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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: October 6, 2018 B. Ramakrishnan 6 Yahoo!, Inc 7 A. Brotman 8 Comcast, Inc 9 J. Jones 10 Microsoft, Inc 11 April 4, 2018 13 SMTP MTA Strict Transport Security (MTA-STS) 14 draft-ietf-uta-mta-sts-15 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 October 6, 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 . . . . . . . . . . . . . . . . . . . . . . 9 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", "MAY", and "OPTIONAL" in this 122 document are to be interpreted as described in [RFC2119]. These 123 words may also appear in this document in lowercase, absent their 124 normative meanings. 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 *(CRLF sts-policy-field *WSP) 293 [CRLF] 295 sts-policy-field = sts-policy-version / ; required once 296 sts-policy-mode / ; required once 297 sts-policy-max-age / ; required once 299 0*(sts-policy-mx *WSP CRLF) / 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-model-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-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) 343 ; chars, excluding "=", ";", SP, and 344 ; control chars 346 Parsers MUST accept TXT records and policy files which are 347 syntactically valid (i.e. valid key/value pairs separated by semi- 348 colons for TXT records) and but containing additional key/value pairs 349 not specified in this document, in which case unknown fields SHALL be 350 ignored. If any non-repeated field--i.e. all fields excepting "mx"-- 351 is duplicated, all entries except for the first SHALL be ignored. If 352 any field is not specified, the policy SHALL be treated as invalid. 354 3.3. HTTPS Policy Fetching 356 When fetching a new policy or updating a policy, the HTTPS endpoint 357 MUST present a X.509 certificate which is valid for the "mta-sts" 358 host (e.g. "mta-sts.example.com") as described below, chain to a 359 root CA that is trusted by the sending MTA, and be non-expired. It 360 is expected that sending MTAs use a set of trusted CAs similar to 361 those in widely deployed Web browsers and operating systems. See 362 [RFC5280] for more details about certificate verification. 364 The certificate is valid for the "mta-sts" host with respect to the 365 rules described in [RFC6125], with the following application-specific 366 considerations: 368 o Matching is performed only against the DNS-ID identifiers. 370 o DNS domain names in server certificates MAY contain the wildcard 371 character '*' as the complete left-most label within the 372 identifier. 374 The certificate MAY be checked for revocation via the Online 375 Certificate Status Protocol (OCSP) [RFC6960], certificate revocation 376 lists (CRLs), or some other mechanism. 378 Policies fetched via HTTPS are only valid if the HTTP response code 379 is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP 380 caching (as specified in [RFC7234]) MUST NOT be used. 382 Senders may wish to rate-limit the frequency of attempts to fetch the 383 HTTPS endpoint even if a valid TXT record for the recipient domain 384 exists. In the case that the HTTPS GET fails, we suggest 385 implementions may limit further attempts to a period of five minutes 386 or longer per version ID, to avoid overwhelming resource-constrained 387 recipients with cascading failures. 389 Senders MAY impose a timeout on the HTTPS GET and/or a limit on the 390 maximum size of the response body to avoid long delays or resource 391 exhaustion during attempted policy updates. A suggested timeout is 392 one minute, and a suggested maximum policy size 64 kilobytes; policy 393 hosts SHOULD respond to requests with a complete policy body within 394 that timeout and size limit. 396 If a valid TXT record is found but no policy can be fetched via HTTPS 397 (for any reason), and there is no valid (non-expired) previously- 398 cached policy, senders MUST continue with delivery as though the 399 domain has not implemented MTA-STS. 401 Conversely, if no "live" policy can be discovered via DNS or fetched 402 via HTTPS, but a valid (non-expired) policy exists in the sender's 403 cache, the sender MUST apply that cached policy. 405 Finally, to mitigate the risk of persistent interference with policy 406 refresh, as discussed in-depth in Section 10, MTAs SHOULD proactively 407 refresh cached policies before they expire; a suggested refresh 408 frequency is once per day. To enable administrators to discover 409 problems with policy refresh, MTAs SHOULD alert administrators 410 (through the use of logs or similar) when such attempts fail, unless 411 the cached policy mode is "none". 413 3.4. Policy Selection for Smart Hosts and Subdomains 415 When sending mail via a "smart host"--an administratively configured 416 intermediate SMTP relay, which is different from the message 417 recipient's server as determined from DNS --compliant senders MUST 418 treat the smart host domain as the policy domain for the purposes of 419 policy discovery and application. 421 When sending mail to a mailbox at a subdomain, compliant senders MUST 422 NOT attempt to fetch a policy from the parent zone. Thus for mail 423 sent to "user@mail.example.com", the policy can be fetched only from 424 "mail.example.com", not "example.com". 426 4. Policy Validation 428 When sending to an MX at a domain for which the sender has a valid 429 and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST 430 validate: 432 1. That the recipient MX supports STARTTLS and offers a valid PKIX- 433 based TLS certificate. 435 2. That at least one of the policy's "mx" patterns matches at least 436 one of the identities presented in the MX's X.509 certificate, as 437 described in "MX Certificate Validation". 439 This section does not dictate the behavior of sending MTAs when 440 policies fail to validate; see Section 5, "Policy Application" for a 441 description of sending MTA behavior when policy validation fails. 443 4.1. MX Certificate Validation 445 The certificate presented by the receiving MX MUST chain to a root CA 446 that is trusted by the sending MTA and be non-expired. The 447 certificate MUST have a subject alternative name (SAN, [RFC5280]) 448 with a DNS-ID ([RFC6125]) matching the "mx" pattern. The MX's 449 certificate MAY also be checked for revocation via OCSP [RFC6960], 450 CRLs [RFC6818], or some other mechanism. 452 Because the "mx" patterns are not hostnames, however, matching is not 453 identical to other common cases of X.509 certificate authentication 454 (as described, for example, in [RFC6125]). Consider the example 455 policy given above, with an "mx" pattern containing ".example.com". 456 In this case, if the MX server's X.509 certificate contains a SAN 457 matching "*.example.com", we are required to implement "wildcard-to- 458 wildcard" matching. 460 To simplify this case, we impose the following constraints on 461 wildcard certificates, identical to those in [RFC7672] section 3.2.3 462 and [RFC6125] section 6.4.3: wildcards are valid in DNS-IDs, but must 463 be the entire first label of the identifier (that is, 464 "*.example.com", not "mail*.example.com"). Senders who are comparing 465 a "suffix" MX pattern with a wildcard identifier should thus strip 466 the wildcard and ensure that the two sides match label-by-label, 467 until all labels of the shorter side (if unequal length) are 468 consumed. 470 Note that a wildcard must match a label; an "mx" pattern of 471 ".example.com" thus does not match a SAN of "example.com", nor does a 472 SAN of "*.example.com" match an "mx" of "example.com". 474 A simple pseudocode implementation of this algorithm is presented in 475 Appendix B. 477 5. Policy Application 479 When sending to an MX at a domain for which the sender has a valid, 480 non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies 481 the result of a policy validation failure one of two ways, depending 482 on the value of the policy "mode" field: 484 1. "enforce": In this mode, sending MTAs MUST NOT deliver the 485 message to hosts which fail MX matching or certificate 486 validation. 488 2. "testing": In this mode, sending MTAs which also implement the 489 TLSRPT specification [I-D.ietf-uta-smtp-tlsrpt] merely send a 490 report indicating policy application failures (so long as TLSRPT 491 is also implemented by the recipient domain). 493 3. "none": In this mode, sending MTAs should treat the policy domain 494 as though it does not have any active policy; see Section 8.3, 495 "Removing MTA-STS", for use of this mode value. 497 When a message fails to deliver due to an "enforce" policy, a 498 compliant MTA MUST NOT permanently fail to deliver messages before 499 checking for the presence of an updated policy at the Policy Domain. 500 (In all cases, MTAs SHOULD treat such failures as transient errors 501 and retry delivery later.) This allows implementing domains to 502 update long-lived policies on the fly. 504 5.1. Policy Application Control Flow 506 An example control flow for a compliant sender consists of the 507 following steps: 509 1. Check for a cached policy whose time-since-fetch has not exceeded 510 its "max_age". If none exists, attempt to fetch a new policy 511 (perhaps asynchronously, so as not to block message delivery). 512 Optionally, sending MTAs may unconditionally check for a new 513 policy at this step. 515 2. For each candidate MX, in order of MX priority, attempt to 516 deliver the message, enforcing STARTTLS and, assuming a policy is 517 present, PKIX certificate validation as described in Section 4.1, 518 "MX Certificate Validation." 520 3. A message delivery MUST NOT be permanently failed until the 521 sender has first checked for the presence of a new policy (as 522 indicated by the "id" field in the "_mta-sts" TXT record). If a 523 new policy is not found, existing rules for the case of temporary 524 message delivery failures apply (as discussed in [RFC5321] 525 section 4.5.4.1). 527 6. Reporting Failures 529 MTA-STS is intended to be used along with TLSRPT 530 [I-D.ietf-uta-smtp-tlsrpt] in order to ensure implementing domains 531 can detect cases of both benign and malicious failures, and to ensure 532 that failures that indicate an active attack are discoverable. As 533 such, senders who also implement TLSRPT SHOULD treat the following 534 events as reportable failures: 536 o HTTPS policy fetch failures when a valid TXT record is present. 538 o Policy fetch failures of any kind when a valid policy exists in 539 the policy cache, except if that policy's mode is "none". 541 o Delivery attempts in which a contacted MX does not support 542 STARTTLS or does not present a certificate which validates 543 according to the applied policy, except if that policy's mode is 544 "none". 546 7. Interoperability Considerations 548 7.1. SNI Support 550 To ensure that the server sends the right certificate chain, the SMTP 551 client MUST have support for the TLS SNI extension [RFC6066]. When 552 connecting to a HTTP server to retrieve the MTA-STS policy, the SNI 553 extension MUST contain the name of the policy host (e.g. "mta- 554 sts.example.com"). When connecting to an SMTP server, the SNI 555 extension MUST contain the MX hostname. 557 HTTP servers used to deliver MTA-STS policies MAY rely on SNI to 558 determine which certificate chain to present to the client. HTTP 559 servers MUST respond with a certificate chain that matches the policy 560 hostname or abort the TLS handshake if unable to do so. Clients that 561 do not send SNI information may not see the expected certificate 562 chain. 564 SMTP servers MAY rely on SNI to determine which certificate chain to 565 present to the client. However servers that have one identity and a 566 single matching certificate do not require SNI support. Servers MUST 567 NOT enforce the use of SNI by clients, as the client may be using 568 unauthenticated opportunistic TLS and may not expect any particular 569 certificate from the server. If the client sends no SNI extension or 570 sends an SNI extension for an unsupported server name, the server 571 MUST simply send a fallback certificate chain of its choice. The 572 reason for not enforcing strict matching of the requested SNI 573 hostname is that MTA-STS TLS clients may be typically willing to 574 accept multiple server names but can only send one name in the SNI 575 extension. The server's fallback certificate may match a different 576 name that is acceptable to the client, e.g., the original next-hop 577 domain. 579 7.2. Minimum TLS Version Support 581 MTAs supporting MTA-STS MUST have support for TLS version 1.2 582 [RFC5246] or higher. The general TLS usage guidance in [RFC7525] 583 SHOULD be followed. 585 8. Operational Considerations 587 8.1. Policy Updates 589 Updating the policy requires that the owner make changes in two 590 places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and 591 at the corresponding HTTPS endpoint. As a result, recipients should 592 expect a policy will continue to be used by senders until both the 593 HTTPS and TXT endpoints are updated and the TXT record's TTL has 594 passed. 596 In other words, a sender who is unable to successfully deliver a 597 message while applying a cache of the recipient's now-outdated policy 598 may be unable to discover that a new policy exists until the DNS TTL 599 has passed. Recipients should therefore ensure that old policies 600 continue to work for message delivery during this period of time, or 601 risk message delays. 603 Recipients should also prefer to update the HTTPS policy body before 604 updating the TXT record; this ordering avoids the risk that senders, 605 seeing a new TXT record, mistakenly cache the old policy from HTTPS. 607 8.2. Policy Delegation 609 Domain owners commonly delegate SMTP hosting to a different 610 organization, such as an ISP or a Web host. In such a case, they may 611 wish to also delegate the MTA-STS policy to the same organization 612 which can be accomplished with two changes. 614 First, the Policy Domain must point the "_mta-sts" record, via CNAME, 615 to the "_mta-sts" record maintained by the hosting organization. 616 This allows the hosting organization to control update signaling. 618 Second, the Policy Domain must point the "well-known" policy location 619 to the hosting organization. This can be done either by setting the 620 "mta-sts" record to an IP address or CNAME specified by the hosting 621 organization and by giving the hosting organization a TLS certificate 622 which is valid for that host, or by setting up a "reverse proxy" 623 (also known as a "gateway") server that serves as the Policy Domain's 624 policy the policy currently served by the hosting organization. 626 For example, given a user domain "user.example" hosted by a mail 627 provider "provider.example", the following configuration would allow 628 policy delegation: 630 DNS: 632 _mta-sts.user.example. IN CNAME _mta-sts.provider.example. 634 Policy: 636 > GET /.well-known/mta-sts.txt 637 > Host: mta-sts.user.example 638 < HTTP/1.1 200 OK # Response proxies content from 639 # https://mta-sts.provider.example 641 Note that while sending MTAs MUST NOT use HTTP caching when fetching 642 policies via HTTPS, such caching may nonetheless be useful to a 643 reverse proxy configured as described in this section. An HTTPS 644 policy endpoint expecting to be proxied for multiple hosted domains-- 645 as with a large mail hosting provider or similar--may wish to 646 indicate an HTTP Cache-Control "max-age" response directive (as 647 specified in [RFC7234]) of 60 seconds as a reasonable value to save 648 reverse proxies an unnecessarily high-rate of proxied policy 649 fetching. 651 8.3. Removing MTA-STS 653 In order to facilitate clean opt-out of MTA-STS by implementing 654 policy domains, and to distinguish clearly between failures which 655 indicate attacks and those which indicate such opt-outs, MTA-STS 656 implements the "none" mode, which allows validated policies to 657 indicate authoritatively that the policy domain wishes to no longer 658 implement MTA-STS and may, in the future, remove the MTA-STS TXT and 659 policy endpoints entirely. 661 A suggested workflow to implement such an opt out is as follows: 663 1. Publish a new policy with "mode" equal to "none" and a small 664 "max_age" (e.g. one day). 666 2. Publish a new TXT record to trigger fetching of the new policy. 668 3. When all previously served policies have expired--normally this 669 is the time the previously published policy was last served plus 670 that policy's "max_age", but note that older policies may have 671 been served with a greater "max_age", allowing overlapping policy 672 caches--safely remove the TXT record and HTTPS endpoint. 674 9. IANA Considerations 676 9.1. Well-Known URIs Registry 678 A new "well-known" URI as described in Section 3 will be registered 679 in the Well-Known URIs registry as described below: 681 URI Suffix: mta-sts.txt Change Controller: IETF 683 9.2. MTA-STS TXT Record Fields 685 IANA is requested to create a new registry titled "MTA-STS TXT Record 686 Fields". The initial entries in the registry are: 688 +------------+--------------------+------------------------+ 689 | Field Name | Description | Reference | 690 +------------+--------------------+------------------------+ 691 | v | Record version | Section 3.1 of RFC XXX | 692 | id | Policy instance ID | Section 3.1 of RFC XXX | 693 +------------+--------------------+------------------------+ 695 New fields are added to this registry using IANA's "Expert Review" 696 policy. 698 9.3. MTA-STS Policy Fields 700 IANA is requested to create a new registry titled "MTA-STS Policy 701 Fields". The initial entries in the registry are: 703 +------------+----------------------+------------------------+ 704 | Field Name | Description | Reference | 705 +------------+----------------------+------------------------+ 706 | version | Policy version | Section 3.2 of RFC XXX | 707 | mode | Enforcement behavior | Section 3.2 of RFC XXX | 708 | max_age | Policy lifetime | Section 3.2 of RFC XXX | 709 | mx | MX identities | Section 3.2 of RFC XXX | 710 +------------+----------------------+------------------------+ 712 New fields are added to this registry using IANA's "Expert Review" 713 policy. 715 10. Security Considerations 717 SMTP MTA Strict Transport Security attempts to protect against an 718 active attacker who wishes to intercept or tamper with mail between 719 hosts who support STARTTLS. There are two classes of attacks 720 considered: 722 o Foiling TLS negotiation, for example by deleting the "250 723 STARTTLS" response from a server or altering TLS session 724 negotiation. This would result in the SMTP session occurring over 725 plaintext, despite both parties supporting TLS. 727 o Impersonating the destination mail server, whereby the sender 728 might deliver the message to an impostor, who could then monitor 729 and/or modify messages despite opportunistic TLS. This 730 impersonation could be accomplished by spoofing the DNS MX record 731 for the recipient domain, or by redirecting client connections 732 intended for the legitimate recipient server (for example, by 733 altering BGP routing tables). 735 MTA-STS can thwart such attacks only if the sender is able to 736 previously obtain and cache a policy for the recipient domain, and 737 only if the attacker is unable to obtain a valid certificate that 738 complies with that policy. Below, we consider specific attacks on 739 this model. 741 10.1. Obtaining a Signed Certificate 743 SMTP MTA-STS relies on certificate validation via PKIX based TLS 744 identity checking [RFC6125]. Attackers who are able to obtain a 745 valid certificate for the targeted recipient mail service (e.g. by 746 compromising a certificate authority) are thus able to circumvent STS 747 authentication. 749 10.2. Preventing Policy Discovery 751 Since MTA-STS uses DNS TXT records for policy discovery, an attacker 752 who is able to block DNS responses can suppress the discovery of an 753 MTA-STS Policy, making the Policy Domain appear not to have an MTA- 754 STS Policy. The sender policy cache is designed to resist this 755 attack by decreasing the frequency of policy discovery and thus 756 reducing the window of vulnerability; it is nonetheless a risk that 757 attackers who can predict or induce policy discovery--for example, by 758 inducing a sending domain to send mail to a never-before-contacted 759 recipient while carrying out a man-in-the-middle attack--may be able 760 to foil policy discovery and effectively downgrade the security of 761 the message delivery. 763 Since this attack depends upon intercepting initial policy discovery, 764 we strongly recommend implementers to prefer policy "max_age" values 765 to be as long as is practical. 767 Because this attack is also possible upon refresh of a cached policy, 768 we suggest implementers do not wait until a cached policy has expired 769 before checking for an update; if senders attempt to refresh the 770 cache regularly (for instance, by checking their cached version 771 string against the TXT record on each successful send, or in a 772 background task that runs daily or weekly), an attacker would have to 773 foil policy discovery consistently over the lifetime of a cached 774 policy to prevent a successful refresh. 776 Additionally, MTAs should alert administrators to repeated policy 777 refresh failures long before cached policies expire (through warning 778 logs or similar applicable mechanisms), allowing administrators to 779 detect such a persistent attack on policy refresh. (However, they 780 should not implement such alerts if the cached policy has a "none" 781 mode, to allow clean MTA-STS removal, as described in Section 8.3.) 783 Resistance to downgrade attacks of this nature--due to the ability to 784 authoritatively determine "lack of a record" even for non- 785 participating recipients--is a feature of DANE, due to its use of 786 DNSSEC for policy discovery. 788 10.3. Denial of Service 790 We additionally consider the Denial of Service risk posed by an 791 attacker who can modify the DNS records for a recipient domain. 792 Absent MTA-STS, such an attacker can cause a sending MTA to cache 793 invalid MX records, but only for however long the sending resolver 794 caches those records. With MTA-STS, the attacker can additionally 795 advertise a new, long-"max_age" MTA-STS policy with "mx" constraints 796 that validate the malicious MX record, causing senders to cache the 797 policy and refuse to deliver messages once the victim has resecured 798 the MX records. 800 This attack is mitigated in part by the ability of a victim domain to 801 (at any time) publish a new policy updating the cached, malicious 802 policy, though this does require the victim domain to both obtain a 803 valid CA-signed certificate and to understand and properly configure 804 MTA-STS. 806 Similarly, we consider the possibility of domains that deliberately 807 allow untrusted users to serve untrusted content on user-specified 808 subdomains. In some cases (e.g. the service Tumblr.com) this takes 809 the form of providing HTTPS hosting of user-registered subdomains; in 810 other cases (e.g. dynamic DNS providers) this takes the form of 811 allowing untrusted users to register custom DNS records at the 812 provider's domain. 814 In these cases, there is a risk that untrusted users would be able to 815 serve custom content at the "mta-sts" host, including serving an 816 illegitimate MTA-STS policy. We believe this attack is rendered more 817 difficult by the need for the attacker to also serve the "_mta-sts" 818 TXT record on the same domain--something not, to our knowledge, 819 widely provided to untrusted users. This attack is additionally 820 mitigated by the aforementioned ability for a victim domain to update 821 an invalid policy at any future date. 823 10.4. Weak Policy Constraints 825 Even if an attacker cannot modify a served policy, the potential 826 exists for configurations that allow attackers on the same domain to 827 receive mail for that domain. For example, an easy configuration 828 option when authoring an MTA-STS Policy for "example.com" is to set 829 the "mx" equal to ".example.com"; recipient domains must consider in 830 this case the risk that any user possessing a valid hostname and CA- 831 signed certificate (for example, "dhcp-123.example.com") will, from 832 the perspective of MTA-STS Policy validation, be a valid MX host for 833 that domain. 835 10.5. Compromise of the Web PKI System 837 A host of risks apply to the PKI system used for certificate 838 authentication, both of the "mta-sts" HTTPS host's certificate and 839 the SMTP servers' certificates. These risks are broadly applicable 840 within the Web PKI ecosystem and are not specific to MTA-STS; 841 nonetheless, they deserve some consideration in this context. 843 Broadly speaking, attackers may compromise the system by obtaining 844 certificates under fraudulent circumstances (i.e. by impersonating 845 the legitimate owner of the victim domain), by compromising a 846 Certificate Authority or Delegate Authority's private keys, by 847 obtaining a legitimate certificate issued to the victim domain, and 848 similar. 850 One approach commonly employed by Web browsers to help mitigate 851 against some of these attacks is to allow for revocation of 852 compromised or fraudulent certificates via OCSP [RFC6960] or CRLs 853 [RFC6818]. Such mechanisms themselves represent tradeoffs and are 854 not universally implemented; we nonetheless recommend implementors of 855 MTA-STS to implement revocation mechanisms which are most applicable 856 to their implementations. 858 11. Contributors 860 Nicolas Lidzborski Google, Inc nlidz (at) google (dot com) 862 Wei Chuang Google, Inc weihaw (at) google (dot com) 864 Brandon Long Google, Inc blong (at) google (dot com) 866 Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com) 868 Klaus Umbach 1&1 Mail & Media Development & Technology GmbH 869 klaus.umbach (at) 1und1 (dot de) 871 Markus Laber 1&1 Mail & Media Development & Technology GmbH 872 markus.laber (at) 1und1 (dot de) 874 12. References 876 12.1. Normative References 878 [I-D.ietf-uta-smtp-tlsrpt] 879 Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J., 880 and M. Risher, "SMTP TLS Reporting", draft-ietf-uta-smtp- 881 tlsrpt-17 (work in progress), March 2018. 883 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 884 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 885 February 2002, . 887 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 888 for Internationalized Domain Names in Applications 889 (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, 890 . 892 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 893 (TLS) Protocol Version 1.2", RFC 5246, 894 DOI 10.17487/RFC5246, August 2008, . 897 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 898 Housley, R., and W. Polk, "Internet X.509 Public Key 899 Infrastructure Certificate and Certificate Revocation List 900 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 901 . 903 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 904 DOI 10.17487/RFC5321, October 2008, . 907 [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known 908 Uniform Resource Identifiers (URIs)", RFC 5785, 909 DOI 10.17487/RFC5785, April 2010, . 912 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 913 Extensions: Extension Definitions", RFC 6066, 914 DOI 10.17487/RFC6066, January 2011, . 917 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 918 Verification of Domain-Based Application Service Identity 919 within Internet Public Key Infrastructure Using X.509 920 (PKIX) Certificates in the Context of Transport Layer 921 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 922 2011, . 924 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 925 RFC 7405, DOI 10.17487/RFC7405, December 2014, 926 . 928 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 929 "Recommendations for Secure Use of Transport Layer 930 Security (TLS) and Datagram Transport Layer Security 931 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 932 2015, . 934 12.2. Informative References 936 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 937 Requirement Levels", BCP 14, RFC 2119, 938 DOI 10.17487/RFC2119, March 1997, . 941 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 942 Rose, "DNS Security Introduction and Requirements", 943 RFC 4033, DOI 10.17487/RFC4033, March 2005, 944 . 946 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 947 DOI 10.17487/RFC5322, October 2008, . 950 [RFC5891] Klensin, J., "Internationalized Domain Names in 951 Applications (IDNA): Protocol", RFC 5891, 952 DOI 10.17487/RFC5891, August 2010, . 955 [RFC6818] Yee, P., "Updates to the Internet X.509 Public Key 956 Infrastructure Certificate and Certificate Revocation List 957 (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January 958 2013, . 960 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 961 Galperin, S., and C. Adams, "X.509 Internet Public Key 962 Infrastructure Online Certificate Status Protocol - OCSP", 963 RFC 6960, DOI 10.17487/RFC6960, June 2013, 964 . 966 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 967 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 968 RFC 7234, DOI 10.17487/RFC7234, June 2014, 969 . 971 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 972 Opportunistic DNS-Based Authentication of Named Entities 973 (DANE) Transport Layer Security (TLS)", RFC 7672, 974 DOI 10.17487/RFC7672, October 2015, . 977 Appendix A. MTA-STS example record & policy 979 The owner of "example.com" wishes to begin using MTA-STS with a 980 policy that will solicit reports from senders without affecting how 981 the messages are processed, in order to verify the identity of MXs 982 that handle mail for "example.com", confirm that TLS is correctly 983 used, and ensure that certificates presented by the recipient MX 984 validate. 986 MTA-STS policy indicator TXT RR: 988 _mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;" 990 MTA-STS Policy file served as the response body at "https://mta- 991 sts.example.com/.well-known/mta-sts.txt": 993 version: STSv1 994 mode: testing 995 mx: mx1.example.com 996 mx: mx2.example.com 997 mx: mx.backup-example.com 998 max_age: 12345678 1000 Appendix B. Message delivery pseudocode 1002 Below is pseudocode demonstrating the logic of a compliant sending 1003 MTA. 1005 While this pseudocode implementation suggests synchronous policy 1006 retrieval in the delivery path, in a working implementation that may 1007 be undesirable, and we expect some implementers to instead prefer a 1008 background fetch that does not block delivery if no cached policy is 1009 present. 1011 func isEnforce(policy) { 1012 // Return true if the policy mode is "enforce". 1013 } 1015 func isNonExpired(policy) { 1016 // Return true if the policy is not expired. 1017 } 1019 func tryStartTls(connection) { 1020 // Attempt to open an SMTP connection with STARTTLS with the MX. 1021 } 1023 func isWildcardMatch(pat, host) { 1024 // Literal matches are true. 1025 if pat == host { 1026 return true 1027 } 1028 // Leading '.' matches a wildcard against the first part, i.e. 1029 // .example.com matches x.example.com but not x.y.example.com. 1030 if pat[0] == '.' { 1031 parts = SplitN(host, '.', 2) // Split on the first '.'. 1032 if len(parts) > 1 && parts[1] == pat[1:] { 1033 return true 1034 } 1035 } 1036 return false 1037 } 1039 func certMatches(connection, policy) { 1040 // Assume a handy function to return DNS-ID SANs. 1041 for san in getDnsIdSansFromCert(connection) { 1042 for mx in policy.mx { 1043 // Return if the server certificate from "connection" matches the 1044 // "mx" host. 1045 if san[0] == '*' { 1046 // Invalid wildcard! 1047 if san[1] != '.' continue 1048 san = san[1:] 1049 } 1050 if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) { 1051 return true 1052 } 1053 } 1054 } 1055 return false 1056 } 1058 func tryDeliverMail(connection, message) { 1059 // Attempt to deliver "message" via "connection". 1060 } 1062 func tryGetNewPolicy(domain) { 1063 // Check for an MTA-STS TXT record for "domain" in DNS, and return the 1064 // indicated policy. 1065 } 1067 func cachePolicy(domain, policy) { 1068 // Store "policy" as the cached policy for "domain". 1069 } 1071 func tryGetCachedPolicy(domain) { 1072 // Return a cached policy for "domain". 1073 } 1075 func reportError(error) { 1076 // Report an error via TLSRPT. 1077 } 1079 func tryMxAccordingTo(message, mx, policy) { 1080 connection := connect(mx) 1081 if !connection { 1082 return false // Can't connect to the MX so it's not an MTA-STS 1083 // error. 1084 } 1085 secure := true 1086 if !tryStartTls(connection) { 1087 secure = false 1088 reportError(E_NO_VALID_TLS) 1089 } else if !certMatches(connection, policy) { 1090 secure = false 1091 reportError(E_CERT_MISMATCH) 1092 } 1093 if secure || !isEnforce(policy) { 1094 return tryDeliverMail(connection, message) 1096 } 1097 return false 1098 } 1100 func tryWithPolicy(message, domain, policy) { 1101 mxes := getMxForDomain(domain) 1102 for mx in mxes { 1103 if tryMxAccordingTo(message, mx, policy) { 1104 return true 1105 } 1106 } 1107 return false 1108 } 1110 func handleMessage(message) { 1111 domain := ... // domain part after '@' from recipient 1112 policy := tryGetNewPolicy(domain) 1113 if policy { 1114 cachePolicy(domain, policy) 1115 } else { 1116 policy = tryGetCachedPolicy(domain) 1117 } 1118 if policy { 1119 return tryWithPolicy(message, domain, policy) 1120 } 1121 // Try to deliver the message normally (i.e. without MTA-STS). 1122 } 1124 Authors' Addresses 1126 Daniel Margolis 1127 Google, Inc 1129 Email: dmargolis (at) google (dot com) 1131 Mark Risher 1132 Google, Inc 1134 Email: risher (at) google (dot com) 1136 Binu Ramakrishnan 1137 Yahoo!, Inc 1139 Email: rbinu (at) yahoo-inc (dot com) 1140 Alexander Brotman 1141 Comcast, Inc 1143 Email: alex_brotman (at) comcast (dot com) 1145 Janet Jones 1146 Microsoft, Inc 1148 Email: janet.jones (at) microsoft (dot com)