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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Obsolete informational reference (is this intentional?): RFC 7001 (ref. 'AUTHRES') (Obsoleted by RFC 7601) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group W. Mills 3 Internet-Draft Yahoo! Inc. 4 Intended status: Standards Track M. Kucherawy 5 Expires: May 23, 2014 Facebook, Inc. 6 November 19, 2013 8 The Require-Recipient-Valid-Since Header Field and SMTP Service 9 Extension 10 draft-ietf-appsawg-rrvs-header-field-04 12 Abstract 14 This document defines an extension for the Simple Mail Transfer 15 Protocol called RRVS, and a header field called Require-Recipient- 16 Valid-Since, to provide a method for senders to indicate to receivers 17 the time when the sender last confirmed the ownership of the target 18 mailbox. This can be used to detect changes of mailbox ownership, 19 and thus prevent mail from being delivered to the wrong party. 21 The intended use of these facilities is on automatically generated 22 messages that might contain sensitive information, though it may also 23 be useful in other applications. 25 Status of This Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at http://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on May 23, 2014. 42 Copyright Notice 44 Copyright (c) 2013 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 61 3. Description . . . . . . . . . . . . . . . . . . . . . . . . . 4 62 3.1. The 'RRVS' SMTP Extension . . . . . . . . . . . . . . . . 5 63 3.2. The 'Require-Recipient-Valid-Since' Header Field . . . . . 5 64 4. Use By Generators . . . . . . . . . . . . . . . . . . . . . . 6 65 5. Handling By Receivers . . . . . . . . . . . . . . . . . . . . 6 66 5.1. SMTP Extension Used . . . . . . . . . . . . . . . . . . . 6 67 5.1.1. Relays . . . . . . . . . . . . . . . . . . . . . . . . 7 68 5.2. Header Field Used . . . . . . . . . . . . . . . . . . . . 7 69 6. Role Accounts . . . . . . . . . . . . . . . . . . . . . . . . 8 70 7. Relaying Without RRVS Support . . . . . . . . . . . . . . . . 8 71 7.1. Header Field Conversion . . . . . . . . . . . . . . . . . 9 72 8. Header Field with Multiple Recipients . . . . . . . . . . . . 9 73 9. Special Use Addresses . . . . . . . . . . . . . . . . . . . . 10 74 9.1. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 10 75 9.2. Single-Recipient Alaises . . . . . . . . . . . . . . . . . 10 76 9.3. Multiple-Recipient Aliases . . . . . . . . . . . . . . . . 11 77 9.4. Confidential Forwarding Addresses . . . . . . . . . . . . 11 78 9.5. Suggested Mailing List Enhancements . . . . . . . . . . . 11 79 10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 12 80 11. Continuous Ownership . . . . . . . . . . . . . . . . . . . . . 12 81 12. Authentication-Results Definitions . . . . . . . . . . . . . . 13 82 13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 83 13.1. SMTP Extension Example . . . . . . . . . . . . . . . . . . 14 84 13.2. Header Field Example . . . . . . . . . . . . . . . . . . . 14 85 14. Security Considerations . . . . . . . . . . . . . . . . . . . 15 86 14.1. Abuse Countermeasures . . . . . . . . . . . . . . . . . . 15 87 14.2. Suggested Use Restrictions . . . . . . . . . . . . . . . . 15 88 15. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 15 89 15.1. Probing Attacks . . . . . . . . . . . . . . . . . . . . . 15 90 15.2. Envelope Recipients . . . . . . . . . . . . . . . . . . . 16 91 15.3. Risks with Use of Header Field . . . . . . . . . . . . . . 16 92 16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 93 16.1. SMTP Extension Registration . . . . . . . . . . . . . . . 17 94 16.2. Header Field Registration . . . . . . . . . . . . . . . . 17 95 16.3. Enhanced Status Code Registration . . . . . . . . . . . . 17 96 16.4. Authentication Results Registration . . . . . . . . . . . 18 97 17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 98 17.1. Normative References . . . . . . . . . . . . . . . . . . . 19 99 17.2. Informative References . . . . . . . . . . . . . . . . . . 19 100 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 19 102 1. Introduction 104 Email addresses sometimes get reassigned to a different person. For 105 example, employment changes at a company can cause an address used 106 for an ex-employee to be assigned to a new employee, or a mail 107 service provider (MSP) might expire an account and then let someone 108 else register for the local-part that was previously used. Those who 109 sent mail to the previous owner of an address might not know that it 110 has been reassigned. This can lead to the sending of email to the 111 correct address, but the wrong recipient. 113 What is needed is a way to indicate an attribute of the recipient 114 that will distinguish between the previous owner of an address and 115 its current owner, if they are different. Further, this needs to be 116 done in a way that respects privacy. 118 The mechanisms specified here allow the sender of the mail to 119 indicate how "old" the address assignment is expected to be. In 120 effect, the sender is saying, "The person to whom I am sending to had 121 this address assigned to as far back as this date-time." A receiving 122 system can then compare this information against the date and time 123 the address was assigned to its current user. If the assignment was 124 made later than the date-time indicated in the message, there is a 125 good chance the current user of the address is not the correct 126 recipient. The receiving system can then choose to prevent delivery 127 and, possibly, to notify the original sender of the problem. 129 The primary application is automatically generated messages rather 130 than user-authored content, though it may be useful in other 131 contexts. 133 2. Definitions 135 For a description of the email architecture, consult [EMAIL-ARCH]. 137 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 138 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 139 document are to be interpreted as described in [KEYWORDS]. 141 3. Description 143 To address the problem described above, a mail sending client needs 144 to indicate to the server to which it is connecting that there is an 145 expectation that the destination of the message has been under 146 continuous ownership since some date-time, presumably the most recent 147 time the message author had confirmed its understanding of who owned 148 that mailbox. Two mechanisms are defined here: an extension to the 149 Simple Mail Transfer Protocol [SMTP], for use between a client and 150 server that both implement the extension, and a header field that can 151 be used when passing a message to a server that appears not to 152 implement this extension. 154 The SMTP extenion is called "RRVS" (Require Recipient Valid Since), 155 and adds a parameter to the SMTP "RCPT" command that indicates the 156 most recent date and time when the message author believed the 157 destination mailbox to be under the continuous ownership (see 158 Section 11) of a specific party. Similarly, the Require-Recipient- 159 Valid-Since header field includes an intended recipient coupled with 160 a timestamp indicating the same thing. Presumably there has been 161 some confirmation process applied to establish this ownership; 162 however, the method of making such determinations is a local matter 163 and outside the scope of this document. 165 3.1. The 'RRVS' SMTP Extension 167 Extensions to SMTP are described in Section 2.2 of [SMTP]. 169 The name of the extension is "RRVS", an abbreviation of "Require 170 Recipient Valid Since". Servers implementing the SMTP extension 171 advertise an additional EHLO keyword of "RRVS", which has no 172 associated parameters, introduces no new SMTP verbs, and does not 173 alter the MAIL verb. 175 An MTA implementing RRVS can transmit or accept a new parameter to 176 the RCPT command. The new parameter is "RRVS", which takes a value 177 that is a timestamp expressed as a "date-time" as defiend in 178 [DATETIME], with the added restriction that a "time-secfrac" MUST NOT 179 be used. Accordingly, this extension increases the maximum command 180 length for the RCPT verb by 31 characters. 182 The meaning of this extension, when used, is described in 183 Section 5.1. 185 3.2. The 'Require-Recipient-Valid-Since' Header Field 187 The general constraints on syntax and placement of header fields in a 188 message are defined in Internet Message Format [MAIL]. 190 Using Augmented Backus-Naur Form [ABNF], the syntax for the field is: 192 rrvs = "Require-Recipient-Valid-Since:" addr-spec; date-time CRLF 194 "CFWS" is defined in Section 3.2.2, "date-time" is defined in Section 195 3.3, and "addr-spec" is defined in Section 3.4.1, of [MAIL]. 197 4. Use By Generators 199 When a message is generated whose content is sufficiently sensitive 200 that an author or author's Administrative Management Domain (ADMD; 201 see [EMAIL-ARCH]) wishes to protect against misdelivery using this 202 protocol, it determines for each recipient mailbox on the message a 203 timestamp at which it last confirmed ownership of that mailbox. It 204 then applies either the SMTP extension or the header field defined 205 above when sending the message to its destination. 207 Use of the SMTP extension provided here is preferable over the header 208 field method, since the additional detail about the relationship 209 between the message author and its intended recipient is at best a 210 property of the message transaction and not part of the message 211 itself. Further, SMTP parameters are not typically recorded in the 212 message upon delivery, so detail about the relationship between the 213 author or author's ADMD and the intended recipient are not recorded. 215 The header field mechanism is defined only to enable passage of the 216 request between and through systems that that do not implement the 217 SMTP extension. 219 5. Handling By Receivers 221 If a receiver implements the RRVS SMTP extension, then there are two 222 possible evaluation paths: 224 1. The sending client implements the extension, and so there was an 225 RRVS parameter on a RCPT TO command in the SMTP session; or 227 2. The sending client does not (or elected not to) implement the 228 extension, so the RRVS parameter was not present on the RCPT TO 229 commands in the SMTP session. 231 5.1. SMTP Extension Used 233 A receiving system that implements the SMTP extension declared above 234 and observes an RRVS parameter on a RCPT TO command checks whether 235 the current owner of the destination mailbox has held it 236 continuously, far enough back to inclue the given date-time, and 237 delivers it unless that check returns in the negative. Expressed as 238 a sequence of steps: 240 1. Ignore the parameter if the named mailbox is a role account as 241 listed in Mailbox Names For Common Services, Roles And Functions 242 [ROLES]. (See Section 6.) 244 2. Determine if the named address is serviced for local delivery. 245 If so, and if that address, has not been under continuous 246 ownership since the specified timestamp, return a 550 error to 247 the RCPT command. (See also Section 16.3.) 249 3. RECOMMENDED: If any Require-Recipient-Valid-Since header fields 250 are present and refer to the named address, remove them prior to 251 delivery or relaying. (See Section 5.2 for discussion.) 253 5.1.1. Relays 255 An MTA that does not make mailbox ownership checks, such as an MTA 256 positioned to do SMTP ingress at an organizational boundary, SHOULD 257 relay the RRVS extension parameter to the next MTA so that it can be 258 processed there. 260 See Section 9.2 for additional discussion. 262 5.2. Header Field Used 264 A receiving system that implements this specification, upon receiving 265 a message bearing a Require-Recipient-Valid-Since header field when 266 no corresponding RRVS SMTP extension was used, checks whether the 267 destination mailbox owner has held it continuously, far enough back 268 to include the given date-time, and delivers it unless that check 269 returns in the negative. Expressed as a sequence of steps: 271 1. Extract the set of Require-Recipient-Valid-Since fields from the 272 message for which no corresponding RRVS SMTP extension was used. 274 2. Discard any such fields that are syntactically invalid. 276 3. Discard any such fields that name a role account as listed in 277 [ROLES] (see Section 6). 279 4. Discard any such fields for which the "addr-spec" portion does 280 not match a current recipient, as listed in the RCPT TO commands 281 in the SMTP session. 283 5. Discard any such fields for which the "addr-spec" portion does 284 not refer to a mailbox handled for local delivery by this MTA. 286 6. For each field remaining, determine if the named address has been 287 under continuous ownership since the corresponding timestamp. If 288 it has not, reject the message. 290 7. RECOMMENDED: If local delivery is being performed, remove all 291 instances of this field prior to delivery to a mailbox; if the 292 message is being forwarded, remove those instances of this header 293 field that were not discarded by steps 1-4 above. 295 Handling proceeds normally upon completion of the above steps if 296 rejection has not been performed. 298 The final step is not mandatory as not all mail handling agents are 299 capable of stripping away header fields, and there are sometimes 300 reasons to keep the field intact such as debugging or presence of 301 digital signatures that might be invalidated by such a change. 303 If a message is to be rejected within the SMTP protocol itself 304 (versus generating a rejection message separately), servers 305 implementing this protocol SHOULD also implement the SMTP extension 306 described in Enhanced Mail System Status Codes [ESC] and use the 307 enhanced status codes described in Section 16.3 as appropriate. 309 Implementation by this method is expected to be transparent to non- 310 participants, since they would typically ignore this header field. 312 This header field is not normally added to a message that is 313 addressed to multiple recipients. The intended use of this field 314 involves an author seeking to protect transactional or otherwise 315 sensitive data intended for a single recipient, and thus generating 316 independent messages for each individual recipient is normal 317 practice. See Section 8 for further discussion. 319 6. Role Accounts 321 It is necessary not to interfere with delivery of messages to role 322 mailboxes (see [ROLES]), but it could be useful to indicate to users 323 handling those mailboxes that a change of ownership might have taken 324 place where doing so is possible. 326 7. Relaying Without RRVS Support 328 When a message is received using the SMTP extension defined here but 329 will not be delivered locally (that is, it needs to be relayed 330 further), the MTA to which the relay will take place might not be 331 compliant with this specification. Where the MTA in possession of 332 the message observes it is going to relay the message to an MTA that 333 does not advertise this extension, it needs to choose one of the 334 following actions: 336 1. Decline to relay the message further, preferably generating a 337 Delivery Status Notification [DSN] to indicate failure 338 (RECOMMENDED); 340 2. Downgrade the data thus provided in the SMTP extension to a 341 header field, as described in Section 7.1 below (RECOMMENDED when 342 the previous option is not available); or 344 3. Silently continue with delivery, dropping the protection offered 345 by this protocol. 347 Using other than the first option needs to be avoided unless there is 348 specific knowledge that further relaying with the degraded 349 protections thus provided does not introduce undue risk. 351 7.1. Header Field Conversion 353 If an SMTP server ("B") that has received mailbox timestamps from a 354 client ("A") using this extension but then needs to relay the 355 corresponding message on to another server ("C") (thereby becoming a 356 client), but "C" does not advertise the SMTP extension and "B" elects 357 not to reject the message, "B" SHOULD add Require-Recipient-Valid- 358 Since header fields matching each mailbox to which relaying is being 359 done, and the corresponding valid-since timestamp for each. 361 Similarly, if "B" receives a message bearing one or more Require- 362 Recipient-Valid-Since header fields from "A" for which it must now 363 relay the message, and "C" advertises support for the SMTP extension, 364 "B" SHOULD delete the header field(s) and instead relay this 365 information by making use of the SMTP extension. 367 8. Header Field with Multiple Recipients 369 Numerous issues arise when using the header field form of this 370 extension, particularly when multiple recipients are specified for a 371 single message resulting in one multiple fields each with a distinct 372 address and timestamp. 374 Because of the nature of SMTP, a message bearing a multiplicity of 375 Require-Recipient-Valid-Since header fields could result in a single 376 delivery attempt for multiple recipients (in particular, if two of 377 the recipients are handled by the same server), and if any one of 378 them fails the test, the delivery fails to all of them; it then 379 becomes necessary to do one of the following: 381 o reject the message on completion of the DATA phase of the SMTP 382 session, which is a rejection of delivery to all recipients; or 384 o accept the message on completion of DATA, and then generate a 385 Delivery Status Notification [DSN] message for each of the failed 386 recipients 388 Additional complexity arises when a message is sent to two 389 recipients, "A" and "B", presumably with different timestamps, both 390 of which are then redirected to a common address "C". The author is 391 not necessarily aware of the current or past ownership of mailbox 392 "C", or indeed that "A" and/or "B" have been redirected. This might 393 result in either or both of the two deliveries failing at "C", which 394 is likely to confuse the message author, who (as far as the author is 395 aware) never sent a message to "C" in the first place. 397 9. Special Use Addresses 399 In [DSN-SMTP], an SMTP extension was defined to allow SMTP clients to 400 request generation of DSNs, and related information to allow such 401 reports to be maximally useful. Section 5.2.7 of that document 402 explored the issue of the use of that extension where the recipient 403 is a mailing list. This extension has similar concerns which are 404 covered here following that document as a model. 406 9.1. Mailing Lists 408 Delivery to a mailing list service is considered a final delivery. 409 Where this protocol is in use, it is evaluated as per any normal 410 delivery: If the same mailing list has been operating in place of the 411 specified recipient mailbox since at least the timestamp given as the 412 RRVS parameter, the message is delivered to the list service 413 normally, and is otherwise not delivered. However, the MTA passing 414 the message to the list service does not convey the RRVS parameter in 415 either form (SMTP extension or header field) to the list service. 416 The emission of a message from the list service to its subscribers 417 constitutes a new message not covered by the previous transaction. 419 9.2. Single-Recipient Alaises 421 Upon delivery of an RRVS-protected message to an alias (acting in 422 place of a mailbox) that results in relaying of the message to a 423 single other destination, the usual RRVS check is performed. The 424 continuous ownership test here might succeed if a conventional user 425 inbox was replaced with an alias on behalf of that same user, and 426 this information is recorded someplace. If the message is thus 427 accepted, the relaying MTA can choose to do one of the following: 429 1. Do not include an RRVS parameter or header field when relaying to 430 the new address. (RECOMMENDED) 432 2. If, however, the relaying system knows the time when the alias 433 was established, it MAY add an RRVS parameter for the new target 434 address that includes that time. 436 3. If a confirmation of the new destination was done, it MAY add an 437 RRVS parameter for the new target address that includes that 438 time. 440 There is risk and additional administrative burden associated with 441 all but the first option in that list which are believed to make them 442 not worth pursuing. 444 9.3. Multiple-Recipient Aliases 446 Upon delivery of an RRVS-protected message to an alias (acting in 447 place of a mailbox) that results in relaying of the message to 448 multiple other destinations, the usual RRVS check is performed as in 449 Section 9.2. The MTA expanding such an alias then decides which of 450 the options enumerated in that section is to be applied for each new 451 recipient. 453 9.4. Confidential Forwarding Addresses 455 In the above cases, the original author could receive message 456 rejections, such as DSNs, from the ultimate destination, where the 457 RRVS check (or indeed, any other) fails and rejection is warranted. 458 This can reveal the existence of a forwarding relationship between 459 the original intended recipient and the actual final recipient. 461 Where this is a concern, the initial delivery attempt is to be 462 treated like a mailing list delivery, with RRVS evaluation done and 463 then all RRVS information removed from the message prior to relaying 464 it to its true destination. 466 9.5. Suggested Mailing List Enhancements 468 Mailing list services could store the timestamp at which a subscriber 469 was added to a mailing list. This specification could then be used 470 in conjunction with that information in order to restrict list 471 traffic to the original subscriber, rather than a different person 472 now in possession of an address under which the original subscriber 473 was added to the list. Upon receiving a rejection caused by this 474 specification, the list service can remove that address from further 475 distribution. 477 A mailing list service that receives a message containing the header 478 field defined here needs to remove it from the message prior to 479 redistributing it, limiting exposure of information regarding the 480 relationship between the message's author and the mailing list. 482 10. Discussion 484 To further obscure account details on the receiving system, the 485 receiver SHOULD ignore the SMTP extension or the header field if the 486 address specified has had one continuous owner since it was created, 487 regardless of the purported confirmation date of the address. This 488 is further discussed in Section 14. 490 The presence of the intended address in the field content supports 491 the case where a message bearing this header field is forwarded. The 492 specific use case is as follows: 494 1. A user subscribes to a service "S" on date "D" and confirms an 495 email address at the user's current location, "A"; 497 2. At some later date, the user intends to leave the current 498 location, and thus creates a new mailbox elsewhere, at "B"; 500 3. The user replaces address "A" with forwarding to "B"; 502 4. "S" constructs a message to "A" claiming that address was valid 503 at date "D" and sends it to "A"; 505 5. The receiving MTA at "A" determines that the forwarding in effect 506 was created by the same party that owned the mailbox there, and 507 thus concludes the continuous ownership test has been satisfied; 509 6. If possible, "A" removes this header field from the message, and 510 in either case, forwards it to "B"; 512 7. On receipt at "B", either the header field has been removed, or 513 the header field does not refer to a current envelope recipient, 514 and in either case delivers the message. 516 SMTP has never required any correspondence between addresses in the 517 RFC5321.MailFrom and RFC5321.RcptTo parameters and header fields of a 518 message, which is why the header field defined here contains the 519 recipient address to which the timestamp applies. 521 11. Continuous Ownership 523 Determining continuous ownership of a mailbox is a local matter at 524 the receiving site. In particular, the only possible answers to the 525 continuous-ownership-since question are "yes", "no", and "unknown"; 526 the action to be taken in the "unknown" case is a matter of local 527 policy. 529 For example, when control of a domain name is transferred, the new 530 domain owner might be unable to determine whether the owner of the 531 subject address has been under continuous ownership since the stated 532 date if the mailbox history is not also transferred (or was not 533 previously maintained). 535 It will also be "unknown" if whatever database contains mailbox 536 ownership data is temporarily unavailable at the time a message 537 arrives for delivery. In this case, typical SMTP temporary failure 538 handling is appropriate. 540 12. Authentication-Results Definitions 542 [AUTHRES] defines a mechanism for indicating, via a header field, the 543 results of message authentication checks. Section 16 registers RRVS 544 as a new method that can be reported in this way, and corresponding 545 result names. The possible result names and their meanings are as 546 follows: 548 none: The message had no recipient mailbox timestamp associated with 549 it, either via the SMTP extension or header field method; this 550 protocol was not in use. 552 unknown: At least one form of this protocol was in use, but 553 continuous ownership of the recipient mailbox could not be 554 determined. 556 temperror: At least one form of this protocol was in use, but some 557 kind of error occurred during evaluation that was transient in 558 nature; a later retry will likely produce a final result. 560 permerror: At least one form of this protocol was in use, but some 561 kind of error occurred during evaluation that was not recoverable; 562 a later retry will not likely produce a final result. 564 pass: At least one form of this protocol was in use, and the 565 destination mailbox was confirmed to have been under constant 566 ownership since the timestamp thus provided. 568 fail: At least one form of this protocol was in use, and the 569 destination mailbox was confirmed not to have been under constant 570 ownership since the timestamp thus provided. 572 Where multiple recipients are present on a message, multiple results 573 can be reported using the mechanism described in [AUTHRES]. 575 13. Examples 577 In the following examples, "C:" indicates data sent by an SMTP 578 client, and "S:" indicates responses by the SMTP server. Message 579 content is CRLF terminated, though these are omitted here for ease of 580 reading. 582 13.1. SMTP Extension Example 584 C: [connection established] 585 S: 220 server.example.com ESMTP ready 586 C: EHLO client.example.net 587 S: 250-server.example.com 588 S: 250 RRVS 589 C: MAIL FROM: 590 S: 250 OK 591 C: RCPT TO: RRVS=1381993177 592 S: 550 5.7.15 receiver@example.com is no longer valid 593 C: QUIT 594 S: 221 So long! 596 13.2. Header Field Example 598 C: [connection established] 599 S: 220 server.example.com ESMTP ready 600 C: HELO client.example.net 601 S: 250 server.example.com 602 C: MAIL FROM: 603 S: 250 OK 604 C: RCPT TO: 605 S: 250 OK 606 C: DATA 607 S: 354 Ready for message content 608 C: From: Mister Sender 609 To: Miss Receiver 610 Subject: Are you still there? 611 Date: Fri, 28 Jun 2013 18:01:01 +0200 612 Require-Recipient-Valid-Since: receiver@example.com; 613 Sat, 1 Jun 2013 09:23:01 -0700 615 Are you still there? 616 . 617 S: 550 5.7.15 receiver@example.com is no longer valid 618 C: QUIT 619 S: 221 So long! 621 If an authentication scheme is applied to claim the added header 622 field is valid, but the scheme fails, a receiver might reject the 623 message with an SMTP reply such as: 625 S: 550-5.7.7 Use of Require-Recipient-Valid-Since header 626 S: 550 field requires a valid signature 628 14. Security Considerations 630 14.1. Abuse Countermeasures 632 The response of a server implementing this protocol can disclose 633 information about the age of existing email mailbox. Implementation 634 of countermeasures against probing attacks is advised. For example, 635 an operator could track appearance of this field with respect to a 636 particular mailbox and observe the timestamps being submitted for 637 testing; if it appears a variety of timestamps is being tried against 638 a single mailbox in short order, the field could be ignored and the 639 message silently discarded. This concern is discussed further in 640 Section 15. 642 14.2. Suggested Use Restrictions 644 If the mailbox named in the field is known to have had only a single 645 continuous owner since creation, or not to have existed at all (under 646 any owner) prior to the date specified in the field, then the field 647 can be silently ignored and normal message handling applied so that 648 this information is not disclosed. Such fields are likely the 649 product of either gross error or an attack. 651 A message author using this specification might restrict inclusion of 652 the header field such that it is only done for recipients known also 653 to implement this specification, in order to reduce the possibility 654 of revealing information about the relationship between the author 655 and the mailbox. 657 If ownership of an entire domain is transferred, the new owner may 658 not know what addresses were assigned in the past by the prior owner. 659 Hence, no address can be known not to have had a single owner, or to 660 have existed (or not) at all. 662 15. Privacy Considerations 664 15.1. Probing Attacks 666 As described above, use of this extension or header field in probing 667 attacks can disclose information about the history of the mailbox. 668 The harm that can be done by leaking any kind of private information 669 is difficult to predict, so it is prudent to be sensitive to this 670 sort of disclosure, either inadvertently or in response to probing by 671 an attacker. It bears restating, then, that implementing 672 countermeasures to abuse of this capability needs strong 673 consideration. 675 That some MSPs allow for expiration of account names when they have 676 been unused for a protracted period forces a choice between two 677 potential types of privacy vulnerabilities, one of which presents 678 significantly greater threats to users than the other. Automatically 679 generated mail is often used to convey authentication credentials 680 that can potentially provide access to extremely sensitive 681 information. Supplying such credentials to the wrong party after a 682 mailbox ownership change could allow the previous owner's data to be 683 exposed without his or her authorization or knowledge. In contrast, 684 the information that may be exposed to a third party via the proposal 685 in this document is limited to information about the mailbox history. 686 Given that MSPs have chosen to allow transfers of mailbox ownership 687 without the prior owner's involvement, the information leakage from 688 the extensions specified here creates far fewer risks than the 689 potential for delivering mail to the wrong party. 691 15.2. Envelope Recipients 693 The email To and Cc header fields are not required to be populated 694 with addresses that match the envelope recipient set, and Cc may even 695 be absent. However, the algorithm in Section 3 requires that this 696 header field contain a match for an envelope recipient in order to be 697 actionable. As such, use of this specification can reveal some or 698 all of the original intended recipient set to any party that can see 699 the message in transit or upon delivery. 701 For a message destined to a single recipient, this is unlikely to be 702 a concern, which is one of the reasons use of this specification on 703 multi-recipient messages is discouraged. 705 15.3. Risks with Use of Header Field 707 MTAs might not implement the recommendation to remove the header 708 field defined here, either out of ignorance or due to error. Since 709 user agents often do not render all of the header fields present, the 710 message could be forwarded to another party that would then 711 inadvertently have the content of this header field. 713 16. IANA Considerations 714 16.1. SMTP Extension Registration 716 Section 2.2.2 of [MAIL] sets out the procedure for registering a new 717 SMTP extension. IANA is requested to register the SMTP extension 718 using the details provided in Section 3.1 of this document. 720 16.2. Header Field Registration 722 IANA is requested to add the following entry to the Permanent Message 723 Header Field registry, as per the procedure found in [IANA-HEADERS]: 725 Header field name: Require-Recipient-Valid-Since 726 Applicable protocol: mail ([MAIL]) 727 Status: Standard 728 Author/Change controller: IETF 729 Specification document(s): [this document] 730 Related information: 731 Requesting review of any proposed changes and additions to 732 this field is recommended. 734 16.3. Enhanced Status Code Registration 736 IANA is requested to register the following in the SMTP Enhanced 737 Status Codes registry: 739 Code: X.7.15 740 Sample Text: Mailbox owner has changed 741 Associated basic status code: 5 742 Description: This status code is returned when a message is 743 received with a Require-Recipient-Valid-Since 744 field or RRVS extension and the receiving 745 system is able to determine that the intended 746 recipient mailbox has not been under 747 continuous ownership since the specified date. 748 Reference: [this document] 749 Submitter: M. Kucherawy 750 Change controller: IESG 751 Code: X.7.16 752 Sample Text: Domain owner has changed 753 Associated basic status code: 5 754 Description: This status code is returned when a message is 755 received with a Require-Recipient-Valid-Since 756 field or RRVS extension and the receiving 757 system wishes to disclose that the owner of 758 the domain name of the recipient has changed 759 since the specified date. 760 Reference: [this document] 761 Submitter: M. Kucherawy 762 Change controller: IESG 764 16.4. Authentication Results Registration 766 IANA is requested to register the following in the "Email 767 Authentication Methods" Registry: 769 Method: rrvs 771 Specifying Document: [this document] 773 ptype: smtp 775 Property: rcptto 777 Value: envelope recipient 779 Status: active 781 Version: 1 783 IANA is also requested to register the following in the "Email 784 Authentication Result Names" Registry: 786 Codes: none, unknown, temperror, permerror, pass, fail 788 Defined: [this document] 790 Auth Method(s): rrvs 792 Meaning: Section 12 of [this document] 794 Status: active 796 17. References 797 17.1. Normative References 799 [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for 800 Syntax Specifications: ABNF", RFC 5234, January 2008. 802 [DATETIME] Klyne, G. and C. Newman, "Date and Time on the 803 Internet: Timestamps", RFC 3339, July 2002. 805 [IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul, 806 "Registration Procedures for Message Header Fields", 807 BCP 90, RFC 3864, September 2004. 809 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate 810 Requirement Levels", BCP 14, RFC 2119, March 1997. 812 [MAIL] Resnick, P., "Internet Message Format", RFC 5322, 813 October 2008. 815 [ROLES] Crocker, D., "Mailbox Names For Common Services, 816 Roles And Functions", RFC 2142, May 1997. 818 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", 819 RFC 5321, October 2008. 821 17.2. Informative References 823 [AUTHRES] Kucherawy, M., "Message Header Field for Indicating 824 Message Authentication Status", RFC 7001, 825 September 2013. 827 [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message 828 Format for Delivery Status Notifications", RFC 3464, 829 January 2003. 831 [DSN-SMTP] Moore, K., "Simple Mail Transfer Protocol (SMTP) 832 Service Extension for Delivery Status Notifications 833 (DSNs)", RFC 3461, January 2003. 835 [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, 836 July 2009. 838 [ESC] Vaudreuil, G., "Enhanced Mail System Status Codes", 839 RFC 3463, January 2003. 841 Appendix A. Acknowledgments 843 Erling Ellingsen proposed the idea. 845 Reviews and comments were provided by Michael Adkins, Kurt Andersen, 846 Alissa Cooper, Dave Cridland, Dave Crocker, Ned Freed, John Levine, 847 Alexey Melnikov, Hector Santos, Gregg Stefancik, Ed Zayas, (others) 849 Authors' Addresses 851 William J. Mills 852 Yahoo! Inc. 854 EMail: wmills_92105@yahoo.com 856 Murray S. Kucherawy 857 Facebook, Inc. 858 1 Hacker Way 859 Menlo Park, CA 94025 860 USA 862 EMail: msk@fb.com