Request for Comments: 7293
Category: Standards Track
The Require-Recipient-Valid-Since Header Field
and SMTP Service Extension
This document defines an extension for the Simple Mail Transfer Protocol (SMTP) called "RRVS" to provide a method for senders to indicate to receivers a point in time when the ownership of the target mailbox was known to the sender. This can be used to detect changes of mailbox ownership and thus prevent mail from being delivered to the wrong party. This document also defines a header field called "Require-Recipient-Valid-Since" that can be used to tunnel the request through servers that do not support the extension.
The intended use of these facilities is on automatically generated messages, such as account statements or password change instructions, that might contain sensitive information, though it may also be useful in other applications.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7293.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Description . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. The "RRVS" SMTP Extension . . . . . . . . . . . . . . . . 5 3.2. The "Require-Recipient-Valid-Since" Header Field . . . . 5 3.3. Timestamps . . . . . . . . . . . . . . . . . . . . . . . 6 4. Use By Generators . . . . . . . . . . . . . . . . . . . . . . 6 5. Handling By Receivers . . . . . . . . . . . . . . . . . . . . 7 5.1. SMTP Extension Used . . . . . . . . . . . . . . . . . . . 7 5.1.1. Relays . . . . . . . . . . . . . . . . . . . . . . . 8 5.2. Header Field Used . . . . . . . . . . . . . . . . . . . . 9 5.2.1. Design Choices . . . . . . . . . . . . . . . . . . . 10 5.3. Clock Synchronization . . . . . . . . . . . . . . . . . . 11 6. Relaying without RRVS Support . . . . . . . . . . . . . . . . 11 6.1. Header Field Conversion . . . . . . . . . . . . . . . . . 11 7. Header Field with Multiple Recipients . . . . . . . . . . . . 12 8. Special Use Addresses . . . . . . . . . . . . . . . . . . . . 13 8.1. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 13 8.2. Single-Recipient Aliases . . . . . . . . . . . . . . . . 13 8.3. Multiple-Recipient Aliases . . . . . . . . . . . . . . . 14 8.4. Confidential Forwarding Addresses . . . . . . . . . . . . 14 8.5. Suggested Mailing List Enhancements . . . . . . . . . . . 14 9. Continuous Ownership . . . . . . . . . . . . . . . . . . . . 15 10. Digital Signatures . . . . . . . . . . . . . . . . . . . . . 15 11. Authentication-Results Definitions . . . . . . . . . . . . . 16 12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16 12.1. SMTP Extension Example . . . . . . . . . . . . . . . . . 17 12.2. Header Field Example . . . . . . . . . . . . . . . . . . 17 12.3. Authentication-Results Example . . . . . . . . . . . . . 17 13. Security Considerations . . . . . . . . . . . . . . . . . . . 18 13.1. Abuse Countermeasures . . . . . . . . . . . . . . . . . 18 13.2. Suggested Use Restrictions . . . . . . . . . . . . . . . 18 13.3. False Sense of Security . . . . . . . . . . . . . . . . 18 13.4. Reassignment of Mailboxes . . . . . . . . . . . . . . . 19 14. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19 14.1. The Tradeoff . . . . . . . . . . . . . . . . . . . . . . 19 14.2. Probing Attacks . . . . . . . . . . . . . . . . . . . . 19 14.3. Envelope Recipients . . . . . . . . . . . . . . . . . . 20 14.4. Risks with Use . . . . . . . . . . . . . . . . . . . . . 20 15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 15.1. SMTP Extension Registration . . . . . . . . . . . . . . 20 15.2. Header Field Registration . . . . . . . . . . . . . . . 20 15.3. Enhanced Status Code Registration . . . . . . . . . . . 21 15.4. Authentication Results Registration . . . . . . . . . . 22 16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 17.1. Normative References . . . . . . . . . . . . . . . . . . 23 17.2. Informative References . . . . . . . . . . . . . . . . . 23
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. This situation is of particular concern with transactional mail related to purchases, online accounts, and the like.
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 mechanisms specified here allow the sender of the mail to indicate how "old" the address assignment is expected to be. In effect, the sender is saying, "I know that the intended recipient was using this address at this point in time. I don't want this message delivered to anyone else". A receiving system can then compare this information against the point in time at which the address was assigned to its current user. If the assignment was made later than the point in time indicated in the message, there is a good chance the current user of the address is not the correct recipient. The receiving system can then prevent delivery and, preferably, notify the original sender of the problem.
The primary application is transactional mail (such as account information, password change requests, and other automatically generated messages) rather than user-authored content. However, it may be useful in other contexts; for example, a personal address book could record the time an email address was added to it, and thus use that time with this extension.
Because the use cases for this extension are strongly tied to privacy issues, attention to the Security Considerations (Section 13) and the Privacy Considerations (Section 14) is particularly important. Note, especially, the limitation described in Section 13.3.
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].
To address the problem described in Section 1, a mail-sending client (usually an automated agent) needs to indicate to the server to which it is connecting that it expects the destination address of the message to have been under continuous ownership (see Section 9) since a specified point time. That specified time would be the time when the intended recipient gave the address to the message author, or perhaps a more recent time when the intended recipient reconfirmed ownership of the address with the sender.
Two mechanisms are defined here: an extension to the Simple Mail Transfer Protocol [SMTP] and a new message header field. The SMTP extension permits strong assurance of enforcement by confirming support at each handling step for a message and the option to demand support at all nodes in the handling path of the message (and returning of the message to the originator otherwise). The header field can be used when the Message Delivery Agent (MDA) supports this function, but an intermediary system between the sending system and the MDA does not. However, the header field does not provide the same strong assurance described above and is more prone to exposure of private information (see Section 14.1).
The SMTP extension is called "RRVS" and adds a parameter to the SMTP "RCPT" command that indicates the most recent point in time when the message author believed the destination mailbox to be under the continuous ownership of a specific party. Similarly, the "Require- Recipient-Valid-Since" header field includes an intended recipient coupled with a timestamp indicating the same thing.
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 commands, and does not alter the MAIL command.
A Message Transfer Agent (MTA) implementing RRVS can transmit or accept one new parameter to the RCPT command. An MDA can also accept this new parameter. The parameter is "RRVS", and the value is a timestamp expressed as "date-time" as defined in [DATETIME], with the added restriction that a "time-secfrac" MUST NOT be used. The timestamp MAY optionally be followed by a semicolon character and a letter (known as the "no-support action"), indicating the action to be taken when a downstream MTA is discovered that does not support the extension. Valid actions are "R" (reject; the default) and "C" (continue).
Formally, the new parameter and its value are defined as follows:
rrvs-param = "RRVS=" date-time [ ";" ( "C" / "R" ) ]
Accordingly, this extension increases the maximum command length for the RCPT command by 33 characters.
The meaning of this extension, when used, is described in Section 5.1.
3.2. The "Require-Recipient-Valid-Since" Header Field
The general constraints on syntax and placement of header fields in a message are defined in "Internet Message Format" [MAIL].
Using Augmented Backus-Naur Form [ABNF], the syntax for the field is:
rrvs = "Require-Recipient-Valid-Since:" addr-spec ";" date-time
"date-time" is defined in Section 3.3, and "addr-spec" is defined in Section 3.4.1 of [MAIL].
The header field version of this protocol has a different format for the date and time expression than the SMTP extension does. This is because message header fields use a format to express date and time that is specific to message header fields, and this is consistent with that usage.
Use of both date and time is done to be consistent with how current implementations typically store the timestamp and to make it easy to include the time zone. In practice, granularity beyond the date may or may not be useful.
4. Use By Generators
When a message is generated whose content is sufficiently sensitive that an author or author's ADministrative Management Domain (ADMD), see [EMAIL-ARCH], wishes to protect against misdelivery using this protocol, it determines for each recipient mailbox on the message a timestamp at which it last confirmed ownership of that mailbox. It then applies the SMTP extension when sending the message to its destination.
In cases where the outgoing MTA does not support the extension, the header field defined above can be used to pass the request through that system. However, use of the header field is only a "best- effort" approach to solving the stated goals, and it has some shortcomings:
- The positive confirmation of support at each handling node, with the option to return the message to the originator when end-to-end support cannot be confirmed, will be unavailable;
- The protocol is focused on affecting delivery (that is, the transaction) rather than content, and therefore use of a header field in the content is generally inappropriate;
- The mechanism cannot be used with multiple recipients without unintentionally exposing information about one recipient to the others (see Section 7); and
- There is a risk of the timestamp parameter being inadvertently forwarded, automatically or intentionally by the user (since user agents might not reveal the presence of the header field), and therefore exposed to unintended recipients. (See Section 14.4.)
Thus, the header field format MUST NOT be used unless the originator or relay has specific knowledge that the receiving MDA or an intermediary MTA will apply it properly. In any case, it SHOULD NOT be used for the multi-recipient case.
Use of the header field mechanism is further restricted by the practices described in Section 7.2 of [SMTP], Section 3.6.3 of [MAIL], and Section 7 of this document.
5. Handling By Receivers
If a receiver implements this specification, then there are two possible evaluation paths:
- The sending client uses the extension, and so there is an RRVS parameter on a RCPT TO command in the SMTP session, and the parameters of interest are taken only from there (and the header field, if present, is disregarded); or
- The sending client does not use the extension, so the RRVS parameter is not present on the RCPT TO commands in the SMTP session, but the corresponding header field might be present in the message.
When the continuous ownership test fails for transient reasons (such as an unavailable database or other condition that is likely temporary), normal transient failure handling for the message is applied.
If the continuous ownership test cannot be completed because the necessary datum (the mailbox creation or reassignment date and time) was not recorded, the MDA doing the evaluation selects a date and time to use that is the latest possible point in time at which the mailbox could have been created or reassigned. For example, this might be the earliest of all recorded mailbox creation/reassignment timestamps, or the time when the host was first installed. If no reasonable substitute for the timestamp can be selected, the MDA rejects the message using an SMTP reply code, preferably with an enhanced mail system status code (see Section 15.3), that indicates the test cannot be completed. A message originator can then decide whether to reissue the message without RRVS protection or find another way to reach the mailbox owner.
5.1. SMTP Extension Used
For an MTA supporting the SMTP extension, the requirement is to continue enforcement of RRVS during the relaying process to the next MTA or the MDA.
A receiving MTA or MDA 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 continuously, far enough back to include the given point in time, and delivers it unless that check returns in the negative. Specifically, an MDA will do the following before continuing with delivery:
- Ignore the parameter if the named mailbox is known to be a role account as listed in "Mailbox Names for Common Services, Roles and Functions" [ROLES].
- If the address is not known to be a role account, and if that address has not been under continuous ownership since the timestamp specified in the extension, return a 550 error to the RCPT command. (See also Section 15.3.)
An MTA that does not make mailbox ownership checks, such as an MTA positioned to do SMTP ingress at an organizational boundary, SHOULD relay the RRVS extension parameter to the next MTA or MDA so that it can be processed there.
For the SMTP extension, the optional RRVS parameter defined in Section 5.1 indicates the action to be taken when relaying a message to another MTA that does not advertise support for this extension. When this is the case and the no-support action was not specified or is "R" (reject), the MTA handling the message MUST reject the message by:
- returning a 550 error to the DATA command, if synchronous service is being provided to the SMTP client that introduced the message, or
- generating a Delivery Status Notification [DSN] to indicate to the originator of the message that the non-delivery occurred and terminating further relay attempts.
An enhanced mail system status code is defined for such rejections in Section 15.3.
See Section 8.2 for additional discussion.
When relaying, an MTA MUST preserve the no-support action if it was used by the SMTP client.
5.2. Header Field Used
A receiving system that implements this specification, upon receiving a message bearing a "Require-Recipient-Valid-Since" header field when no corresponding RRVS SMTP extension was used, checks whether the destination mailbox owner has held it continuously, far enough back to include the given date-time, and delivers it unless that check returns in the negative. Expressed as a sequence of steps:
1. Extract those Require-Recipient-Valid-Since fields from the message that contain a recipient for which no corresponding RRVS SMTP extension was used.
- Discard any such fields that match any of these criteria:
- are syntactically invalid;
* name a role account as listed in [ROLES];
- the "addr-spec" portion does not match a current recipient, as listed in the RCPT TO commands in the SMTP session; or
- the "addr-spec" portion does not refer to a mailbox handled for local delivery by this ADMD.
- For each field remaining, determine if the named address has been under continuous ownership since the corresponding timestamp. If it has not, reject the message.
- RECOMMENDED: If local delivery is being performed, remove all instances of this field prior to delivery to a mailbox; if the message is being forwarded, remove those instances of this header field that were not discarded by step 2 above.
Handling proceeds normally upon completion of the above steps if rejection has not been performed.
The final step is not mandatory as not all mail handling agents are capable of stripping away header fields, and there are sometimes reasons to keep the field intact such as debugging or presence of digital signatures that might be invalidated by such a change. See Section 10 for additional discussion.
If a message is to be rejected within the SMTP protocol itself (versus generating a rejection message separately), servers implementing this protocol SHOULD also implement the SMTP extension described in "Enhanced Mail System Status Codes" [ESC] and use the enhanced status codes described in Section 15.3 as appropriate.
Implementation by this method is expected to be transparent to non- participants, since they would typically ignore this header field.
This header field is not normally added to a message that is addressed to multiple recipients. The intended use of this field involves an author seeking to protect transactional or otherwise sensitive data intended for a single recipient, and thus generating independent messages for each individual recipient is normal practice. See Section 7 for further discussion and restrictions.
5.2.1. Design Choices
The presence of the address in the field content supports the case where a message bearing this header field is forwarded. The specific use case is as follows:
- A user subscribes to a service "S" at date-time "D" and confirms an email address at the user's current location, "A";
- At some later date, the user intends to leave the current location and thus creates a new mailbox elsewhere, at "B";
- The user configures address "A" to forward to "B";
- "S" constructs a message to "A" claiming that the address was valid at date-time "D" and sends it to "A";
- The receiving MTA for "A" determines that the forwarding in effect was created by the same party that owned the mailbox there and thus concludes that the continuous ownership test has been satisfied;
- If possible, the MTA for "A" removes this header field from the message, and in either case, forwards it to "B"; and
- On receipt at "B", either the header field has been removed or the header field does not refer to a current envelope recipient, and in either case the MTA delivers the message.
Section 8 discusses some interesting use cases, such as the case where "B" above results in further forwarding of the message.
SMTP has never required any correspondence between addresses in the RFC5321.MailFrom and RFC5321.RcptTo parameters and header fields of a message, which is why the header field defined here contains the recipient address to which the timestamp applies.
5.3. Clock Synchronization
The timestamp portion of this specification supports a precision at the seconds level. Although uncommon, it is not impossible for a clock at either a generator or a receiver to be incorrect, leading to an incorrect result in the RRVS evaluation.
To minimize the risk of such incorrect results, both generators and receivers implementing this specification MUST use a standard clock synchronization protocol such as [NTP] to synchronize to a common clock.
6. Relaying without RRVS Support
When a message is received using the SMTP extension defined here but will not be delivered locally (that is, it needs to be relayed further), the MTA to which the relay will take place might not be compliant with this specification. Where the MTA in possession of the message observes it is going to relay the message to an MTA that does not advertise this extension, it needs to choose one of the following actions:
- Decline to relay the message further, preferably generating a Delivery Status Notification [DSN] to indicate failure (RECOMMENDED);
- Downgrade the data thus provided in the SMTP extension to a header field, as described in Section 6.1 below (SHOULD NOT unless the conditions in that section are satisfied, and only when the previous option is not available); or
- Silently continue with delivery, dropping the protection offered by this protocol.
Using options other than the first option needs to be avoided unless there is specific knowledge that further relaying with the degraded protections thus provided does not introduce undue risk.
6.1. Header Field Conversion
If an SMTP server ("B") receives a message bearing one or more "Require-Recipient-Valid-Since" header fields from a client ("A"), presumably because "A" does not support the SMTP extension, and needs to relay the corresponding message on to another server ("C") (thereby becoming a client), and "C" advertises support for the SMTP extension, "B" SHOULD delete the header field(s) and instead relay this information by making use of the SMTP extension. Note that such modification of the header might affect later validation of the header upon delivery; for example, a hash of the modified header would produce a different result. This might be a valid cause for some operators to skip this delete operation.
Conversely, if "B" has received a mailbox timestamp from "A" using the SMTP extension for which it must now relay the message on to "C", but "C" does not advertise the SMTP extension, and "B" does not reject the message because rejection was specifically declined by the client (see Section 5.1.1), "B" SHOULD add a Require-Recipient-Valid- Since header field matching the mailbox to which relaying is being done, and the corresponding valid-since timestamp for it, if it has prior information that the eventual MDA or another intermediate MTA supports this mechanism and will be able to process the header field as described in this specification.
The admonitions about very cautious use of the header field described in Section 4 apply to this relaying mechanism as well. If multiple mailbox timestamps are received from "A", the admonitions in Section 7 also apply.
7. Header Field with Multiple Recipients
Numerous issues arise when using the header field form of this extension, particularly when multiple recipients are specified for a single message resulting in multiple fields each with a distinct address and timestamp.
Because of the nature of SMTP, a message bearing a multiplicity of Require-Recipient-Valid-Since header fields could result in a single delivery attempt for multiple recipients (in particular, if two of the recipients are handled by the same server), and if any one of them fails the test, the delivery fails to all of them; it then becomes necessary to do one of the following:
- reject the message on completion of the DATA phase of the SMTP session, which is a rejection of delivery to all recipients, or
- accept the message on completion of DATA, and then generate a Delivery Status Notification [DSN] message for each of the failed recipients.
Additional complexity arises when a message is sent to two recipients, "A" and "B", presumably with different timestamps, both of which are then redirected to a common address "C". The author is not necessarily aware of the current or past ownership of mailbox "C", or indeed that "A" and/or "B" have been redirected. This might result in either or both of the two deliveries failing at "C", which is likely to confuse the message author, who (as far as the author is aware) never sent a message to "C" in the first place.
Finally, there is an obvious concern with the fan-out of a message bearing the timestamps of multiple users; tight control over the handling of the timestamp information is very difficult to assure as the number of handling agents increases.
8. Special Use Addresses
In [DSN-SMTP], an SMTP extension was defined to allow SMTP clients to request generation of DSNs and related information to allow such reports to be maximally useful. Section 5.2.7 of that document explored the issue of the use of that extension where the recipient is a mailing list. This extension has similar concerns, which are covered here following that document as a model.
For all cases described below, a receiving MTA SHOULD NOT introduce RRVS in either form (SMTP extension or header field) if the message did not arrive with RRVS in use. This would amount to second guessing the message originator's intention and might lead to an undesirable outcome.
8.1. Mailing Lists
Delivery to a mailing list service is considered a final delivery. Where this protocol is in use, it is evaluated as per any normal delivery: if the same mailing list has been operating in place of the specified recipient mailbox since at least the timestamp given as the RRVS parameter, the message is delivered to the list service normally, and is otherwise not delivered.
It is important, however, that the participating MDA passing the message to the list service needs to omit the RRVS parameter in either form (SMTP extension or header field) when doing so. The emission of a message from the list service to its subscribers constitutes a new message not covered by the previous transaction.
8.2. Single-Recipient Aliases
Upon delivery of an RRVS-protected message to an alias (acting in place of a mailbox) that results in relaying of the message to a single other destination, the usual RRVS check is performed. The continuous ownership test here might succeed if, for example, a conventional user inbox was replaced with an alias on behalf of that same user, and the time when this was done is recorded in a way that can be queried by the relaying MTA.
If the relaying system also performs some kind of step where ownership of the new destination address is confirmed, it SHOULD apply RRVS using the later of that timestamp and the one that was used inbound. This also allows for changes to the alias without disrupting the protection offered by RRVS.
If the relaying system has no such time records related to the new destination address, the RRVS SMTP extension is not used on the relaying SMTP session, and the header field relative to the local alias is removed, in accordance with Section 5.
8.3. Multiple-Recipient Aliases
Upon delivery of an RRVS-protected message to an alias (acting in place of a mailbox) that results in relaying of the message to multiple other destinations, the usual RRVS check is performed as in Section 8.2. The MTA expanding such an alias then decides which of the options enumerated in that section is to be applied for each new recipient.
8.4. Confidential Forwarding Addresses
In the above cases, the original author could receive message rejections, such as DSNs, from the ultimate destination, where the RRVS check (or indeed, any other) fails and rejection is warranted. This can reveal the existence of a forwarding relationship between the original intended recipient and the actual final recipient.
Where this is a concern, the initial delivery attempt is to be treated like a mailing list delivery, with RRVS evaluation done and then all RRVS information removed from the message prior to relaying it to its true destination.
8.5. Suggested Mailing List Enhancements
Mailing list services could store the timestamp at which a subscriber was added to a mailing list. This specification could then be used in conjunction with that information in order to restrict list traffic to the original subscriber, rather than a different person now in possession of an address under which the original subscriber was added to the list. Upon receiving a rejection caused by this specification, the list service can remove that address from further distribution.
A mailing list service that receives a message containing the header field defined here needs to remove it from the message prior to redistributing it, limiting exposure of information regarding the relationship between the message's author and the mailing list.
9. Continuous Ownership
For the purposes of this specification, an address is defined as having been under continuous ownership since a given date-time if a message sent to the address at any point since the given date-time would not go to anyone except the owner at that given date-time. That is, while an address may have been suspended or otherwise disabled for some period, any mail actually delivered would have been delivered exclusively to the same owner. It is presumed that some sort of relationship exists between the message sender and the intended recipient. Presumably, there has been some confirmation process applied to establish this ownership of the receiver's mailbox; however, the method of making such determinations is a local matter and outside the scope of this document.
Evaluating the notion of continuous ownership is accomplished by doing any query that establishes whether the above condition holds for a given mailbox.
Determining continuous ownership of a mailbox is a local matter at the receiving site. 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-time 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 latter case, typical SMTP temporary failure handling is appropriate.
To avoid exposing account details unnecessarily, if the address specified has had one continuous owner since it was created, any confirmation date-time SHOULD be considered to pass the test, even if that date-time is earlier than the account creation date and time. This is further discussed in Section 13.
10. Digital Signatures
This protocol mandates removal of the header field (when used) upon delivery in all but exceptional circumstances. If a message with the header field were digitally signed in a way that included the header field, altering a message in this way would invalidate the signature. However, the header field is strictly for tunneling purposes and should be regarded by the rest of the transport system as purely trace information.
Accordingly, the header field MUST NOT be included in the content covered by digital signatures.
11. Authentication-Results Definitions
[AUTHRES] defines a mechanism for indicating, via a header field, the results of message authentication checks. Section 15 registers RRVS as a new method that can be reported in this way, as well as corresponding result names. The possible result names and their meanings are as follows:
none: The message had no recipient mailbox timestamp associated with it, either via the SMTP extension or header field method; this protocol was not in use. unknown: At least one form of this protocol was in use, but continuous ownership of the recipient mailbox could not be determined. temperror: At least one form of this protocol was in use, but some kind of error occurred during evaluation that was transient in nature; a later retry will likely produce a final result. permerror: At least one form of this protocol was in use, but some kind of error occurred during evaluation that was not recoverable; a later retry will not likely produce a final result. pass: At least one form of this protocol was in use, and the destination mailbox was confirmed to have been under continuous ownership since the timestamp thus provided. fail: At least one form of this protocol was in use, and the destination mailbox was confirmed not to have been under continuous ownership since the timestamp thus provided.
Where multiple recipients are present on a message, multiple results can be reported using the mechanism described in [AUTHRES].
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.
12.1. 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:<email@example.com> S: 250 OK C: RCPT TO:<firstname.lastname@example.org> RRVS=2014-04-03T23:01:00Z S: 550 5.7.17 email@example.com is no longer valid C: QUIT S: 221 So long!
12.2. Header Field Example
C: [connection established] S: 220 server.example.com ESMTP ready C: HELO client.example.net S: 250 server.example.com C: MAIL FROM:<firstname.lastname@example.org> S: 250 OK C: RCPT TO:<email@example.com> S: 250 OK C: DATA S: 354 Ready for message content C: From: Mister Sender <firstname.lastname@example.org> To: Miss Receiver <email@example.com> Subject: Are you still there? Date: Fri, 28 Jun 2013 18:01:01 +0200 Require-Recipient-Valid-Since: firstname.lastname@example.org; Sat, 1 Jun 2013 09:23:01 -0700 Are you still there? . S: 550 5.7.17 email@example.com is no longer valid C: QUIT S: 221 So long!
12.3. Authentication-Results Example
Here is an example use of the Authentication-Results header field used to yield the results of an RRVS evaluation:
Authentication-Results: mx.example.com; rrvs=pass firstname.lastname@example.org
This indicates that the message arrived addressed to the mailbox email@example.com, the continuous ownership test was applied with the provided timestamp, and the check revealed that the test was satisfied. The timestamp is not revealed.
13. Security Considerations
13.1. Abuse Countermeasures
The response of a server implementing this protocol can disclose information about the age of an existing email mailbox. Implementation of countermeasures against probing attacks is RECOMMENDED. For example, an operator could track appearance of this field with respect to a particular mailbox and observe the timestamps being submitted for testing; if it appears that a variety of timestamps are being tried against a single mailbox in short order, the field could be ignored and the message silently discarded. This concern is discussed further in Section 14.
13.2. Suggested Use Restrictions
If the mailbox named in the field 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 date-time specified in the field, then the field SHOULD be silently ignored and normal message handling applied so that this information is not disclosed. Such fields are likely the product of either gross error or an attack.
A message author using this specification might restrict inclusion of the header field 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. In this case, the "unknown" result is likely appropriate.
13.3. False Sense of Security
Senders implementing this protocol likely believe their content is being protected by doing so. It has to be considered, however, that receiving systems might not implement this protocol correctly, or at all. Furthermore, use of RRVS by a sending system constitutes nothing more than a request to the receiving system; that system could choose not to prevent delivery for some local policy, for legal or operational reasons, which compromises the security the sending system believed was a benefit to using RRVS. This could mean the timestamp information involved in the protocol becomes inadvertently revealed.
This concern lends further support to the notion that senders would do well to avoid using this protocol other than when sending to known, trusted receivers.
13.4. Reassignment of Mailboxes
This specification is a direct response to the risks involved with reassignment or recycling of email addresses, an inherently dangerous practice. It is typically expected that email addresses will not have a high rate of turnover or ownership change.
It is RECOMMENDED to have a substantial period of time between mailbox owners during which the mailbox accepts no mail, giving message generators an opportunity to detect that the previous owner is no longer at that address.
14. Privacy Considerations
14.1. The Tradeoff
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 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 extensions specified here creates far lower overall risk than the potential for delivering mail to the wrong party.
14.2. Probing Attacks
As described above, use of this extension or header field 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, either inadvertently or in response to probing by an attacker. It bears restating, then, that implementing countermeasures against abuse of this capability needs strong consideration.
14.3. Envelope Recipients
The email To and Cc header fields are not required to be populated with addresses that match the envelope recipient set, and Cc may even be absent. However, the algorithm in Section 3 requires that this header field contain a match for an envelope recipient in order to be actionable. As such, use of this specification can reveal some or all of the original intended recipient set to any party that can see the message in transit or upon delivery.
For a message destined to a single recipient, this is unlikely to be a concern, which is one of the reasons use of this specification on multi-recipient messages is discouraged.
14.4. Risks with Use
MDAs might not implement the recommendation to remove the header field defined here when messages are delivered, either out of ignorance or due to error. Since user agents often do not render all of the header fields present, the message could be forwarded to another party that would then inadvertently have the content of this header field.
A bad actor may detect use of either form of the RRVS protocol and interpret it as an indication of high-value content.
15. IANA Considerations
15.1. SMTP Extension Registration
Section 2.2.2 of [SMTP] sets out the procedure for registering a new SMTP extension. IANA has registered the SMTP extension using the details provided in Section 3.1 of this document.
15.2. Header Field Registration
IANA has added the following entry to the "Permanent Message Header Field Names" registry, as per the procedure found in [IANA-HEADERS]:
Header field name: Require-Recipient-Valid-Since Applicable protocol: mail ([MAIL]) Status: standard Author/Change controller: IETF Specification document(s): RFC 7293
Requesting review of any proposed changes and additions to this field is recommended.
15.3. Enhanced Status Code Registration
IANA has registered the following in the Enumerated Status Codes table of the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry":
Code: X.7.17 Sample Text: Mailbox owner has changed Associated basic status code: 5XX Description: This status code is returned when a message is received with a Require-Recipient-Valid-Since field or RRVS extension and the receiving system is able to determine that the intended recipient mailbox has not been under continuous ownership since the specified date-time. Reference: RFC 7293 Submitter: M. Kucherawy Change controller: IESG Code: X.7.18 Sample Text: Domain owner has changed Associated basic status code: 5XX Description: This status code is returned when a message is received with a Require-Recipient-Valid-Since field or RRVS extension and the receiving system wishes to disclose that the owner of the domain name of the recipient has changed since the specified date-time. Reference: RFC 7293 Submitter: M. Kucherawy Change controller: IESG Code: X.7.19 Sample Text: RRVS test cannot be completed Associated basic status code: 5XX Description: This status code is returned when a message is received with a Require-Recipient-Valid-Since field or RRVS extension and the receiving system cannot complete the requested evaluation because the required timestamp was not recorded. The message originator needs to decide whether to reissue the message without RRVS protection. Reference: RFC 7293 Submitter: M. Kucherawy Change controller: IESG
15.4. Authentication Results Registration
IANA has registered the following in the "Email Authentication Methods" registry:
Method: rrvs Specifying Document: RFC 7293 ptype: smtp Property: rcptto Value: envelope recipient Status: active Version: 1
IANA has also registered the following in the "Email Authentication Result Names" registry:
Erling Ellingsen proposed the idea.
Reviews and comments were provided by Michael Adkins, Kurt Andersen, Eric Burger, Alissa Cooper, Dave Cridland, Dave Crocker, Ned Freed, John Levine, Alexey Melnikov, Jay Nancarrow, Hector Santos, Gregg Stefancik, and Ed Zayas.
17.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [DATETIME] Klyne, G., Ed. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002.
Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [NTP] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [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.
17.2. Informative References
[AUTHRES] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 7001, September 2013. [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications", RFC 3464, January 2003. [DSN-SMTP] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)", RFC 3461, January 2003.
William J. Mills
firstname.lastname@example.org Murray S. Kucherawy Facebook, Inc. 1 Hacker Way Menlo Park, CA 94025 USA