658 8.3. Removing MTA-STS 660 In order to facilitate clean opt-out of MTA-STS by implementing 661 policy domains, and to distinguish clearly between failures which 662 indicate attacks and those which indicate such opt-outs, MTA-STS 663 implements the "none" mode, which allows validated policies to 664 indicate authoritatively that the policy domain wishes to no longer 665 implement MTA-STS and may, in the future, remove the MTA-STS TXT and 666 policy endpoints entirely. 668 A suggested workflow to implement such an opt out is as follows: 670 1. Publish a new policy with "mode" equal to "none" and a small 671 "max_age" (e.g. one day). 673 2. Publish a new TXT record to trigger fetching of the new policy. 675 3. When all previously served policies have expired--normally this 676 is the time the previously published policy was last served plus 677 that policy's "max_age", but note that older policies may have 678 been served with a greater "max_age", allowing overlapping policy 679 caches--safely remove the TXT record and HTTPS endpoint. 681 9. IANA Considerations 683 9.1. Well-Known URIs Registry 685 A new "well-known" URI as described in Section 3 will be registered 686 in the Well-Known URIs registry as described below: 688 URI Suffix: mta-sts.txt Change Controller: IETF 690 9.2. MTA-STS TXT Record Fields 692 IANA is requested to create a new registry titled "MTA-STS TXT Record 693 Fields". The initial entries in the registry are: 695 +------------+--------------------+------------------------+ 696 | Field Name | Description | Reference | 697 +------------+--------------------+------------------------+ 698 | v | Record version | Section 3.1 of RFC XXX | 699 | id | Policy instance ID | Section 3.1 of RFC XXX | 700 +------------+--------------------+------------------------+ 702 New fields are added to this registry using IANA's "Expert Review" 703 policy. 705 9.3. MTA-STS Policy Fields 707 IANA is requested to create a new registry titled "MTA-STS Policy 708 Fields". The initial entries in the registry are: 710 +------------+----------------------+------------------------+ 711 | Field Name | Description | Reference | 712 +------------+----------------------+------------------------+ 713 | version | Policy version | Section 3.2 of RFC XXX | 714 | mode | Enforcement behavior | Section 3.2 of RFC XXX | 715 | max_age | Policy lifetime | Section 3.2 of RFC XXX | 716 | mx | MX identities | Section 3.2 of RFC XXX | 717 +------------+----------------------+------------------------+ 719 New fields are added to this registry using IANA's "Expert Review" 720 policy. 722 10. Security Considerations 724 SMTP MTA Strict Transport Security attempts to protect against an 725 active attacker who wishes to intercept or tamper with mail between 726 hosts who support STARTTLS. There are two classes of attacks 727 considered: 729 o Foiling TLS negotiation, for example by deleting the "250 730 STARTTLS" response from a server or altering TLS session 731 negotiation. This would result in the SMTP session occurring over 732 plaintext, despite both parties supporting TLS. 734 o Impersonating the destination mail server, whereby the sender 735 might deliver the message to an impostor, who could then monitor 736 and/or modify messages despite opportunistic TLS. This 737 impersonation could be accomplished by spoofing the DNS MX record 738 for the recipient domain, or by redirecting client connections 739 intended for the legitimate recipient server (for example, by 740 altering BGP routing tables). 742 MTA-STS can thwart such attacks only if the sender is able to 743 previously obtain and cache a policy for the recipient domain, and 744 only if the attacker is unable to obtain a valid certificate that 745 complies with that policy. Below, we consider specific attacks on 746 this model. 748 10.1. Obtaining a Signed Certificate 750 SMTP MTA-STS relies on certificate validation via PKIX based TLS 751 identity checking [RFC6125]. Attackers who are able to obtain a 752 valid certificate for the targeted recipient mail service (e.g. by 753 compromising a certificate authority) are thus able to circumvent STS 754 authentication. 756 10.2. Preventing Policy Discovery 758 Since MTA-STS uses DNS TXT records for policy discovery, an attacker 759 who is able to block DNS responses can suppress the discovery of an 760 MTA-STS Policy, making the Policy Domain appear not to have an MTA- 761 STS Policy. The sender policy cache is designed to resist this 762 attack by decreasing the frequency of policy discovery and thus 763 reducing the window of vulnerability; it is nonetheless a risk that 764 attackers who can predict or induce policy discovery--for example, by 765 inducing a sending domain to send mail to a never-before-contacted 766 recipient while carrying out a man-in-the-middle attack--may be able 767 to foil policy discovery and effectively downgrade the security of 768 the message delivery. 770 Since this attack depends upon intercepting initial policy discovery, 771 we strongly recommend implementers to prefer policy "max_age" values 772 to be as long as is practical. 774 Because this attack is also possible upon refresh of a cached policy, 775 we suggest implementers do not wait until a cached policy has expired 776 before checking for an update; if senders attempt to refresh the 777 cache regularly (for instance, by checking their cached version 778 string against the TXT record on each successful send, or in a 779 background task that runs daily or weekly), an attacker would have to 780 foil policy discovery consistently over the lifetime of a cached 781 policy to prevent a successful refresh. 783 Additionally, MTAs should alert administrators to repeated policy 784 refresh failures long before cached policies expire (through warning 785 logs or similar applicable mechanisms), allowing administrators to 786 detect such a persistent attack on policy refresh. (However, they 787 should not implement such alerts if the cached policy has a "none" 788 mode, to allow clean MTA-STS removal, as described in Section 8.3.) 790 Resistance to downgrade attacks of this nature--due to the ability to 791 authoritatively determine "lack of a record" even for non- 792 participating recipients--is a feature of DANE, due to its use of 793 DNSSEC for policy discovery. 795 10.3. Denial of Service 797 We additionally consider the Denial of Service risk posed by an 798 attacker who can modify the DNS records for a recipient domain. 799 Absent MTA-STS, such an attacker can cause a sending MTA to cache 800 invalid MX records, but only for however long the sending resolver 801 caches those records. With MTA-STS, the attacker can additionally 802 advertise a new, long-"max_age" MTA-STS policy with "mx" constraints 803 that validate the malicious MX record, causing senders to cache the 804 policy and refuse to deliver messages once the victim has resecured 805 the MX records. 807 This attack is mitigated in part by the ability of a victim domain to 808 (at any time) publish a new policy updating the cached, malicious 809 policy, though this does require the victim domain to both obtain a 810 valid CA-signed certificate and to understand and properly configure 811 MTA-STS. 813 Similarly, we consider the possibility of domains that deliberately 814 allow untrusted users to serve untrusted content on user-specified 815 subdomains. In some cases (e.g. the service Tumblr.com) this takes 816 the form of providing HTTPS hosting of user-registered subdomains; in 817 other cases (e.g. dynamic DNS providers) this takes the form of 818 allowing untrusted users to register custom DNS records at the 819 provider's domain. 821 In these cases, there is a risk that untrusted users would be able to 822 serve custom content at the "mta-sts" host, including serving an 823 illegitimate MTA-STS policy. We believe this attack is rendered more 824 difficult by the need for the attacker to also serve the "_mta-sts" 825 TXT record on the same domain--something not, to our knowledge, 826 widely provided to untrusted users. This attack is additionally 827 mitigated by the aforementioned ability for a victim domain to update 828 an invalid policy at any future date. 830 10.4. Weak Policy Constraints 832 Even if an attacker cannot modify a served policy, the potential 833 exists for configurations that allow attackers on the same domain to 834 receive mail for that domain. For example, an easy configuration 835 option when authoring an MTA-STS Policy for "example.com" is to set 836 the "mx" equal to ".example.com"; recipient domains must consider in 837 this case the risk that any user possessing a valid hostname and CA- 838 signed certificate (for example, "dhcp-123.example.com") will, from 839 the perspective of MTA-STS Policy validation, be a valid MX host for 840 that domain. 842 10.5. Compromise of the Web PKI System 844 A host of risks apply to the PKI system used for certificate 845 authentication, both of the "mta-sts" HTTPS host's certificate and 846 the SMTP servers' certificates. These risks are broadly applicable 847 within the Web PKI ecosystem and are not specific to MTA-STS; 848 nonetheless, they deserve some consideration in this context. 850 Broadly speaking, attackers may compromise the system by obtaining 851 certificates under fraudulent circumstances (i.e. by impersonating 852 the legitimate owner of the victim domain), by compromising a 853 Certificate Authority or Delegate Authority's private keys, by 854 obtaining a legitimate certificate issued to the victim domain, and 855 similar. 857 One approach commonly employed by Web browsers to help mitigate 858 against some of these attacks is to allow for revocation of 859 compromised or fraudulent certificates via OCSP [RFC6960] or CRLs 860 [RFC6818]. Such mechanisms themselves represent tradeoffs and are 861 not universally implemented; we nonetheless recommend implementors of 862 MTA-STS to implement revocation mechanisms which are most applicable 863 to their implementations. 865 11. Contributors 867 Wei Chuang Google, Inc weihaw (at) google (dot com) 869 Viktor Dukhovni ietf-dane (at) dukhovni (dot org) 871 Markus Laber 1&1 Mail & Media Development & Technology GmbH 872 markus.laber (at) 1und1 (dot de) 874 Nicolas Lidzborski Google, Inc nlidz (at) google (dot com) 876 Brandon Long Google, Inc blong (at) google (dot com) 878 Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com) 880 Klaus Umbach 1&1 Mail & Media Development & Technology GmbH 881 klaus.umbach (at) 1und1 (dot de) 883 12. References 885 12.1. Normative References 887 [I-D.ietf-uta-smtp-tlsrpt] 888 Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J., 889 and M. Risher, "SMTP TLS Reporting", draft-ietf-uta-smtp- 890 tlsrpt-18 (work in progress), April 2018. 892 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 893 Requirement Levels", BCP 14, RFC 2119, 894 DOI 10.17487/RFC2119, March 1997, . 897 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 898 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 899 February 2002, . 901 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 902 for Internationalized Domain Names in Applications 903 (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, 904 . 906 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 907 (TLS) Protocol Version 1.2", RFC 5246, 908 DOI 10.17487/RFC5246, August 2008, . 911 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 912 Housley, R., and W. Polk, "Internet X.509 Public Key 913 Infrastructure Certificate and Certificate Revocation List 914 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 915 . 917 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 918 DOI 10.17487/RFC5321, October 2008, . 921 [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known 922 Uniform Resource Identifiers (URIs)", RFC 5785, 923 DOI 10.17487/RFC5785, April 2010, . 926 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 927 Extensions: Extension Definitions", RFC 6066, 928 DOI 10.17487/RFC6066, January 2011, . 931 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 932 Verification of Domain-Based Application Service Identity 933 within Internet Public Key Infrastructure Using X.509 934 (PKIX) Certificates in the Context of Transport Layer 935 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 936 2011, . 938 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 939 RFC 7405, DOI 10.17487/RFC7405, December 2014, 940 . 942 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 943 "Recommendations for Secure Use of Transport Layer 944 Security (TLS) and Datagram Transport Layer Security 945 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 946 2015, . 948 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 949 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 950 May 2017, . 952 12.2. Informative References 954 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 955 Rose, "DNS Security Introduction and Requirements", 956 RFC 4033, DOI 10.17487/RFC4033, March 2005, 957 . 959 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 960 DOI 10.17487/RFC5322, October 2008, . 963 [RFC5891] Klensin, J., "Internationalized Domain Names in 964 Applications (IDNA): Protocol", RFC 5891, 965 DOI 10.17487/RFC5891, August 2010, . 968 [RFC6818] Yee, P., "Updates to the Internet X.509 Public Key 969 Infrastructure Certificate and Certificate Revocation List 970 (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January 971 2013, . 973 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 974 Galperin, S., and C. Adams, "X.509 Internet Public Key 975 Infrastructure Online Certificate Status Protocol - OCSP", 976 RFC 6960, DOI 10.17487/RFC6960, June 2013, 977 . 979 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 980 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 981 RFC 7234, DOI 10.17487/RFC7234, June 2014, 982 . 984 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 985 Opportunistic DNS-Based Authentication of Named Entities 986 (DANE) Transport Layer Security (TLS)", RFC 7672, 987 DOI 10.17487/RFC7672, October 2015, . 990 Appendix A. MTA-STS example record & policy 992 The owner of "example.com" wishes to begin using MTA-STS with a 993 policy that will solicit reports from senders without affecting how 994 the messages are processed, in order to verify the identity of MXs 995 that handle mail for "example.com", confirm that TLS is correctly 996 used, and ensure that certificates presented by the recipient MX 997 validate. 999 MTA-STS policy indicator TXT RR: 1001 _mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;" 1003 MTA-STS Policy file served as the response body at "https://mta- 1004 sts.example.com/.well-known/mta-sts.txt": 1006 version: STSv1 1007 mode: testing 1008 mx: mx1.example.com 1009 mx: mx2.example.com 1010 mx: mx.backup-example.com 1011 max_age: 12345678 1013 Appendix B. Message delivery pseudocode 1015 Below is pseudocode demonstrating the logic of a compliant sending 1016 MTA. 1018 While this pseudocode implementation suggests synchronous policy 1019 retrieval in the delivery path, in a working implementation that may 1020 be undesirable, and we expect some implementers to instead prefer a 1021 background fetch that does not block delivery if no cached policy is 1022 present. 1024 func isEnforce(policy) { 1025 // Return true if the policy mode is "enforce". 1026 } 1028 func isNonExpired(policy) { 1029 // Return true if the policy is not expired. 1030 } 1032 func tryStartTls(connection) { 1033 // Attempt to open an SMTP connection with STARTTLS with the MX. 1034 } 1036 func isWildcardMatch(pat, host) { 1037 // Literal matches are true. 1038 if pat == host { 1039 return true 1040 } 1041 // Leading '.' matches a wildcard against the first part, i.e. 1042 // .example.com matches x.example.com but not x.y.example.com. 1043 if pat[0] == '.' { 1044 parts = SplitN(host, '.', 2) // Split on the first '.'. 1045 if len(parts) > 1 && parts[1] == pat[1:] { 1046 return true 1047 } 1048 } 1049 return false 1050 } 1052 func certMatches(connection, policy) { 1053 // Assume a handy function to return DNS-ID SANs. 1054 for san in getDnsIdSansFromCert(connection) { 1055 for mx in policy.mx { 1056 // Return if the server certificate from "connection" matches the 1057 // "mx" host. 1058 if san[0] == '*' { 1059 // Invalid wildcard! 1060 if san[1] != '.' continue 1061 san = san[1:] 1062 } 1063 if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) { 1064 return true 1065 } 1066 } 1067 } 1068 return false 1069 } 1071 func tryDeliverMail(connection, message) { 1072 // Attempt to deliver "message" via "connection". 1073 } 1075 func tryGetNewPolicy(domain) { 1076 // Check for an MTA-STS TXT record for "domain" in DNS, and return the 1077 // indicated policy. 1078 } 1080 func cachePolicy(domain, policy) { 1081 // Store "policy" as the cached policy for "domain". 1082 } 1084 func tryGetCachedPolicy(domain) { 1085 // Return a cached policy for "domain". 1086 } 1088 func reportError(error) { 1089 // Report an error via TLSRPT. 1090 } 1092 func tryMxAccordingTo(message, mx, policy) { 1093 connection := connect(mx) 1094 if !connection { 1095 return false // Can't connect to the MX so it's not an MTA-STS 1096 // error. 1097 } 1098 secure := true 1099 if !tryStartTls(connection) { 1100 secure = false 1101 reportError(E_NO_VALID_TLS) 1102 } else if !certMatches(connection, policy) { 1103 secure = false 1104 reportError(E_CERT_MISMATCH) 1105 } 1106 if secure || !isEnforce(policy) { 1107 return tryDeliverMail(connection, message) 1108 } 1109 return false 1110 } 1112 func tryWithPolicy(message, domain, policy) { 1113 mxes := getMxForDomain(domain) 1114 for mx in mxes { 1115 if tryMxAccordingTo(message, mx, policy) { 1116 return true 1117 } 1118 } 1119 return false 1120 } 1122 func handleMessage(message) { 1123 domain := ... // domain part after '@' from recipient 1124 policy := tryGetNewPolicy(domain) 1125 if policy { 1126 cachePolicy(domain, policy) 1127 } else { 1128 policy = tryGetCachedPolicy(domain) 1129 } 1130 if policy { 1131 return tryWithPolicy(message, domain, policy) 1132 } 1133 // Try to deliver the message normally (i.e. without MTA-STS). 1134 } 1136 Authors' Addresses 1138 Daniel Margolis 1139 Google, Inc 1141 Email: dmargolis (at) google (dot com) 1143 Mark Risher 1144 Google, Inc 1146 Email: risher (at) google (dot com) 1147 Binu Ramakrishnan 1148 Yahoo!, Inc 1150 Email: rbinu (at) yahoo-inc (dot com) 1152 Alexander Brotman 1153 Comcast, Inc 1155 Email: alex_brotman@comcast.com 1157 Janet Jones 1158 Microsoft, Inc 1160 Email: janet.jones (at) microsoft (dot com)