RFC 5322
Internet Message Format
Pages: 57
Draft Standard
→ Errata
Obsoletes: 2822
Updates: 4021
Updated by: 6854
Draft Standard
→ Errata
Obsoletes: 2822
Updates: 4021
Updated by: 6854
Part 3 of 3 – Pages 38 to 57
5. Security Considerations
Care needs to be taken when displaying messages on a terminal or terminal emulator. Powerful terminals may act on escape sequences and other combinations of US-ASCII control characters with a variety of consequences. They can remap the keyboard or permit other modifications to the terminal that could lead to denial of service or even damaged data. They can trigger (sometimes programmable)
answerback messages that can allow a message to cause commands to be issued on the recipient's behalf. They can also affect the operation of terminal attached devices such as printers. Message viewers may wish to strip potentially dangerous terminal escape sequences from the message prior to display. However, other escape sequences appear in messages for useful purposes (cf. [ISO.2022.1994], [RFC2045], [RFC2046], [RFC2047], [RFC2049], [RFC4288], [RFC4289]) and therefore should not be stripped indiscriminately. Transmission of non-text objects in messages raises additional security issues. These issues are discussed in [RFC2045], [RFC2046], [RFC2047], [RFC2049], [RFC4288], and [RFC4289]. Many implementations use the "Bcc:" (blind carbon copy) field, described in section 3.6.3, to facilitate sending messages to recipients without revealing the addresses of one or more of the addressees to the other recipients. Mishandling this use of "Bcc:" may disclose confidential information that could eventually lead to security problems through knowledge of even the existence of a particular mail address. For example, if using the first method described in section 3.6.3, where the "Bcc:" line is removed from the message, blind recipients have no explicit indication that they have been sent a blind copy, except insofar as their address does not appear in the header section of a message. Because of this, one of the blind addressees could potentially send a reply to all of the shown recipients and accidentally reveal that the message went to the blind recipient. When the second method from section 3.6.3 is used, the blind recipient's address appears in the "Bcc:" field of a separate copy of the message. If the "Bcc:" field sent contains all of the blind addressees, all of the "Bcc:" recipients will be seen by each "Bcc:" recipient. Even if a separate message is sent to each "Bcc:" recipient with only the individual's address, implementations still need to be careful to process replies to the message as per section 3.6.3 so as not to accidentally reveal the blind recipient to other recipients.6. IANA Considerations
This document updates the registrations that appeared in [RFC4021] that referred to the definitions in [RFC2822]. IANA has updated the Permanent Message Header Field Repository with the following header fields, in accordance with the procedures set out in [RFC3864]. Header field name: Date Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.1)
Header field name: From Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.2) Header field name: Sender Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.2) Header field name: Reply-To Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.2) Header field name: To Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.3) Header field name: Cc Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.3) Header field name: Bcc Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.3) Header field name: Message-ID Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.4) Header field name: In-Reply-To Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.4)
Header field name: References Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.4) Header field name: Subject Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.5) Header field name: Comments Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.5) Header field name: Keywords Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.5) Header field name: Resent-Date Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.6) Header field name: Resent-From Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.6) Header field name: Resent-Sender Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.6) Header field name: Resent-To Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.6)
Header field name: Resent-Cc Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.6) Header field name: Resent-Bcc Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.6) Header field name: Resent-Reply-To Applicable protocol: Mail Status: obsolete Author/Change controller: IETF Specification document(s): This document (section 4.5.6) Header field name: Resent-Message-ID Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.6) Header field name: Return-Path Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.7) Header field name: Received Applicable protocol: Mail Status: standard Author/Change controller: IETF Specification document(s): This document (section 3.6.7) Related information: [RFC5321]
Appendix A. Example Messages
This section presents a selection of messages. These are intended to assist in the implementation of this specification, but should not be taken as normative; that is to say, although the examples in this section were carefully reviewed, if there happens to be a conflict between these examples and the syntax described in sections 3 and 4 of this document, the syntax in those sections is to be taken as correct. In the text version of this document, messages in this section are delimited between lines of "----". The "----" lines are not part of the message itself.
Appendix A.1. Addressing Examples
The following are examples of messages that might be sent between two individuals.Appendix A.1.1. A Message from One Person to Another with Simple Addressing
This could be called a canonical message. It has a single author, John Doe, a single recipient, Mary Smith, a subject, the date, a message identifier, and a textual message in the body. ---- From: John Doe <jdoe@machine.example> To: Mary Smith <mary@example.net> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <1234@local.machine.example> This is a message just to say hello. So, "Hello". ---- If John's secretary Michael actually sent the message, even though John was the author and replies to this message should go back to him, the sender field would be used: ---- From: John Doe <jdoe@machine.example> Sender: Michael Jones <mjones@machine.example> To: Mary Smith <mary@example.net> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <1234@local.machine.example> This is a message just to say hello. So, "Hello". ----
Appendix A.1.2. Different Types of Mailboxes
This message includes multiple addresses in the destination fields and also uses several different forms of addresses. ---- From: "Joe Q. Public" <john.q.public@example.com> To: Mary Smith <mary@x.test>, jdoe@example.org, Who? <one@y.test> Cc: <boss@nil.test>, "Giant; \"Big\" Box" <sysservices@example.net> Date: Tue, 1 Jul 2003 10:52:37 +0200 Message-ID: <5678.21-Nov-1997@example.com> Hi everyone. ---- Note that the display names for Joe Q. Public and Giant; "Big" Box needed to be enclosed in double-quotes because the former contains the period and the latter contains both semicolon and double-quote characters (the double-quote characters appearing as quoted-pair constructs). Conversely, the display name for Who? could appear without them because the question mark is legal in an atom. Notice also that jdoe@example.org and boss@nil.test have no display names associated with them at all, and jdoe@example.org uses the simpler address form without the angle brackets.Appendix A.1.3. Group Addresses
---- From: Pete <pete@silly.example> To: A Group:Ed Jones <c@a.test>,joe@where.test,John <jdoe@one.test>; Cc: Undisclosed recipients:; Date: Thu, 13 Feb 1969 23:32:54 -0330 Message-ID: <testabcd.1234@silly.example> Testing. ---- In this message, the "To:" field has a single group recipient named "A Group", which contains 3 addresses, and a "Cc:" field with an empty group recipient named Undisclosed recipients.
Appendix A.2. Reply Messages
The following is a series of three messages that make up a conversation thread between John and Mary. John first sends a message to Mary, Mary then replies to John's message, and then John replies to Mary's reply message. Note especially the "Message-ID:", "References:", and "In-Reply-To:" fields in each message. ---- From: John Doe <jdoe@machine.example> To: Mary Smith <mary@example.net> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <1234@local.machine.example> This is a message just to say hello. So, "Hello". ---- When sending replies, the Subject field is often retained, though prepended with "Re: " as described in section 3.6.5. ---- From: Mary Smith <mary@example.net> To: John Doe <jdoe@machine.example> Reply-To: "Mary Smith: Personal Account" <smith@home.example> Subject: Re: Saying Hello Date: Fri, 21 Nov 1997 10:01:10 -0600 Message-ID: <3456@example.net> In-Reply-To: <1234@local.machine.example> References: <1234@local.machine.example> This is a reply to your hello. ---- Note the "Reply-To:" field in the above message. When John replies to Mary's message above, the reply should go to the address in the "Reply-To:" field instead of the address in the "From:" field.
---- To: "Mary Smith: Personal Account" <smith@home.example> From: John Doe <jdoe@machine.example> Subject: Re: Saying Hello Date: Fri, 21 Nov 1997 11:00:00 -0600 Message-ID: <abcd.1234@local.machine.test> In-Reply-To: <3456@example.net> References: <1234@local.machine.example> <3456@example.net> This is a reply to your reply. ----Appendix A.3. Resent Messages
Start with the message that has been used as an example several times: ---- From: John Doe <jdoe@machine.example> To: Mary Smith <mary@example.net> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <1234@local.machine.example> This is a message just to say hello. So, "Hello". ---- Say that Mary, upon receiving this message, wishes to send a copy of the message to Jane such that (a) the message would appear to have come straight from John; (b) if Jane replies to the message, the reply should go back to John; and (c) all of the original information, like the date the message was originally sent to Mary, the message identifier, and the original addressee, is preserved. In this case, resent fields are prepended to the message:
---- Resent-From: Mary Smith <mary@example.net> Resent-To: Jane Brown <j-brown@other.example> Resent-Date: Mon, 24 Nov 1997 14:22:01 -0800 Resent-Message-ID: <78910@example.net> From: John Doe <jdoe@machine.example> To: Mary Smith <mary@example.net> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <1234@local.machine.example> This is a message just to say hello. So, "Hello". ---- If Jane, in turn, wished to resend this message to another person, she would prepend her own set of resent header fields to the above and send that. (Note that for brevity, trace fields are not shown.)Appendix A.4. Messages with Trace Fields
As messages are sent through the transport system as described in [RFC5321], trace fields are prepended to the message. The following is an example of what those trace fields might look like. Note that there is some folding white space in the first one since these lines can be long. ---- Received: from x.y.test by example.net via TCP with ESMTP id ABC12345 for <mary@example.net>; 21 Nov 1997 10:05:43 -0600 Received: from node.example by x.y.test; 21 Nov 1997 10:01:22 -0600 From: John Doe <jdoe@node.example> To: Mary Smith <mary@example.net> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <1234@local.node.example> This is a message just to say hello. So, "Hello". ----
Appendix A.5. White Space, Comments, and Other Oddities
White space, including folding white space, and comments can be inserted between many of the tokens of fields. Taking the example from A.1.3, white space and comments can be inserted into all of the fields. ---- From: Pete(A nice \) chap) <pete(his account)@silly.test(his host)> To:A Group(Some people) :Chris Jones <c@(Chris's host.)public.example>, joe@example.org, John <jdoe@one.test> (my dear friend); (the end of the group) Cc:(Empty list)(start)Hidden recipients :(nobody(that I know)) ; Date: Thu, 13 Feb 1969 23:32 -0330 (Newfoundland Time) Message-ID: <testabcd.1234@silly.test> Testing. ---- The above example is aesthetically displeasing, but perfectly legal. Note particularly (1) the comments in the "From:" field (including one that has a ")" character appearing as part of a quoted-pair); (2) the white space absent after the ":" in the "To:" field as well as the comment and folding white space after the group name, the special character (".") in the comment in Chris Jones's address, and the folding white space before and after "joe@example.org,"; (3) the multiple and nested comments in the "Cc:" field as well as the comment immediately following the ":" after "Cc"; (4) the folding white space (but no comments except at the end) and the missing seconds in the time of the date field; and (5) the white space before (but not within) the identifier in the "Message-ID:" field.
Appendix A.6. Obsoleted Forms
The following are examples of obsolete (that is, the "MUST NOT generate") syntactic elements described in section 4 of this document.Appendix A.6.1. Obsolete Addressing
Note in the example below the lack of quotes around Joe Q. Public, the route that appears in the address for Mary Smith, the two commas that appear in the "To:" field, and the spaces that appear around the "." in the jdoe address. ---- From: Joe Q. Public <john.q.public@example.com> To: Mary Smith <@node.test:mary@example.net>, , jdoe@test . example Date: Tue, 1 Jul 2003 10:52:37 +0200 Message-ID: <5678.21-Nov-1997@example.com> Hi everyone. ----Appendix A.6.2. Obsolete Dates
The following message uses an obsolete date format, including a non- numeric time zone and a two digit year. Note that although the day- of-week is missing, that is not specific to the obsolete syntax; it is optional in the current syntax as well. ---- From: John Doe <jdoe@machine.example> To: Mary Smith <mary@example.net> Subject: Saying Hello Date: 21 Nov 97 09:55:06 GMT Message-ID: <1234@local.machine.example> This is a message just to say hello. So, "Hello". ----
Appendix A.6.3. Obsolete White Space and Comments
White space and comments can appear between many more elements than in the current syntax. Also, folding lines that are made up entirely of white space are legal. ---- From : John Doe <jdoe@machine(comment). example> To : Mary Smith __ <mary@example.net> Subject : Saying Hello Date : Fri, 21 Nov 1997 09(comment): 55 : 06 -0600 Message-ID : <1234 @ local(blah) .machine .example> This is a message just to say hello. So, "Hello". ---- Note especially the second line of the "To:" field. It starts with two space characters. (Note that "__" represent blank spaces.) Therefore, it is considered part of the folding, as described in section 4.2. Also, the comments and white space throughout addresses, dates, and message identifiers are all part of the obsolete syntax.
Appendix B. Differences from Earlier Specifications
This appendix contains a list of changes that have been made in the Internet Message Format from earlier specifications, specifically [RFC0822], [RFC1123], and [RFC2822]. Items marked with an asterisk (*) below are items which appear in section 4 of this document and therefore can no longer be generated. The following are the changes made from [RFC0822] and [RFC1123] to [RFC2822] that remain in this document: 1. Period allowed in obsolete form of phrase. 2. ABNF moved out of document, now in [RFC5234]. 3. Four or more digits allowed for year. 4. Header field ordering (and lack thereof) made explicit. 5. Encrypted header field removed. 6. Specifically allow and give meaning to "-0000" time zone. 7. Folding white space is not allowed between every token. 8. Requirement for destinations removed. 9. Forwarding and resending redefined. 10. Extension header fields no longer specifically called out. 11. ASCII 0 (null) removed.* 12. Folding continuation lines cannot contain only white space.* 13. Free insertion of comments not allowed in date.* 14. Non-numeric time zones not allowed.* 15. Two digit years not allowed.* 16. Three digit years interpreted, but not allowed for generation.* 17. Routes in addresses not allowed.* 18. CFWS within local-parts and domains not allowed.* 19. Empty members of address lists not allowed.* 20. Folding white space between field name and colon not allowed.* 21. Comments between field name and colon not allowed. 22. Tightened syntax of in-reply-to and references.* 23. CFWS within msg-id not allowed.* 24. Tightened semantics of resent fields as informational only. 25. Resent-Reply-To not allowed.* 26. No multiple occurrences of fields (except resent and received).* 27. Free CR and LF not allowed.* 28. Line length limits specified. 29. Bcc more clearly specified.
The following are changes from [RFC2822]. 1. Assorted typographical/grammatical errors fixed and clarifications made. 2. Changed "standard" to "document" or "specification" throughout. 3. Made distinction between "header field" and "header section". 4. Removed NO-WS-CTL from ctext, qtext, dtext, and unstructured.* 5. Moved discussion of specials to the "Atom" section. Moved text to "Overall message syntax" section. 6. Simplified CFWS syntax. 7. Fixed unstructured syntax. 8. Changed date and time syntax to deal with white space in obsolete date syntax. 9. Removed quoted-pair from domain literals and message identifiers.* 10. Clarified that other specifications limit domain syntax. 11. Simplified "Bcc:" and "Resent-Bcc:" syntax. 12. Allowed optional-field to appear within trace information. 13. Removed no-fold-quote from msg-id. Clarified syntax limitations. 14. Generalized "Received:" syntax to fix bugs and move definition out of this document. 15. Simplified obs-qp. Fixed and simplified obs-utext (which now only appears in the obsolete syntax). Removed obs-text and obs- char, adding obs-body. 16. Fixed obsolete date syntax to allow for more (or less) comments and white space. 17. Fixed all obsolete list syntax (obs-domain-list, obs-mbox-list, obs-addr-list, obs-phrase-list, and the newly added obs-group- list). 18. Fixed obs-reply-to syntax. 19. Fixed obs-bcc and obs-resent-bcc to allow empty lists. 20. Removed obs-path.Appendix C. Acknowledgements
Many people contributed to this document. They included folks who participated in the Detailed Revision and Update of Messaging Standards (DRUMS) Working Group of the Internet Engineering Task Force (IETF), the chair of DRUMS, the Area Directors of the IETF, and people who simply sent their comments in via email. The editor is deeply indebted to them all and thanks them sincerely. The below list includes everyone who sent email concerning both this document and [RFC2822]. Hopefully, everyone who contributed is named here: +--------------------+----------------------+---------------------+ | Matti Aarnio | Tanaka Akira | Russ Allbery | | Eric Allman | Harald Alvestrand | Ran Atkinson | | Jos Backus | Bruce Balden | Dave Barr |
| Alan Barrett | John Beck | J Robert von Behren | | Jos den Bekker | D J Bernstein | James Berriman | | Oliver Block | Norbert Bollow | Raj Bose | | Antony Bowesman | Scott Bradner | Randy Bush | | Tom Byrer | Bruce Campbell | Larry Campbell | | W J Carpenter | Michael Chapman | Richard Clayton | | Maurizio Codogno | Jim Conklin | R Kelley Cook | | Nathan Coulter | Steve Coya | Mark Crispin | | Dave Crocker | Matt Curtin | Michael D'Errico | | Cyrus Daboo | Michael D Dean | Jutta Degener | | Mark Delany | Steve Dorner | Harold A Driscoll | | Michael Elkins | Frank Ellerman | Robert Elz | | Johnny Eriksson | Erik E Fair | Roger Fajman | | Patrik Faltstrom | Claus Andre Faerber | Barry Finkel | | Erik Forsberg | Chuck Foster | Paul Fox | | Klaus M Frank | Ned Freed | Jochen Friedrich | | Randall C Gellens | Sukvinder Singh Gill | Tim Goodwin | | Philip Guenther | Arnt Gulbrandsen | Eric A Hall | | Tony Hansen | John Hawkinson | Philip Hazel | | Kai Henningsen | Robert Herriot | Paul Hethmon | | Jim Hill | Alfred Hoenes | Paul E Hoffman | | Steve Hole | Kari Hurtta | Marco S Hyman | | Ofer Inbar | Olle Jarnefors | Kevin Johnson | | Sudish Joseph | Maynard Kang | Prabhat Keni | | John C Klensin | Graham Klyne | Brad Knowles | | Shuhei Kobayashi | Peter Koch | Dan Kohn | | Christian Kuhtz | Anand Kumria | Steen Larsen | | Eliot Lear | Barry Leiba | Jay Levitt | | Bruce Lilly | Lars-Johan Liman | Charles Lindsey | | Pete Loshin | Simon Lyall | Bill Manning | | John Martin | Mark Martinec | Larry Masinter | | Denis McKeon | William P McQuillan | Alexey Melnikov | | Perry E Metzger | Steven Miller | S Moonesamy | | Keith Moore | John Gardiner Myers | Chris Newman | | John W Noerenberg | Eric Norman | Mike O'Dell | | Larry Osterman | Paul Overell | Jacob Palme | | Michael A Patton | Uzi Paz | Michael A Quinlan | | Robert Rapplean | Eric S Raymond | Sam Roberts | | Hugh Sasse | Bart Schaefer | Tom Scola | | Wolfgang Segmuller | Nick Shelness | John Stanley | | Einar Stefferud | Jeff Stephenson | Bernard Stern | | Peter Sylvester | Mark Symons | Eric Thomas | | Lee Thompson | Karel De Vriendt | Matthew Wall | | Rolf Weber | Brent B Welch | Dan Wing | | Jack De Winter | Gregory J Woodhouse | Greg A Woods | | Kazu Yamamoto | Alain Zahm | Jamie Zawinski | | Timothy S Zurcher | | | +--------------------+----------------------+---------------------+
7. References
7.1. Normative References
[ANSI.X3-4.1986] American National Standards Institute, "Coded Character Set - 7-bit American Standard Code for Information Interchange", ANSI X3.4, 1986. [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.7.2. Informative References
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet text messages", STD 11, RFC 822, August 1982. [RFC1305] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation", RFC 1305, March 1992. [ISO.2022.1994] International Organization for Standardization, "Information technology - Character code structure and extension techniques", ISO Standard 2022, 1994. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC 2049, November 1996. [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. [RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME Header Fields", RFC 4021, March 2005. [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December 2005. [RFC4289] Freed, N. and J. Klensin, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", BCP 13, RFC 4289, December 2005. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008.Author's Address
Peter W. Resnick (editor) Qualcomm Incorporated 5775 Morehouse Drive San Diego, CA 92121-1714 US Phone: +1 858 651 4478 EMail: presnick@qualcomm.com URI: http://www.qualcomm.com/~presnick/
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