idnits 2.17.1 draft-ietf-uta-smtp-tlsrpt-14.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (January 26, 2018) is 2275 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'CFWS' is mentioned on line 627, but not defined == Outdated reference: A later version (-21) exists of draft-ietf-uta-mta-sts-14 ** Obsolete normative reference: RFC 6125 (Obsoleted by RFC 9525) ** Obsolete normative reference: RFC 7231 (Obsoleted by RFC 9110) -- Obsolete informational reference (is this intentional?): RFC 3501 (Obsoleted by RFC 9051) Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Using TLS in Applications D. Margolis 3 Internet-Draft Google, Inc 4 Intended status: Standards Track A. Brotman 5 Expires: July 30, 2018 Comcast, Inc 6 B. Ramakrishnan 7 Yahoo!, Inc 8 J. Jones 9 Microsoft, Inc 10 M. Risher 11 Google, Inc 12 January 26, 2018 14 SMTP TLS Reporting 15 draft-ietf-uta-smtp-tlsrpt-14 17 Abstract 19 A number of protocols exist for establishing encrypted channels 20 between SMTP Mail Transfer Agents, including STARTTLS, DANE TLSA, and 21 MTA-STS. These protocols can fail due to misconfiguration or active 22 attack, leading to undelivered messages or delivery over unencrypted 23 or unauthenticated channels. This document describes a reporting 24 mechanism and format by which sending systems can share statistics 25 and specific information about potential failures with recipient 26 domains. Recipient domains can then use this information to both 27 detect potential attackers and diagnose unintentional 28 misconfigurations. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on July 30, 2018. 47 Copyright Notice 49 Copyright (c) 2018 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (http://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 65 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 66 2. Related Technologies . . . . . . . . . . . . . . . . . . . . 4 67 3. Reporting Policy . . . . . . . . . . . . . . . . . . . . . . 4 68 3.1. Example Reporting Policy . . . . . . . . . . . . . . . . 6 69 3.1.1. Report using MAILTO . . . . . . . . . . . . . . . . . 6 70 3.1.2. Report using HTTPS . . . . . . . . . . . . . . . . . 6 71 4. Reporting Schema . . . . . . . . . . . . . . . . . . . . . . 6 72 4.1. Report Time-frame . . . . . . . . . . . . . . . . . . . . 7 73 4.2. Delivery Summary . . . . . . . . . . . . . . . . . . . . 7 74 4.2.1. Success Count . . . . . . . . . . . . . . . . . . . . 7 75 4.2.2. Failure Count . . . . . . . . . . . . . . . . . . . . 7 76 4.3. Result Types . . . . . . . . . . . . . . . . . . . . . . 8 77 4.3.1. Negotiation Failures . . . . . . . . . . . . . . . . 8 78 4.3.2. Policy Failures . . . . . . . . . . . . . . . . . . . 8 79 4.3.3. General Failures . . . . . . . . . . . . . . . . . . 9 80 4.3.4. Transient Failures . . . . . . . . . . . . . . . . . 9 81 4.4. JSON Report Schema . . . . . . . . . . . . . . . . . . . 9 82 5. Report Delivery . . . . . . . . . . . . . . . . . . . . . . . 12 83 5.1. Report Filename . . . . . . . . . . . . . . . . . . . . . 12 84 5.2. Compression . . . . . . . . . . . . . . . . . . . . . . . 13 85 5.3. Email Transport . . . . . . . . . . . . . . . . . . . . . 13 86 5.3.1. Example Report . . . . . . . . . . . . . . . . . . . 15 87 5.4. HTTPS Transport . . . . . . . . . . . . . . . . . . . . . 15 88 5.5. Delivery Retry . . . . . . . . . . . . . . . . . . . . . 16 89 5.6. Metadata Variances . . . . . . . . . . . . . . . . . . . 16 90 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 91 6.1. Message headers . . . . . . . . . . . . . . . . . . . . . 16 92 6.2. Report Type . . . . . . . . . . . . . . . . . . . . . . . 17 93 6.3. application/tlsrpt+json Media Type . . . . . . . . . . . 17 94 6.4. application/tlsrpt+gzip Media Type . . . . . . . . . . . 18 95 6.5. STARTTLS Validation Result Types . . . . . . . . . . . . 19 96 7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 97 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 98 8.1. Normative References . . . . . . . . . . . . . . . . . . 21 99 8.2. Informative References . . . . . . . . . . . . . . . . . 23 100 Appendix A. Example Reporting Policy . . . . . . . . . . . . . . 24 101 A.1. Report using MAILTO . . . . . . . . . . . . . . . . . . . 24 102 A.2. Report using HTTPS . . . . . . . . . . . . . . . . . . . 24 103 Appendix B. Example JSON Report . . . . . . . . . . . . . . . . 24 104 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 106 1. Introduction 108 The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and 109 hosts to establish secure SMTP sessions over TLS. The protocol 110 design is based on "Opportunistic Security" (OS) [RFC7435], which 111 maintains interoperability with clients that do not support STARTTLS 112 but means that any attacker who can delete parts of the SMTP session 113 (such as the "250 STARTTLS" response) or redirect the entire SMTP 114 session (perhaps by overwriting the resolved MX record of the 115 delivery domain) can perform a downgrade or interception attack. 117 Because such "downgrade attacks" are not necessarily apparent to the 118 receiving MTA, this document defines a mechanism for sending domains 119 to report on failures at multiple stages of the MTA-to-MTA 120 conversation. 122 Recipient domains may also use the mechanisms defined by MTA-STS 123 [I-D.ietf-uta-mta-sts] or DANE [RFC6698] to publish additional 124 encryption and authentication requirements; this document defines a 125 mechanism for sending domains that are compatible with MTA-STS or 126 DANE to share success and failure statistics with recipient domains. 128 Specifically, this document defines a reporting schema that covers 129 failures in routing, STARTTLS negotiation, and both DANE [RFC6698] 130 and MTA-STS [I-D.ietf-uta-mta-sts] policy validation errors, and a 131 standard TXT record that recipient domains can use to indicate where 132 reports in this format should be sent. 134 This document is intended as a companion to the specification for 135 SMTP MTA Strict Transport Security [I-D.ietf-uta-mta-sts]. 137 1.1. Terminology 139 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 140 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 141 document are to be interpreted as described in [RFC2119]. 143 We also define the following terms for further use in this document: 145 o MTA-STS Policy: A definition of the expected TLS availability, 146 behavior, and desired actions for a given domain when a sending 147 MTA encounters problems in negotiating a secure channel. MTA-STS 148 is defined in [I-D.ietf-uta-mta-sts]. 150 o DANE Policy: A mechanism by which administrators can supply a 151 record that can be used to validate the certificate presented by 152 an MTA. DANE is defined in [RFC6698]. 154 o TLSRPT Policy: A policy specifying the endpoint to which sending 155 MTAs should deliver reports. 157 o Policy Domain: The domain against which an MTA-STS or DANE Policy 158 is defined. 160 o Sending MTA: The MTA initiating the delivery of an email message. 162 2. Related Technologies 164 o This document is intended as a companion to the specification for 165 SMTP MTA Strict Transport Security [I-D.ietf-uta-mta-sts]. 167 o SMTP-TLSRPT defines a mechanism for sending domains that are 168 compatible with MTA-STS or DANE to share success and failure 169 statistics with recipient domains. DANE is defined in [RFC6698] 170 and MTA-STS is defined in [I-D.ietf-uta-mta-sts]. 172 3. Reporting Policy 174 A domain publishes a record to its DNS indicating that it wishes to 175 receive reports. These SMTP TLSRPT policies are distributed via DNS 176 from the Policy Domain's zone, as TXT records (similar to DMARC 177 policies) under the name "_smtp-tlsrpt". For example, for the Policy 178 Domain "example.com", the recipient's TLSRPT policy can be retrieved 179 from "_smtp-tlsrpt.example.com". 181 Policies consist of the following directives: 183 o "v": This value MUST be equal to "TLSRPTv1". 185 o "rua": A URI specifying the endpoint to which aggregate 186 information about policy validation results should be sent (see 187 Section 4, "Reporting Schema", for more information). Two URI 188 schemes are supported: "mailto" and "https". As with DMARC 189 [RFC7489], the policy domain can specify a comma-separated list of 190 URIs. 192 o In the case of "https", reports should be submitted via POST 193 ([RFC7231]) to the specified URI. Report submitters MAY ignore 194 certificate validation errors when submitting reports via https. 196 o In the case of "mailto", reports should be submitted to the 197 specified email address ([RFC6068]). When sending failure reports 198 via SMTP, sending MTAs MUST deliver reports despite any TLS- 199 related failures and SHOULD NOT include this SMTP session in the 200 next report. This may mean that the reports are delivered in the 201 clear. Additionally, reports sent via SMTP MUST contain a valid 202 DKIM [RFC6376] signature by the reporting domain. Reports lacking 203 such a signature MUST be ignored by the recipient. DKIM 204 signatures must not use the "l=" attribute to limit the body 205 length used in the signature. 207 The formal definition of the "_smtp-tlsrpt" TXT record, defined using 208 [RFC5234] & [RFC7405], is as follows: 210 tlsrpt-record = tlsrpt-version 1*(field-delim tlsrpt-field) 211 [field-delim] 213 field-delim = *WSP ";" *WSP 215 tlsrpt-field = tlsrpt-rua / ; Note that the 216 tlsrpt-extension ; tlsrpt-rua record is 217 ; required. 219 tlsrpt-version = %s"v=TLSRPTv1" 221 tlsrpt-rua = %s"rua=" 222 tlsrpt-uri *(*WSP "," *WSP tlsrpt-uri) 224 tlsrpt-uri = URI 225 ; "URI" is imported from [RFC3986]; 226 ; commas (ASCII 0x2C) and exclamation 227 ; points (ASCII 0x21) MUST be encoded 229 tlsrpt-extension = tlsrpt-ext-name "=" tlsrpt-ext-value 231 tlsrpt-ext-name = (ALPHA / DIGIT) *31(ALPHA / 232 DIGIT / "_" / "-" / ".") 234 tlsrpt-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) 235 ; chars excluding "=", ";", SP, and control 236 ; chars 238 If multiple TXT records for "_smtp-tlsrpt" are returned by the 239 resolver, records which do not begin with "v=TLSRPTv1;" are 240 discarded. If the number of resulting records is not one, senders 241 MUST assume the recipient domain does not implement TLSRPT. If the 242 resulting TXT record contains multiple strings, then the record MUST 243 be treated as if those strings are concatenated together without 244 adding spaces. 246 Parsers MUST accept TXT records which are syntactically valid (i.e. 247 valid key-value pairs separated by semi-colons) and implementing a 248 superset of this specification, in which case unknown fields SHALL be 249 ignored. 251 3.1. Example Reporting Policy 253 3.1.1. Report using MAILTO 255 _smtp-tlsrpt.example.com. IN TXT \ 256 "v=TLSRPTv1;rua=mailto:reports@example.com" 258 3.1.2. Report using HTTPS 260 _smtp-tlsrpt.example.com. IN TXT \ 261 "v=TLSRPTv1; \ 262 rua=https://reporting.example.com/v1/tlsrpt" 264 4. Reporting Schema 266 The report is composed as a plain text file encoded in the I-JSON 267 format ([RFC7493]). 269 Aggregate reports contain the following fields: 271 o Report metadata: 273 * The organization responsible for the report 275 * Contact information for one or more responsible parties for the 276 contents of the report 278 * A unique identifier for the report 280 * The reporting date range for the report 282 o Policy, consisting of: 284 * One of the following policy types: (1) The MTA-STS policy 285 applied (as a string) (2) The DANE TLSA record applied (as a 286 string, with each RR entry of the RRset listed and separated by 287 a semicolon) (3) The literal string "no-policy-found", if 288 neither a DANE nor MTA-STS policy could be found. 290 * The domain for which the policy is applied 292 * The MX host 294 * An identifier for the policy (where applicable) 296 o Aggregate counts, comprising result type, sending MTA IP, 297 receiving MTA hostname, session count, and an optional additional 298 information field containing a URI for recipients to review 299 further information on a failure type. 301 Note that the failure types are non-exclusive; an aggregate report 302 may contain overlapping "counts" of failure types when a single send 303 attempt encountered multiple errors. Reporters may report multiple 304 applied policies (for example, an MTA-STS policy and a DANE TLSA 305 record for the same domain and MX); even in the case where only a 306 single policy was applied, the "policies" field of the report body 307 MUST be an array and not a singular value. 309 4.1. Report Time-frame 311 The report SHOULD cover a full day, from 0000-2400 UTC. This should 312 allow for easier correlation of failure events. To avoid a Denial of 313 Service against the system processing the reports, the reports should 314 be delivered after some delay, perhaps several hours. 316 4.2. Delivery Summary 318 4.2.1. Success Count 320 o "success-count": This indicates that the sending MTA was able to 321 successfully negotiate a policy-compliant TLS connection, and 322 serves to provide a "heartbeat" to receiving domains that 323 reporting is functional and tabulating correctly. This field 324 contains an aggregate count of successful connections for the 325 reporting system. 327 4.2.2. Failure Count 329 o "failure-count": This indicates that the sending MTA was unable to 330 successfully establish a connection with the receiving platform. 331 Section 4.3, "Result Types", will elaborate on the failed 332 negotiation attempts. This field contains an aggregate count of 333 failed connections. 335 4.3. Result Types 337 The list of result types will start with the minimal set below, and 338 is expected to grow over time based on real-world experience. The 339 initial set is: 341 4.3.1. Negotiation Failures 343 o "starttls-not-supported": This indicates that the recipient MX did 344 not support STARTTLS. 346 o "certificate-host-mismatch": This indicates that the certificate 347 presented did not adhere to the constraints specified in the MTA- 348 STS or DANE policy, e.g. if the MX does not match any identities 349 listed in the Subject Alternate Name (SAN) [RFC5280]. 351 o "certificate-expired": This indicates that the certificate has 352 expired. 354 o "certificate-not-trusted": This a label that covers multiple 355 certificate related failures that include, but not limited to 356 errors such as untrusted/unknown CAs, certificate name 357 constraints, certificate chain errors etc. When using this 358 declaration, the reporting MTA SHOULD utilize the "failure-reason" 359 to provide more information to the receiving entity. 361 o "validation-failure": This indicates a general failure for a 362 reason not matching a category above. When using this 363 declaration, the reporting MTA SHOULD utilize the "failure-reason" 364 to provide more information to the receiving entity. 366 4.3.2. Policy Failures 368 4.3.2.1. DANE-specific Policy Failures 370 o "tlsa-invalid": This indicates a validation error in the TLSA 371 record associated with a DANE policy. None of the records in the 372 RRset were found to be valid. 374 o "dnssec-invalid": This would indicate that no valid records were 375 returned from the recursive resolver. The request returned with 376 SERVFAIL for the requested TLSA record. 378 o "dane-required": This indicates that the sending system is 379 configured to require DANE TLSA records for all the MX hosts of 380 the destination domain, but no DNSSEC-validated TLSA records were 381 present for the MX host that is the subject of the report. 382 Mandatory DANE for SMTP is described in section 6 of [RFC7672]. 384 Such policies may be created by mutual agreement between two 385 organizations that frequently exchange sensitive content via 386 email. 388 4.3.2.2. MTA-STS-specific Policy Failures 390 o "sts-policy-invalid": This indicates a validation error for the 391 overall MTA-STS policy. 393 o "sts-webpki-invalid": This indicates that the MTA-STS policy could 394 not be authenticated using PKIX validation. 396 4.3.3. General Failures 398 When a negotiation failure can not be categorized into one of the 399 "Negotiation Failures" stated above, the reporter SHOULD use the 400 "validation-failure" category. As TLS grows and becomes more 401 complex, new mechanisms may not be easily categorized. This allows 402 for a generic feedback category. When this category is used, the 403 reporter SHOULD also use the "failure-reason-code" to give some 404 feedback to the receiving entity. This is intended to be a short 405 text field, and the contents of the field should be an error code or 406 error text, such as "X509_V_ERR_UNHANDLED_CRITICAL_CRL_EXTENSION". 408 4.3.4. Transient Failures 410 Transient errors due to too-busy network, TCP timeouts, etc. are not 411 required to be reported. 413 4.4. JSON Report Schema 415 The JSON schema is derived from the HPKP JSON schema [RFC7469] (cf. 416 Section 3) 418 { 419 "organization-name": organization-name, 420 "date-range": { 421 "start-datetime": date-time, 422 "end-datetime": date-time 423 }, 424 "contact-info": email-address, 425 "report-id": report-id, 426 "policies": [{ 427 "policy": { 428 "policy-type": policy-type, 429 "policy-string": policy-string, 430 "policy-domain": domain, 431 "mx-host": mx-host-pattern 432 }, 433 "summary": { 434 "total-successful-session-count": total-successful-session-count, 435 "total-failure-session-count": total-failure-session-count 436 }, 437 "failure-details": [ 438 { 439 "result-type": result-type, 440 "sending-mta-ip": ip-address, 441 "receiving-mx-hostname": receiving-mx-hostname, 442 "receiving-mx-helo": receiving-mx-helo, 443 "failed-session-count": failed-session-count, 444 "additional-information": additional-info-uri, 445 "failure-reason-code": failure-reason-code 446 } 447 ] 448 } 449 ] 450 } 452 JSON Report Format 454 o "organization-name": The name of the organization responsible for 455 the report. It is provided as a string. 457 o "date-time": The date-time indicates the start- and end-times for 458 the report range. It is provided as a string formatted according 459 to Section 5.6, "Internet Date/Time Format", of [RFC3339]. The 460 report should be for a full UTC day, 0000-2400. 462 o "email-address": The contact information for a responsible party 463 of the report. It is provided as a string formatted according to 464 Section 3.4.1, "Addr-Spec", of [RFC5321]. 466 o "report-id": A unique identifier for the report. Report authors 467 may use whatever scheme they prefer to generate a unique 468 identifier. It is provided as a string. 470 o "policy-type": The type of policy that was applied by the sending 471 domain. Presently, the only three valid choices are "tlsa", 472 "sts", and the literal string "no-policy-found". It is provided 473 as a string. 475 o "policy-string": A string representation of the policy, whether 476 TLSA record ([RFC6698] section 2.3) or MTA-STS policy. Examples: 477 TLSA: ""_25._tcp.mx.example.com. 3 0 1 \ 1F850A337E6DB9C609C522D13 478 6A475638CC43E1ED424F8EEC8513D747D1D085D)"" MTA-STS: ""version: 479 STSv1\nmode: report\nmx: mx1.example.com\nmx: \ 480 mx2.example.com\nmx: mx.backup-example.com\nmax_age: 12345678"" 482 o "domain": The Policy Domain is the domain against which the MTA- 483 STS or DANE policy is defined. In the case of Internationalized 484 Domain Names ([RFC5891]), the domain MUST consist of the Punycode- 485 encoded A-labels ([RFC3492]) and not the U-labels. 487 o "mx-host-pattern": The pattern of MX hostnames from the applied 488 policy. It is provided as a string, and is interpreted in the 489 same manner as the "Checking of Wildcard Certificates" rules in 490 Section 6.4.3 of [RFC6125]. In the case of Internationalized 491 Domain Names ([RFC5891]), the domain MUST consist of the Punycode- 492 encoded A-labels ([RFC3492]) and not the U-labels. 494 o "result-type": A value from Section 4.3, "Result Types", above. 496 o "ip-address": The IP address of the sending MTA that attempted the 497 STARTTLS connection. It is provided as a string representation of 498 an IPv4 (see below) or IPv6 ([RFC5952]) address in dot-decimal or 499 colon-hexadecimal notation. 501 o "receiving-mx-hostname": The hostname of the receiving MTA MX 502 record with which the sending MTA attempted to negotiate a 503 STARTTLS connection. 505 o "receiving-mx-helo": (optional) The HELO or EHLO string from the 506 banner announced during the reported session. 508 o "receiving-ip": The destination IP address that was using when 509 creating the outbound session. It is provided as a string 510 representation of an IPv4 (see below) or IPv6 ([RFC5952]) address 511 in dot-decimal or colon-hexadecimal notation. 513 o "total-successful-session-count": The aggregate number (integer) 514 of successfully negotiated TLS-enabled connections to the 515 receiving site. 517 o "total-failure-session-count": The aggregate number (integer) of 518 failures to negotiate a TLS-enabled connection to the receiving 519 site. 521 o "failed-session-count": The number of (attempted) sessions that 522 match the relevant "result-type" for this section. 524 o "additional-info-uri": An optional URI [RFC3986] pointing to 525 additional information around the relevant "result-type". For 526 example, this URI might host the complete certificate chain 527 presented during an attempted STARTTLS session. 529 o "failure-reason-code": A text field to include a TLS-related error 530 code or error message. 532 For report purposes, an IPv4 Address is defined as: IPv4address = 533 dec-octet "." dec-octet "." dec-octet "." dec-octet 534 dec-octet = DIGIT ; 0-9 / %x31-39 DIGIT ; 10-99 / "1" 2DIGIT ; 535 100-199 / "2" %x30-34 DIGIT ; 200-249 / "25" %x30-35 ; 250-255 537 5. Report Delivery 539 Reports can be delivered either as an email message via SMTP or via 540 HTTP POST. 542 5.1. Report Filename 544 The filename is RECOMMENDED to be constructed using the following 545 ABNF: 547 filename = sender "!" policy-domain "!" begin-timestamp 548 "!" end-timestamp [ "!" unique-id ] "." extension 550 unique-id = 1*(ALPHA / DIGIT) 552 sender = domain ; From the [RFC5321] that is used 553 ; as the domain for the `contact-info` 554 ; address in the report body 556 policy-domain = domain 558 begin-timestamp = 1*DIGIT 559 ; seconds since 00:00:00 UTC January 1, 1970 560 ; indicating start of the time range contained 561 ; in the report 563 end-timestamp = 1*DIGIT 564 ; seconds since 00:00:00 UTC January 1, 1970 565 ; indicating end of the time range contained 566 ; in the report 568 extension = "json" 570 "unique-id" allows an optional unique ID generated by the Sending MTA 571 to distinguish among multiple reports generated simultaneously by 572 different sources within the same Policy Domain. For example, this 573 is a possible filename for the gzip file of a report to the Policy 574 Domain "example.net" from the Sending MTA "mail.sender.example.com": 576 "mail.sender.example.com!example.net!1470013207!1470186007!001.json" 578 5.2. Compression 580 The report SHOULD be subjected to GZIP compression for both email and 581 HTTPS transport. Declining to apply compression can cause the report 582 to be too large for a receiver to process (a commonly observed 583 receiver limit is ten megabytes); compressing the file increases the 584 chances of acceptance of the report at some compute cost. 586 5.3. Email Transport 588 The report MAY be delivered by email. To make the reports machine- 589 parsable for the receivers, we define a top-level media type 590 "multipart/report" with a new parameter "report-type="tlsrpt"". 591 Inside it, there are two parts: The first part is human readable, 592 typically "text/plain", and the second part is machine readable with 593 a new media type defined called "application/tlsrpt+json". If 594 compressed, the report should use the media type "application/ 595 tlsrpt+gzip". 597 In addition, the following two new top level message header fields 598 are defined: 600 "TLS-Report-Domain: Receiver-Domain TLS-Report-Submitter: Sender- 601 Domain" 603 The "TLS-Report-Submitter" value MUST match the value found in the 604 filename and the [RFC5321] domain from the "contact-info" from the 605 report body. These message headers MUST be included and should allow 606 for easy searching for all reports submitted by a report domain or a 607 particular submitter, for example in IMAP [RFC3501]: 609 "s SEARCH HEADER "TLS-Report-Domain" "example.com"" 611 It is presumed that the aggregate reporting address will be equipped 612 to process new message header fields and extract MIME parts with the 613 prescribed media type and filename, and ignore the rest. These 614 additional headers SHOULD be included in the DKIM [RFC6376] signature 615 for the message. 617 The [RFC5322].Subject field for report submissions SHOULD conform to 618 the following ABNF: 620 tlsrpt-subject = %s"Report" FWS ; "Report" 621 %s"Domain:" FWS ; "Domain:" 622 domain-name FWS ; per [RFC6376] 623 %s"Submitter:" FWS ; "Submitter:" 624 domain-name FWS ; per [RFC6376] 625 %s"Report-ID:" FWS ; "Report-ID: 626 "<" id-left "@" id-right ">" ; per [RFC5322] 627 [CFWS] ; per [RFC5322] 628 ; (as with FWS) 630 The first domain-name indicates the DNS domain name about which the 631 report was generated. The second domain-name indicates the DNS 632 domain name representing the Sending MTA generating the report. The 633 purpose of the Report-ID: portion of the field is to enable the 634 Policy Domain to identify and ignore duplicate reports that might be 635 sent by a Sending MTA. 637 For instance, this is a possible Subject field for a report to the 638 Policy Domain "example.net" from the Sending MTA 639 "mail.sender.example.com". It is line-wrapped as allowed by 640 [RFC5322]: 642 Subject: Report Domain: example.net 643 Submitter: mail.sender.example.com 644 Report-ID: <735ff.e317+bf22029@mailexample.net> 646 5.3.1. Example Report 648 From: tlsrpt@mail.sender.example.com 649 Date: Fri, May 09 2017 16:54:30 -0800 650 To: mts-sts-tlsrpt@example.net 651 Subject: Report Domain: example.net 652 Submitter: mail.sender.example.com 653 Report-ID: <735ff.e317+bf22029@example.net> 654 TLS-Report-Domain: example.net 655 TLS-Report-Submitter: mail.sender.example.com 656 MIME-Version: 1.0 657 Content-Type: multipart/report; report-type="tlsrpt"; 658 boundary="----=_NextPart_000_024E_01CC9B0A.AFE54C00" 659 Content-Language: en-us 661 This is a multipart message in MIME format. 663 ------=_NextPart_000_024E_01CC9B0A.AFE54C00 664 Content-Type: text/plain; charset="us-ascii" 665 Content-Transfer-Encoding: 7bit 667 This is an aggregate TLS report from mail.sender.example.com 669 ------=_NextPart_000_024E_01CC9B0A.AFE54C00 670 Content-Type: application/tlsrpt+gzip 671 Content-Transfer-Encoding: base64 672 Content-Disposition: attachment; 673 filename="mail.sender.example!example.com! 674 1013662812!1013749130.gz" 676 678 ------=_NextPart_000_024E_01CC9B0A.AFE54C00-- 679 ... 681 Note that, when sending failure reports via SMTP, sending MTAs MUST 682 NOT honor MTA-STS or DANE TLSA failures. 684 5.4. HTTPS Transport 686 The report MAY be delivered by POST to HTTPS. If compressed, the 687 report SHOULD use the media type "application/tlsrpt+gzip", and 688 The report MAY be delivered by POST to HTTPS. When doing so, the 689 report should use the media-type "application/tlsrpt" (see section 690 Section 6, "IANA Considerations"). If the reporting entity 691 compresses the report, that should be noted in the Content-Type 692 "conversions" field: 694 Content-Type: application/tlsrpt; conversions=gzip Content-Type: 695 application/tlsrpt; conversions=none Content-Type: application/tlsrpt 697 The second and third items in the list are equivalent. 699 A reporting entity SHOULD expect a "successful" response from the 700 accepting HTTPS server, typically a 200 or 201 HTTP code [RFC7231]. 701 Other codes could indicate a delivery failure, and may be retried as 702 per local policy. The receiving system is not expected to process 703 reports at receipt time, and MAY store them for processing at a later 704 time. 706 5.5. Delivery Retry 708 In the event of a delivery failure, regardless of the delivery 709 method, a sender SHOULD attempt redelivery for up to 24hrs after the 710 initial attempt. As previously stated the reports are optional, so 711 while it is ideal to attempt redelivery, it is not required. If 712 multiple retries are attempted, ideally they SHOULD be done with 713 exponential backoff. 715 5.6. Metadata Variances 717 As stated above, there are a variable number of ways to declare 718 information about the data therein. If any of items declared via 719 subject or filename disagree with the report, the report MUST be 720 considered the authoritative source. 722 6. IANA Considerations 724 The following are the IANA considerations discussed in this document. 726 6.1. Message headers 728 Below is the Internet Assigned Numbers Authority (IANA) Permanent 729 Message Header Field registration information per [RFC3864]. 731 Header field name: TLS-Report-Domain 732 Applicable protocol: mail 733 Status: standard 734 Author/Change controller: IETF 735 Specification document(s): this one 737 Header field name: TLS-Report-Submitter 738 Applicable protocol: mail 739 Status: standard 740 Author/Change controller: IETF 741 Specification document(s): this one 743 6.2. Report Type 745 This document registers a new parameter "report-type="tlsrpt"" under 746 "multipart/report" top-level media type for use with [RFC6522]. 748 The media type suitable for use as a report-type is defined in the 749 following section. 751 6.3. application/tlsrpt+json Media Type 753 This document registers multiple media types, beginning with Table 1 754 below. 756 +-------------+----------------+-------------+-------------------+ 757 | Type | Subtype | File extn | Specification | 758 +-------------+----------------+-------------+-------------------+ 759 | application | tlsrpt+json | .json | Section 5.3 | 760 +-------------+----------------+-------------+-------------------+ 761 Table 1: SMTP TLS Reporting Media Type 763 Type name: application 765 Subtype name: tlsrpt+json 767 Required parameters: n/a 769 Optional parameters: n/a 771 Encoding considerations: Encoding considerations are identical to 772 those specified for the "application/json" media type. See 773 [RFC7493]. 775 Security considerations: Security considerations relating to SMTP TLS 776 Reporting are discussed in Section 7. 778 Interoperability considerations: This document specifies format of 779 conforming messages and the interpretation thereof. 781 Published specification: Section 5.3 of this document. 783 Applications that use this media type: Mail User Agents (MUA) and 784 Mail Transfer Agents. 786 Additional information: 788 Magic number(s): n/a 790 File extension(s): ".json" 792 Macintosh file type code(s): n/a 794 Person & email address to contact for further information: See 795 Authors' Addresses section. 797 Intended usage: COMMON 799 Restrictions on usage: n/a 801 Author: See Authors' Addresses section. 803 Change controller: Internet Engineering Task Force 804 (mailto:iesg@ietf.org). 806 6.4. application/tlsrpt+gzip Media Type 808 +-------------+----------------+-------------+-------------------+ 809 | Type | Subtype | File extn | Specification | 810 +-------------+----------------+-------------+-------------------+ 811 | application | tlsrpt+gzip | .gz | Section 5.3 | 812 +-------------+----------------+-------------+-------------------+ 813 Table 2: SMTP TLS Reporting Media Type 815 Type name: application 817 Subtype name: tlsrpt+gzip 819 Required parameters: n/a 821 Optional parameters: n/a 823 Encoding considerations: Binary 824 Security considerations: Security considerations relating to SMTP TLS 825 Reporting are discussed in Section 7. 827 Interoperability considerations: This document specifies format of 828 conforming messages and the interpretation thereof. 830 Published specification: Section 5.3 of this document. 832 Applications that use this media type: Mail User Agents (MUA) and 833 Mail Transfer Agents. 835 Additional information: 837 Magic number(s): n/a 839 File extension(s): ".gz" 841 Macintosh file type code(s): n/a 843 Person & email address to contact for further information: See 844 Authors' Addresses section. 846 Intended usage: COMMON 848 Restrictions on usage: n/a 850 Author: See Authors' Addresses section. 852 Change controller: Internet Engineering Task Force 853 (mailto:iesg@ietf.org). 855 6.5. STARTTLS Validation Result Types 857 This document creates a new registry, "STARTTLS Validation Result 858 Types". The initial entries in the registry are: 860 +-------------------------------+ 861 | Result Type | 862 +-------------------------------+ 863 | "starttls-not-supported" | 864 | "certificate-host-mismatch" | 865 | "certificate-expired" | 866 | "tlsa-invalid" | 867 | "dnssec-invalid" | 868 | "sts-policy-invalid" | 869 | "sts-webpki-invalid" | 870 | "validation-failure" | 871 +-------------------------------+ 873 The above entries are described in section Section 4.3, "Result 874 Types." New result types can be added to this registry using "Expert 875 Review" IANA registration policy. 877 7. Security Considerations 879 SMTP TLS Reporting provides transparency into misconfigurations or 880 attempts to intercept or tamper with mail between hosts who support 881 STARTTLS. There are several security risks presented by the 882 existence of this reporting channel: 884 o Flooding of the Aggregate report URI (rua) endpoint: An attacker 885 could flood the endpoint with excessive reporting traffic and 886 prevent the receiving domain from accepting additional reports. 887 This type of Denial-of-Service attack would limit visibility into 888 STARTTLS failures, leaving the receiving domain blind to an 889 ongoing attack. 891 o Untrusted content: An attacker could inject malicious code into 892 the report, opening a vulnerability in the receiving domain. 893 Implementers are advised to take precautions against evaluating 894 the contents of the report. 896 o Report snooping: An attacker could create a bogus TLSRPT record to 897 receive statistics about a domain the attacker does not own. 898 Since an attacker able to poison DNS is already able to receive 899 counts of SMTP connections (and, absent DANE or MTA-STS policies, 900 actual SMTP message payloads), this does not present a significant 901 new vulnerability. 903 o Reports as DDoS: TLSRPT allows specifying destinations for the 904 reports that are outside the authority of the Policy Domain, which 905 allows domains to delegate processing of reports to a partner 906 organization. However, an attacker who controls the Policy Domain 907 DNS could also use this mechanism to direct the reports to an 908 unwitting victim, flooding that victim with excessive reports. 909 DMARC [RFC7489] defines a solution for verifying delegation to 910 avoid such attacks; the need for this is greater with DMARC, 911 however, because DMARC allows an attacker to trigger reports to a 912 target from an innocent third party by sending that third party 913 mail (which triggers a report from the third party to the target). 914 In the case of TLSRPT, the attacker would have to induce the third 915 party to send the attacker mail in order to trigger reports from 916 the third party to the victim; this reduces the risk of such an 917 attack and the need for a verification mechanism. 919 Finally, because TLSRPT is intended to help administrators discover 920 man-in-the-middle attacks against transport-layer encryption, 921 including attacks designed to thwart negotiation of encrypted 922 connections (by downgrading opportunistic encryption or, in the case 923 of MTA-STS, preventing discovery of a new MTA-STS policy), we must 924 also consider the risk that an adversary who can induce such a 925 downgrade attack can also prevent discovery of the TLSRPT TXT record 926 (and thus prevent discovery of the successful downgrade attack). 927 Administrators are thus encouraged to deploy TLSRPT TXT records with 928 a large TTL (reducing the window for successful attacks against DNS 929 resolution of the record) or to deploy DNSSEC on the deploying zone. 931 8. References 933 8.1. Normative References 935 [I-D.ietf-uta-mta-sts] 936 Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., 937 and J. Jones, "SMTP MTA Strict Transport Security (MTA- 938 STS)", draft-ietf-uta-mta-sts-14 (work in progress), 939 January 2018. 941 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 942 Requirement Levels", BCP 14, RFC 2119, 943 DOI 10.17487/RFC2119, March 1997, . 946 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: 947 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, 948 . 950 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 951 for Internationalized Domain Names in Applications 952 (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, 953 . 955 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 956 Resource Identifier (URI): Generic Syntax", STD 66, 957 RFC 3986, DOI 10.17487/RFC3986, January 2005, 958 . 960 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 961 Specifications: ABNF", STD 68, RFC 5234, 962 DOI 10.17487/RFC5234, January 2008, . 965 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 966 Housley, R., and W. Polk, "Internet X.509 Public Key 967 Infrastructure Certificate and Certificate Revocation List 968 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 969 . 971 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 972 DOI 10.17487/RFC5321, October 2008, . 975 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 976 DOI 10.17487/RFC5322, October 2008, . 979 [RFC5891] Klensin, J., "Internationalized Domain Names in 980 Applications (IDNA): Protocol", RFC 5891, 981 DOI 10.17487/RFC5891, August 2010, . 984 [RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 985 Address Text Representation", RFC 5952, 986 DOI 10.17487/RFC5952, August 2010, . 989 [RFC6068] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto' 990 URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010, 991 . 993 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 994 Verification of Domain-Based Application Service Identity 995 within Internet Public Key Infrastructure Using X.509 996 (PKIX) Certificates in the Context of Transport Layer 997 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 998 2011, . 1000 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 1001 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 1002 RFC 6376, DOI 10.17487/RFC6376, September 2011, 1003 . 1005 [RFC6522] Kucherawy, M., Ed., "The Multipart/Report Media Type for 1006 the Reporting of Mail System Administrative Messages", 1007 STD 73, RFC 6522, DOI 10.17487/RFC6522, January 2012, 1008 . 1010 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 1011 of Named Entities (DANE) Transport Layer Security (TLS) 1012 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 1013 2012, . 1015 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1016 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 1017 DOI 10.17487/RFC7231, June 2014, . 1020 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 1021 RFC 7405, DOI 10.17487/RFC7405, December 2014, 1022 . 1024 [RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493, 1025 DOI 10.17487/RFC7493, March 2015, . 1028 8.2. Informative References 1030 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 1031 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 1032 February 2002, . 1034 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 1035 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 1036 . 1038 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 1039 Procedures for Message Header Fields", BCP 90, RFC 3864, 1040 DOI 10.17487/RFC3864, September 2004, . 1043 [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection 1044 Most of the Time", RFC 7435, DOI 10.17487/RFC7435, 1045 December 2014, . 1047 [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning 1048 Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 1049 2015, . 1051 [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based 1052 Message Authentication, Reporting, and Conformance 1053 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, 1054 . 1056 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 1057 Opportunistic DNS-Based Authentication of Named Entities 1058 (DANE) Transport Layer Security (TLS)", RFC 7672, 1059 DOI 10.17487/RFC7672, October 2015, . 1062 Appendix A. Example Reporting Policy 1064 A.1. Report using MAILTO 1066 _smtp-tlsrpt.mail.example.com. IN TXT \ 1067 "v=TLSRPTv1;rua=mailto:reports@example.com" 1069 A.2. Report using HTTPS 1071 _smtp-tlsrpt.mail.example.com. IN TXT \ 1072 "v=TLSRPTv1; \ 1073 rua=https://reporting.example.com/v1/tlsrpt" 1075 Appendix B. Example JSON Report 1076 { 1077 "organization-name": "Company-X", 1078 "date-range": { 1079 "start-datetime": "2016-04-01T00:00:00Z", 1080 "end-datetime": "2016-04-01T23:59:59Z" 1081 }, 1082 "contact-info": "sts-reporting@company-x.example", 1083 "report-id": "5065427c-23d3-47ca-b6e0-946ea0e8c4be", 1084 "policies": [{ 1085 "policy": { 1086 "policy-type": "sts", 1087 "policy-string": "version: STSv1\r\nmode: report\r\n 1088 mx: .mail.company-y.example\r\nmax_age: 86400", 1089 "policy-domain": "company-y.example", 1090 "mx-host": ".mail.company-y.example" 1091 }, 1092 "summary": { 1093 "total-successful-session-count": 5326, 1094 "total-failure-session-count": 303 1095 }, 1096 "failure-details": [{ 1097 "result-type": "certificate-expired", 1098 "sending-mta-ip": "98.136.216.25", 1099 "receiving-mx-hostname": "mx1.mail.company-y.example", 1100 "failed-session-count": 100 1101 }, { 1102 "result-type": "starttls-not-supported", 1103 "sending-mta-ip": "98.22.33.99", 1104 "receiving-mx-hostname": "mx2.mail.company-y.example", 1105 "receiving-ip": "192.168.14.72", 1106 "failed-session-count": 200, 1107 "additional-information": "https://reports.company-x.example/ 1108 report_info ? id = 5065427 c - 23 d3# StarttlsNotSupported " 1109 }, { 1110 "result-type": "validation-failure", 1111 "sending-mta-ip": "47.97.15.2", 1112 "receiving-ip": "10.72.84.12", 1113 "receiving-mx-hostname": "mx-backup.mail.company-y.example", 1114 "failed-session-count": 3, 1115 "failure-error-code": "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED" 1116 }] 1117 }] 1118 } 1120 Figure: Example JSON report for a messages from Company-X to Company- 1121 Y, where 100 sessions were attempted to Company Y servers with an 1122 expired certificate and 200 sessions were attempted to Company Y 1123 servers that did not successfully respond to the "STARTTLS" command. 1124 Additionally 3 sessions failed due to 1125 "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED". 1127 Authors' Addresses 1129 Daniel Margolis 1130 Google, Inc 1132 Email: dmargolis@google.com 1134 Alexander Brotman 1135 Comcast, Inc 1137 Email: alex_brotman@comcast.com 1139 Binu Ramakrishnan 1140 Yahoo!, Inc 1142 Email: rbinu@oath.com 1144 Janet Jones 1145 Microsoft, Inc 1147 Email: janet.jones@microsoft.com 1149 Mark Risher 1150 Google, Inc 1152 Email: risher@google.com