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Network Working Group T. Hansen
Request for Comments: 5703 AT&T Laboratories
Category: Standards Track C. Daboo
Apple Inc.
October 2009
Sieve Email Filtering: MIME Part Tests, Iteration, Extraction,
Replacement, and Enclosure
Abstract
This document defines extensions to the Sieve email filtering
language to permit analysis and manipulation of the MIME body parts
of an email message.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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Hansen & Daboo Standards Track [Page 1]
RFC 5703 Sieve MIME Operations October 2009 outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction ....................................................2 2. Conventions Used in This Document ...............................3 3. Sieve Loops: Actions "foreverypart" and "break" .................3 4. Changes to Sieve Tests ..........................................4 4.1. Test "header" ..............................................4 4.2. Test "address" .............................................7 4.3. Test "exists" ..............................................8 5. Action "replace" ................................................8 6. Action "enclose" ...............................................10 7. Action "extracttext" ...........................................11 8. Sieve Capability Strings .......................................11 9. Examples .......................................................12 9.1. Example 1 .................................................12 9.2. Example 2 .................................................12 9.3. Example 3 .................................................13 10. Acknowledgements ..............................................13 11. Security Considerations .......................................14 12. IANA Considerations ...........................................14 12.1. foreverypart capability ..................................15 12.2. mime capability ..........................................15 12.3. replace capability .......................................15 12.4. enclose capability .......................................16 12.5. extracttext capability ...................................16 13. References ....................................................16 13.1. Normative References .....................................16 13.2. Informative References ...................................17 1. Introduction MIME messages ([RFC2045]) are often complex objects, consisting of many parts and sub-parts. This Sieve ([RFC5228]) extension defines mechanisms for performing tests on MIME body parts, looping through the MIME body parts, extracting information from a MIME body part, changing the contents of a MIME body part, and enclosing the entire message within a wrapper. Hansen & Daboo Standards Track [Page 2]
RFC 5703 Sieve MIME Operations October 2009 2. Conventions Used in This Document Conventions for notations are as in [RFC5228], Section 1.1. 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 [RFC2119]. 3. Sieve Loops: Actions "foreverypart" and "break" The base Sieve language has no looping mechanism. Given that messages may contain multiple parts, in order to support filters that apply to any and all parts, we introduce a new control command: "foreverypart", which is an iterator that walks though every MIME part of a message, including nested parts, depth first, and applies the commands in the specified block to each of them. The iterator will start with the first MIME part (as its current context) and will execute a command block (Sieve commands enclosed by {...}). Upon completion of this command block, the iterator advances to the next MIME part (as its current context) and executes the same command block again. The iterator can be terminated prematurely by a new Sieve control command, "break". Usage: foreverypart [":name" string] block Usage: break [":name" string]; "foreverypart" commands can be nested inside other "foreverypart" commands. When this occurs, the nested "foreverypart" iterates over the MIME parts contained within the MIME part currently being targeted by the nearest enclosing "foreverypart" command. (That is, the inner loop only operates on children of the bodypart currently accessed by the outer loop.) If that MIME part is a terminal MIME part (i.e., does not contain other MIME parts), then the nested "foreverypart" loop is simply ignored. Sieve implementations MAY limit the number of nested loops that occur within one another; however, they MUST support at least one nested loop inside another loop. If a name is given to a "break" command, it terminates the closest enclosing loop with the identical matching name. (If a nested "foreverypart" name is the same as a "foreverypart" name in an outer level, the outer level name is hidden.) It is an error if there is no enclosing loop with that name. Hansen & Daboo Standards Track [Page 3]
RFC 5703 Sieve MIME Operations October 2009 If no name is given in a "break" command (i.e., the ":name" parameter is omitted), the break command terminates the closest enclosing loop. 4. Changes to Sieve Tests This specification extends the base Sieve "header", "address", and "exists" tests to support targeting those tests at a specific MIME part or at all MIME parts in the enclosing scope. 4.1. Test "header" The "header" test is extended with the addition of new ":mime" and ":anychild" tagged arguments and their associated options. Usage: header [":mime"] [":anychild"] [MIMEOPTS] [COMPARATOR] [MATCH-TYPE] <header-names: string-list> <key-list: string-list> The definition of [MIMEOPTS] is: Syntax: ":type" / ":subtype" / ":contenttype" / ":param" <param-list: string-list> When the ":mime" tagged argument is present in the "header" test, it will parse the MIME header lines in the message so that tests can be performed on specific elements. The ":anychild" tagged argument may only appear when the ":mime" tagged argument is present, and only modifies the semantics of the ":mime" tagged argument. That is, presence of the ":anychild" in absence of ":mime" is an error. When used outside the context of a "foreverypart" iterator, and without an ":anychild" tagged argument, the "header" test will examine only the outer top-level [RFC5322] headers of the message. When used inside the context of a "foreverypart" iterator, and without an ":anychild" tagged argument, the "header" test will examine the headers associated with the current MIME part context from the loop. When used outside the context of a "foreverypart" iterator, and with an ":anychild" tagged argument, the "header" test will examine all MIME body parts and return true if any of them satisfies the test. When used inside the context of a "foreverypart" iterator, and with an ":anychild" tagged argument, the "header" test will examine the current MIME part context and all its nested MIME body parts, returning true if any of them satisfies the test. Hansen & Daboo Standards Track [Page 4]
RFC 5703 Sieve MIME Operations October 2009 The "header" test with the ":mime" tagged argument can test various aspects of certain structured MIME headers. Implementations SHOULD support desegmentation, decoding, and charset translation of parameter values encoded according to [RFC2231] as part of this test. Additionally, [RFC2047] describes a process whereby [RFC5322] headers can be encoded in various ways. That encoding is not strictly allowed in MIME parameters; however, in practice, it has been used in many email implementations. So, Sieve implementations MAY decode [RFC2047]-encoded words in parameter values as part of this test. These options are available: :type for a "Content-Type" MIME header field, parses and tests the value of the MIME type specified in the header; for a "Content-Disposition" MIME header field, parses and tests the value of the disposition specified in the header; for other MIME headers, uses a blank string for the test. :subtype for a "Content-Type" MIME header field, parses and tests the value of the MIME subtype specified in the header; for a "Content-Disposition" MIME header field, uses a blank string for the test; for other MIME headers, uses a blank string for the test. :contenttype for a "Content-Type" MIME header field, parses and tests the combined value of the MIME type and subtype specified in the header; for a "Content-Disposition" MIME header field, behaves the same as the ":type" option; for other MIME headers, uses a blank string for the test. :param parses the header looking for MIME parameters in the header. The supplied string-list lists the names of any parameters to be tested. If any one named parameter value matches any of the test string values, the test will return true. When the ":count" option from [RFC5231] is used, the following applies: a. for ":type", ":subtype", or ":contenttype", return a count of the number of headers that parsed successfully b. for ":param", return a count of the number of parameters with the given name that were found Hansen & Daboo Standards Track [Page 5]
RFC 5703 Sieve MIME Operations October 2009 Example: require ["mime", "fileinto"]; if header :mime :type "Content-Type" "image" { fileinto "INBOX.images"; } In this example, any message that contains a MIME image type part at the top-level is saved to the mailbox "INBOX.images". Example: require ["mime", "fileinto"]; if header :mime :anychild :contenttype "Content-Type" "text/html" { fileinto "INBOX.html"; } In this example, any message that contains any MIME part with a content-type of "text/html" is saved to the mailbox "INBOX.html". Example: require ["mime", "foreverypart", "fileinto"]; foreverypart { if allof ( header :mime :param "filename" :contains "Content-Disposition" "important", header :mime :subtype "Content-Type" "pdf", size :over "100K") { fileinto "INBOX.important"; break; } } In this example, any message that contains a MIME part that has a content-disposition with a filename parameter containing the text "important", has a content-subtype of "pdf" and is bigger than 100 Kb is saved to the mailbox "INBOX.important". Hansen & Daboo Standards Track [Page 6]
RFC 5703 Sieve MIME Operations October 2009 4.2. Test "address" The "address" test is extended with the addition of new ":mime" and ":anychild" tagged arguments and their associated options. Usage: address [":mime"] [":anychild"] [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE] <header-list: string-list> <key-list: string-list> When the ":mime" tagged argument is present in the "address" test, it will parse the MIME header lines as if they were standard address header lines in a message so that tests can be performed on specific elements. The behavior of the ":anychild" tagged argument and the interaction with the "foreverypart" iterator is the same as for the extended "header" test in Section 4.1. That is, the use of "address" when both the ":mime" and ":anychild" tagged arguments are omitted is the test defined in [RFC5228], i.e., it will *only* operate on top-level header fields, whether or not it is inside "foreverypart". the use of "address" with ":mime" and no ":anychild" operates on the current MIME part only (or on the top-level header fields, if outside "foreverypart"). the use of "address" with ":mime" and ":anychild" operates on the current MIME part and all of its descendants. Example: require ["mime", "fileinto"]; if address :mime :is :all "content-from" "tim@example.com" { fileinto "INBOX.part-from-tim"; } In this example, any message that contains a MIME Content-From header at the top-level matching the text "tim@example.com" is saved to the mailbox "INBOX.part-from-tim". Hansen & Daboo Standards Track [Page 7]
RFC 5703 Sieve MIME Operations October 2009 4.3. Test "exists" The "exists" test is extended with the addition of the new ":mime" and ":anychild" tagged arguments and their associated options. Usage: exists [":mime"] [":anychild"] <header-names: string-list> When the ":mime" tagged argument is present in the "exists" test, the test is extended to check for the existence of MIME headers in MIME parts. The behavior of the ":anychild" tagged argument and the interaction with the "foreverypart" iterator is the same as for the extended "header" test Section 4.1. That is, the use of "exists" when both the ":mime" and ":anychild" tagged arguments are omitted is the test defined in [RFC5228], i.e., it will *only* operate on top-level header fields, whether or not it is inside "foreverypart". the use of "exists" with ":mime" and no ":anychild" operates on the current MIME part only (or on the top-level header fields, if outside "foreverypart"). the use of "exists" with ":mime" and ":anychild" operates on the current MIME part and all of its descendants. Example: require ["mime", "fileinto"]; if exists :mime :anychild "content-md5" { fileinto "INBOX.md5"; } In this example, any message that contains a MIME Content-MD5 header in any MIME part is saved to the mailbox "INBOX.md5". 5. Action "replace" Usage: replace [":mime"] [":subject" string] [":from" string] <replacement: string> The "replace" command is defined to allow a MIME part to be replaced with the text supplied in the command. Hansen & Daboo Standards Track [Page 8]
RFC 5703 Sieve MIME Operations October 2009 When used in the context of a "foreverypart" iterator, the MIME part to be replaced is the "current" MIME part. If the current MIME context is a multipart MIME part, the entire multipart MIME part is replaced, which would alter the MIME structure of the message by eliminating all of the children of the multipart part. (Replacing a non-multipart MIME part within a "foreverypart" loop context does not alter the overall message structure.) If the MIME structure is altered, the change takes effect immediately: the "foreverypart" iterator that is executing does not go into the no-longer existing body parts, and subsequent "foreverypart" iterators would use the new message structure. When used outside the context of a "foreverypart" loop, the MIME part to be replaced is the entire message. If the ":mime" parameter is not specified, the replacement string is a text/plain part in UTF-8 [RFC3629]. If the ":mime" parameter is specified, then the replacement string is, in fact, a MIME entity as defined in [RFC2045], Section 2.4, including both MIME headers and content. If the entire message is being replaced, the optional ":subject" parameter specifies a subject line to attach to the message that is generated. UTF-8 characters can be used in the string argument; implementations MUST convert the string to [RFC2047]-encoded words if and only if non-ASCII characters are present. If the ":subject" parameter is used, implementations MUST preserve any previous Subject header as an Original-Subject header. Implementations MUST preserve all other header fields from the original message with the exception of those relating to the MIME structure that is being replaced. If the entire message is being replaced, as an indication that the message is no longer as created by the original author of the message, the optional ":from" parameter may be used to specify an alternate address to use in the From field of the message that is generated. The string must specify a valid [RFC5322] mailbox-list. Implementations SHOULD check the syntax and generate an error when a syntactically invalid ":from" parameter is specified. Implementations MAY also impose restrictions on what addresses can be specified in a ":from" parameter; it is suggested that values that fail such a validity check simply be ignored rather than causing the "replace" action to fail. If the From header is changed, implementations MUST preserve the previous From header as an Original-From header. Hansen & Daboo Standards Track [Page 9]
RFC 5703 Sieve MIME Operations October 2009 Implementations that support the "editheader" extension [RFC5293] MUST ensure that any Original-Subject or Original-From headers added by the system cannot be modified or removed. Implementations MAY prevent the addition of Original-Subject and Orignal-From headers via the "editheader" extension. If ":mime" is specified and either ":subject" or ":from" is specified, the ":subject:" or ":from" parameter MUST be ignored. This SHOULD be flagged as a compilation error. 6. Action "enclose" Usage: enclose <:subject string> <:headers string-list> string A new Sieve action command is defined to allow an entire message to be enclosed as an attachment to a new message. After enclosure, subsequent actions affecting the message header or content, as well as tests operating on the MIME structure or accessing MIME header fields, use the newly created message instead of the original message; this means that any use of a "replace" action or other similar actions should be executed before the "enclose" action. If multiple "enclose" actions are executed by a script, the message is enclosed multiple times. (If a Sieve script desires to choose between different enclosures, or wants to delay the enclosure to the end of the script, it can use variables with appropriate tests [RFC5229].) This action does not affect messages that are forwarded via a "redirect" action. Specifically, the original message becomes a multipart/mixed message with two parts: a text/plain portion with the string argument as its body, and a message/rfc822 portion with the original message enclosed. The Content-Type: header field becomes multipart/mixed. The optional Subject: header is specified by the ":subject" argument; if not present, the subject will be taken from the enclosed message. Any headers specified by ":headers" are copied from the old message into the new message. If not specified by ":headers", Date: and From: headers should be synthesized to reflect the current date and the user running the Sieve action. Hansen & Daboo Standards Track [Page 10]
RFC 5703 Sieve MIME Operations October 2009 7. Action "extracttext" Usage: extracttext [MODIFIER] [":first" number] <varname: string> The "extracttext" action may be used within the context of a "foreverypart" loop and is used to store text into a variable as defined by [RFC5229]. Servers MUST support transcoding of any textual body part into UTF-8 for use with this action. This requires decoding any transfer encoding as well as transcoding from the indicated character set into UTF-8. It stores at most ":first" characters of the transcoded content of the current MIME body part in the variable identified by varname. If the ":first" parameter is not present, the whole content of the current MIME body part is stored. In either case, the actually stored data MAY be truncated to conform to implementation specific limit on variable length and/or on MIME body part length. If the transfer encoding or character set is unrecognized by the implementation or recognized but invalid, an empty string will result. If "extracttext" is used outside the context of a "foreverypart" loop, the action will set the variable identified by varname to the empty string. This SHOULD be flagged as a compilation error. Modifiers are applied on the extracted text before it is stored in the variable. 8. Sieve Capability Strings A Sieve implementation that defines the "foreverypart" and "break" actions will advertise the capability string "foreverypart". A Sieve implementation that defines the ":mime" and ":anychild" tagged arguments to the "header", "address", and "exists" commands will advertise the capability string "mime". A Sieve implementation that defines the "replace" action will advertise the capability string "replace". A Sieve implementation that defines the "enclose" action will advertise the capability string "enclose". A Sieve implementation that defines the "extracttext" action will advertise the capability string "extracttext". Note that to be useful, the "extracttext" action also requires the "variables" [RFC5229] and "foreverypart" capabilities. Hansen & Daboo Standards Track [Page 11]
RFC 5703 Sieve MIME Operations October 2009 9. Examples 9.1. Example 1 Consider a Sieve script to replace some of the Windows executable attachments in a message. (The actual list of executable types and extensions is considerably longer and constantly changing. The tests shown here are an example only.) Such a script might look like this: require [ "foreverypart", "mime", "replace" ]; foreverypart { if anyof ( header :mime :contenttype :is "Content-Type" "application/exe", header :mime :param "filename" :matches ["Content-Type", "Content-Disposition"] "*.com" ) { replace "Executable attachment removed by user filter"; } } 9.2. Example 2 Consider a Sieve script to warn the user about some of the executable attachment types. (The actual list of executable types and extensions is considerably longer and constantly changing. The tests shown here are an example only.) Such a script might look like this: require [ "foreverypart", "mime", "enclose" ]; foreverypart { if header :mime :param "filename" :matches ["Content-Type", "Content-Disposition"] ["*.com", "*.exe", "*.vbs", "*.scr", "*.pif", "*.hta", "*.bat", "*.zip" ] { # these attachment types are executable enclose :subject "Warning" :text WARNING! The enclosed message contains executable attachments. These attachment types may contain a computer virus program that can infect your computer and potentially damage your data. Before clicking on these message attachments, you should verify with the sender that this message was sent by them and not a computer virus. Hansen & Daboo Standards Track [Page 12]
RFC 5703 Sieve MIME Operations October 2009 . ; break; } } 9.3. Example 3 A Sieve script to extract subject and text out of messages from the boss might look like this: require ["mime", "variables", "extracttext"]; if header :contains "from" "boss@example.org" { # :matches is used to get the value of the Subject header if header :matches "Subject" "*" { set "subject" "${1}"; } # extract the first 100 characters of the first text/* part foreverypart { if header :mime :type :is "Content-Type" "text" { extracttext :first 100 "msgcontent"; break; } } # if it's not a 'for your information' message if not header :contains "subject" "FYI:" { # do something using ${subject} and ${msgcontent} # such as sending a notification using a # notification extension } } 10. Acknowledgements Comments from members of the MTA Filters Working Group, in particular Ned Freed, Kjetil Torgrim Homme, Mark Mallett, Alexey Melnikov, Aaron Stone, and Nigel Swinson are gratefully acknowledged. Hansen & Daboo Standards Track [Page 13]
RFC 5703 Sieve MIME Operations October 2009 11. Security Considerations The "enclose" action creates an entirely new message, as compared to just redirecting or forwarding the existing message. Therefore, any site policies applicable to message submission should be enforced. The looping specification specified here provides easier access to information about the message contents, which may also be achieved through other sieve tests. This is not believed to raise any additional security issues beyond those for the Sieve "envelope" and "body" [RFC5173] tests. Any change in message content may interfere with digital signature mechanisms that include that content in the signed material. In particular, using "replace" makes direct changes to the body content and will affect the body hash included in Domain Keys Identified Mail (DKIM) signatures [RFC4871], or the message signature used for Secure MIME (S/MIME) [RFC3851], Pretty Good Privacy (PGP) [RFC1991] or OpenPGP [RFC4880]. It is not possible to examine the MIME structure of decrypted content in a multipart/encrypted MIME part. When "enclose" is used on a message containing a multipart/signed MIME part, the Sieve implementation MUST ensure that the original message is copied octet-for-octet to maintain the validity of the digital signature. The system MUST be sized and restricted in such a manner that even malicious use of MIME part matching does not deny service to other users of the host system. All of the security considerations given in the base Sieve specification also apply to these extensions. 12. IANA Considerations The Original-Subject and Original-From headers have been registered in the Permanent Message Header Fields registry. The following templates specify the IANA registrations of the Sieve extensions specified in this document. This information has been added to the IANA registry of Sieve Extensions (currently found at http://www.iana.org). Hansen & Daboo Standards Track [Page 14]
RFC 5703 Sieve MIME Operations October 2009 12.1. foreverypart capability To: iana@iana.org Subject: Registration of new Sieve extension Capability name: foreverypart Description: adds the "foreverypart" and "break" actions for iterating through MIME parts of a message. RFC number: RFC 5703 Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>. 12.2. mime capability To: iana@iana.org Subject: Registration of new Sieve extension Capability name: mime Description: adds the ":mime" and ":anychild" tagged arguments to the "header", "address", and "exists" tests. Adds the ":type", ":subtype", ":contenttype", and ":param" options when ":mime" is used with the "header" test. RFC number: RFC 5703 Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>. 12.3. replace capability To: iana@iana.org Subject: Registration of new Sieve extension Capability name: replace Description: adds the "replace" action for replacing a MIME body part of a message. RFC number: RFC 5703 Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>. Hansen & Daboo Standards Track [Page 15]
RFC 5703 Sieve MIME Operations October 2009 12.4. enclose capability To: iana@iana.org Subject: Registration of new Sieve extension Capability name: enclose Description: adds the "enclose" action for enclosing a message with a wrapper. RFC number: RFC 5703 Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>. 12.5. extracttext capability To: iana@iana.org Subject: Registration of new Sieve extension Capability name: extracttext Description: adds the "extracttext" action for extracting text from a MIME body part. RFC number: RFC 5703 Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>. 13. References 13.1. Normative References [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations", RFC 2231, November 1997. Hansen & Daboo Standards Track [Page 16]
RFC 5703 Sieve MIME Operations October 2009 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC5173] Degener, J. and P. Guenther, "Sieve Email Filtering: Body Extension", RFC 5173, April 2008. [RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering Language", RFC 5228, January 2008. [RFC5229] Homme, K., "Sieve Email Filtering: Variables Extension", RFC 5229, January 2008. [RFC5231] Segmuller, W. and B. Leiba, "Sieve Email Filtering: Relational Extension", RFC 5231, January 2008. [RFC5293] Degener, J. and P. Guenther, "Sieve Email Filtering: Editheader Extension", RFC 5293, August 2008. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. 13.2. Informative References [RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message Exchange Formats", RFC 1991, August 1996. [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 3851, July 2004. [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures", RFC 4871, May 2007. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", RFC 4880, November 2007. Hansen & Daboo Standards Track [Page 17]
RFC 5703 Sieve MIME Operations October 2009 Authors' Addresses Tony Hansen AT&T Laboratories 200 Laurel Ave. Middletown, NJ 07748 USA EMail: tony+sieveloop@maillennium.att.com Cyrus Daboo Apple Inc. 1 Infinite Loop Cupertino, CA 95014 USA EMail: cyrus@daboo.name URI: http://www.apple.com/ Hansen & Daboo Standards Track [Page 18]