Network Working Group W. Mills Internet-Draft Yahoo! Inc. Intended status: Standards Track M. Kucherawy Expires: May 10, 2014 Facebook, Inc. November 6, 2013 The "Require Recipient Valid Since" SMTP Service Extension draft-ietf-appsawg-rrvs-header-field-02 Abstract This document defines an extension for the Simple Mail Transfer Protocol, called RRVS (for "Require Recipient Valid Since"), to provide a method for senders to indicate to receivers the time when the sender last confirmed the ownership of the target mailbox. This can be used to detect changes of mailbox ownership, and thus prevent mail from being delivered to the wrong party. The intended use of this facility is on automatically generated messages that might contain sensitive information, though it may also be useful in other applications. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on May 10, 2014. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of Mills & Kucherawy Expires May 10, 2014 [Page 1] Internet-Draft Require-Recipient-Valid-Since November 2013 publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Description . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. The 'RRVS' SMTP Extension . . . . . . . . . . . . . . . . . 4 4. Handling By Receivers . . . . . . . . . . . . . . . . . . . . . 4 4.1. SMTP Extension Offered . . . . . . . . . . . . . . . . . . 4 4.2. SMTP Extension Not Offered . . . . . . . . . . . . . . . . 5 5. Role Accounts . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Use with Mailing Lists . . . . . . . . . . . . . . . . . . . . 5 7. Continuous Ownership . . . . . . . . . . . . . . . . . . . . . 6 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. SMTP Extension Example . . . . . . . . . . . . . . . . . . 6 9. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 9.1. Abuse Countermeasures . . . . . . . . . . . . . . . . . . . 6 9.2. Suggested Use Restrictions . . . . . . . . . . . . . . . . 7 10. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 7 10.1. Probing Attacks . . . . . . . . . . . . . . . . . . . . . . 7 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 11.1. SMTP Extension Registration . . . . . . . . . . . . . . . . 8 11.2. Enhanced Status Code Registration . . . . . . . . . . . . . 8 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 12.1. Normative References . . . . . . . . . . . . . . . . . . . 8 12.2. Informative References . . . . . . . . . . . . . . . . . . 9 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 9 Mills & Kucherawy Expires May 10, 2014 [Page 2] Internet-Draft Require-Recipient-Valid-Since November 2013 1. Introduction Email addresses sometimes get reassigned to a different person. For example, employment changes at a company can cause an address used for an ex-employee to be assigned to a new employee, or a mail service provider (MSP) might expire an account and then let someone else register for the local-part that was previously used. Those who sent mail to the previous owner of an address might not know that it has been reassigned. This can lead to the sending of email to the correct address, but the wrong recipient. What is needed is a way to indicate an attribute of the recipient that will distinguish between the previous owner of an address and its current owner, if they are different. Further, this needs to be done in a way that respects privacy. The mechanism specified here allows the sender of the mail to indicate how "old" the address assignment is expected to be. In effect, the sender is saying, "The person to whom I am sending to had this address assigned to as far back as this date-time." A receiving system can then compare this information against the date and time the address was assigned to its current user. If the assignment was made later than the date-time indicated in the message, there is a good chance the current user of the address is not the intended recipient. The receiving system can then choose to prevent delivery and, possibly, to notify the original sender of the problem. The primary application is automatically generated messages rather than user-authored content, though it may be useful in other contexts. 2. Definitions For a description of the email architecture, consult [EMAIL-ARCH]. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS]. 3. Description To address the problem described above, a mail sending client needs to indicate to the server to which it is connecting that there is an expectation that the destination of the message has been under continuous ownership since some date-time, presumably the most recent time the message author had confirmed its understanding of who owned that mailbox. The mechanism defined here is an extension to the Simple Mail Transfer Protocol [SMTP] for use between a client and Mills & Kucherawy Expires May 10, 2014 [Page 3] Internet-Draft Require-Recipient-Valid-Since November 2013 server that both implement the extension. The SMTP extenion is called "RRVS" (Require Recipient Valid Since), and adds a parameter to the SMTP "RCPT" command that indicates the most recent date and time when the message author believed the destination mailbox to be under the continuous ownership (see Section 7) of a specific party. Presumably there has been some confirmation process applied to establish this ownership; however, the method of making such determinations is a local matter and outside the scope of this document. 3.1. The 'RRVS' SMTP Extension Extensions to SMTP are described in Section 2.2 of [SMTP]. The name of the extension is "RRVS", an abbreviation of "Require Recipient Valid Since". Servers implementing the SMTP extension advertise an additional EHLO keyword of "RRVS", which has no associated parameters, introduces no new SMTP verbs, and does not alter the MAIL verb. An MTA implementing RRVS can transmit or accept a new parameter to the RCPT command. The new parameter is "RRVS", which takes a value that is a timestamp expressed as a "date-time" as defiend in [DATETIME], with the added restriction that a "time-secfrac" MUST NOT be used. Accordingly, this extension increases the maximum command length for the RCPT verb by 31 characters. The meaning of this extension, when used, is described in Section 4.1. 4. Handling By Receivers If a receiver implements the RRVS SMTP extension, then there are two possible evaluation paths: 1. The sending client implements the extension, and so there was an RRVS parameter on a RCPT TO command in the SMTP session; or 2. The sending client does not (or elected not to) implement the extension, so the RRVS parameter was not present on the RCPT TO commands in the SMTP session. 4.1. SMTP Extension Offered A receiving system that implements the SMTP extension declared above and observes an RRVS parameter on a RCPT TO command checks whether the current owner of the destination mailbox has held it Mills & Kucherawy Expires May 10, 2014 [Page 4] Internet-Draft Require-Recipient-Valid-Since November 2013 continuously, far enough back to inclue the given date-time, and delivers it unless that check returns in the negative. Expressed as a sequence of steps: 1. Ignore the parameter if the named mailbox is a role account as listed in Mailbox Names For Common Services, Roles And Functions [ROLES]. (See Section 5.) 2. Determine if the named address is serviced for local delivery. If so, and if that address has not been under continuous ownership since the specified timestamp, return a 550 error to the RCPT command. If the server implements Enhanced Mail System Status Codes [ESC], it SHOULD use the code defined in Section 11.2. 3. Otherwise, continue with normal delivery. To further obscure account details on the receiving system, the receiver SHOULD ignore the SMTP extension if the address specified has had one continuous owner since it was created, regardless of the purported confirmation date of the address. This is further discussed in Section 9. 4.2. SMTP Extension Not Offered For a message being sent that includes content meant to be protected by this extension, the client MUST NOT continue to deliver a message to a server where the server does not advertise the RRVS SMTP extension. 5. Role Accounts It is necessary not to interfere with delivery of messages to role mailboxes (see [ROLES]), but it could be useful to indicate to users handling those mailboxes that a change of ownership might have taken place where such notification is possible. 6. Use with Mailing Lists Mailing list services can store the timestamp at which a subscriber was added to a mailing list. This specification can be used in conjunction with that information in order to restrict traffic to the original subscriber, rather than a different person now in possession of an address under which the original subscriber registered. Upon receiving a rejection caused by this specification, the list service can remove that address from further distribution. Mills & Kucherawy Expires May 10, 2014 [Page 5] Internet-Draft Require-Recipient-Valid-Since November 2013 7. Continuous Ownership Determining continuous ownership of a mailbox is a local matter at the receiving site. In particular, the only possible answers to the continuous-ownership-since question are "yes", "no", and "unknown"; the action to be taken in the "unknown" case is a matter of local policy. For example, when control of a domain name is transferred, the new domain owner might be unable to determine whether the owner of the subject address has been under continuous ownership since the stated date if the mailbox history is not also transferred (or was not previously maintained). It will also be "unknown" if whatever database contains mailbox ownership data is temporarily unavailable at the time a message arrives for delivery. In this case, typical SMTP temporary failure handling is appropriate. 8. Examples In the following examples, "C:" indicates data sent by an SMTP client, and "S:" indicates responses by the SMTP server. Message content is CRLF terminated, though these are omitted here for ease of reading. 8.1. SMTP Extension Example C: [connection established] S: 220 server.example.com ESMTP ready C: EHLO client.example.net S: 250-server.example.com S: 250 RRVS C: MAIL FROM: S: 250 OK C: RCPT TO: RRVS=2013-12-31T23:59:59 S: 550 5.7.15 receiver@example.com is no longer valid C: QUIT S: 221 So long! 9. Security Considerations 9.1. Abuse Countermeasures The response of a server implementing this protocol can disclose information about the age of existing email mailbox. Implementation of countermeasures against probing attacks is advised. For example, Mills & Kucherawy Expires May 10, 2014 [Page 6] Internet-Draft Require-Recipient-Valid-Since November 2013 an operator could track appearance of this extension with respect to a particular mailbox and observe the timestamps being submitted for testing; if it appears a variety of timestamps is being tried against a single mailbox in short order, the extension could be ignored and the message silently discarded. This concern is discussed further in Section 10. 9.2. Suggested Use Restrictions If the mailbox named in the RCPT TO command is known to have had only a single continuous owner since creation, or not to have existed at all (under any owner) prior to the time specified in the parameter, then the field can be silently ignored and normal message handling applied so that this information is not disclosed. Such parameters are likely the product of either gross error or an attack. A message author using this specification might restrict use of the extension such that it is only done for recipients known also to implement this specification, in order to reduce the possibility of revealing information about the relationship between the author and the mailbox. If ownership of an entire domain is transferred, the new owner may not know what addresses were assigned in the past by the prior owner. Hence, no address can be known not to have had a single owner, or to have existed (or not) at all. 10. Privacy Considerations 10.1. Probing Attacks As described above, use of this extension in probing attacks can disclose information about the history of the mailbox. The harm that can be done by leaking any kind of private information is difficult to predict, so it is prudent to be sensitive to this sort of disclosure, be it inadvertently or in response to probing by an attacker. It bears restating, then, that implementing countermeasures to abuse of this capability needs strong consideration. That some MSPs allow for expiration of account names when they have been unused for a protracted period forces a choice between two potential types of privacy vulnerabilities, one of which presents significantly greater threats to users than the other. Automatically generated mail is often used to convey authentication credentials that can potentially provide access to extremely sensitive information. Supplying such credentials to the wrong party after a mailbox ownership change could allow the previous owner's data to be Mills & Kucherawy Expires May 10, 2014 [Page 7] Internet-Draft Require-Recipient-Valid-Since November 2013 exposed without his or her authorization or knowledge. In contrast, the information that may be exposed to a third party via the proposal in this document is limited to information about the mailbox history. Given that MSPs have chosen to allow transfers of mailbox ownership without the prior owner's involvement, the information leakage from the header field specified here creates far fewer risks than the potential for delivering mail to the wrong party. 11. IANA Considerations 11.1. SMTP Extension Registration IANA is requested to register the SMTP extension described in Section 3.1. 11.2. Enhanced Status Code Registration IANA is requested to register the following in the SMTP Enhanced Status Codes registry: Code: X.7.15 Sample Text: Mailbox owner has changed Associated basic status code: 5 Description: This status code is returned when a message is received with a Require-Recipient-Valid-Since field and the receiving system is able to determine that the intended recipient mailbox has not been under continuous ownership since the specified date. Reference: [this document] Submitter: M. Kucherawy Change controller: IESG 12. References 12.1. Normative References [DATETIME] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [ROLES] Crocker, D., "Mailbox Names For Common Services, Roles And Functions", RFC 2142, May 1997. [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. Mills & Kucherawy Expires May 10, 2014 [Page 8] Internet-Draft Require-Recipient-Valid-Since November 2013 12.2. Informative References [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. [ESC] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463, January 2003. Appendix A. Acknowledgments Erling Ellingsen proposed the idea. Reviews and comments were provided by Michael Adkins, Kurt Andersen, Alissa Cooper, Dave Cridland, Dave Crocker, Ned Freed, John Levine, Hector Santos, Gregg Stefancik, Ed Zayas, (others) Authors' Addresses William J. Mills Yahoo! Inc. EMail: wmills_92105@yahoo.com Murray S. Kucherawy Facebook, Inc. 1 Hacker Way Menlo Park, CA 94025 USA EMail: msk@fb.com Mills & Kucherawy Expires May 10, 2014 [Page 9]