Sieve Email Filtering: Extension for Processing Calendar Attachments

Status of This Memo

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Table of Contents

3. Capability Identifier

Sieve interpreters that implement this extension MUST have an identifier of "processcalendar" for use with the capability mechanism.¶

4. Process Calendar Action

Usage: processcalendar [ :allowpublic ]
                       [ :addresses <string-list> ]
                       [ :updatesonly / :calendarid <string> ]
                       [ :deletecancelled ]
                       [ :organizers <ext-list-name: string> ]
                       [ :outcome <variablename: string> ]
                       [ :reason <variablename: string> ]

The "processcalendar" action is used to parse encapsulated calendar data and perform the appropriate action based on the content. If the calendar data is malformed in any way, it MUST be ignored and no action is taken. Otherwise, based on the iTIP method (see Section 1.4 of [RFC5546]) of the message, calendar objects are created, updated, or deleted from a given calendar.¶

This action can be used with or without the "extlists" [RFC6134] extension. When the "extlists" extension is enabled in a script using <require "extlists">, the script can use the :organizers (Section 4.6) argument to the "processcalendar" action as described below. When the "extlists" extension is not enabled, the :organizers argument MUST NOT be used and MUST cause an error according to [RFC5228].¶

This action can be used with or without the "variables" [RFC5229] extension. When the "variables" extension is enabled in a script using <require "variables">, the script can use the :outcome (Section 4.7) and :reason (Section 4.8) arguments to the "processcalendar" action as described below. When the "variables" extension is not enabled, the :outcome and :reason arguments MUST NOT be used and MUST cause an error according to [RFC5228].¶

If a mail messages contains calendar data in multiple MIME [RFC2045] parts, this action MUST verify that the calendar data in each part are semantically equivalent to one another. If the data is found to be semantically different, the action MUST NOT process the message. Otherwise, the action MUST only process one representation of the data.¶

This action MUST NOT make any changes to the participant status of the recipient when processing the calendar data. The mechanism for a recipient to change their participant status to an event is out of scope for this document.¶

This action SHOULD remove alarms from calendar data before applying it to a calendar.¶

require [ "processcalendar" ];

processcalendar :addresses [ "me@example.com", "alsome@example.com" ]
                :calendarid "1ea6d86b-6c7f-48a2-bed3-2a4c40ec281a";

require [ "processcalendar" ];

if allof (address ["from", "sender"] "airline@example.com",
          header :contains "subject" "itinerary") {
   processcalendar :allowpublic;
}

require [ "processcalendar", "variables", "editheader" ];

set "processcal_outcome" "no_action";
set "processcal_reason" "";

processcalendar :outcome "processcal_outcome"
                :reason "processcal_reason";

if not string :is "${processcal_outcome}" ["added", "updated"] {
   addheader "X-ProcessCal-Outcome" "${processcal_outcome}";
   addheader "X-ProcessCal-Reason" "${processcal_reason}";
}

5. Implementation Status

< RFC Editor: before publication please remove this section and the reference to [RFC7942] >¶

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶

According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶

6. Security Considerations

This document describes a method for altering an electronic calendar without user interaction. As such, unless proper precautions are undertaken, it can be used as a vector for calendar abuse.¶

It is critical that implementations correctly implement the behavior and restrictions described throughout this document. Security issues associated with processing unsolicited calendar data, and methods for mitigating them are discussed in [CALSPAM]. Specifically:¶

• Processcalendar MUST NOT process any calendar data enclosed in a message flagged as spam and/or malicious. The spamtest and virustest [RFC5235] extensions (or the header [RFC5228] test if messages are scanned outside of the Sieve interpreter) can be used to make processcalendar conditional on "safe" content.¶
• Processcalendar SHOULD NOT process calendar data received from a potentially malicious sender. The address and envelope [RFC5228] tests (optionally along with the extlists [RFC6134] extension) can be used to create a "deny list" and make processcalendar conditional on the sender not being a member of that list.¶
• Similarly, processcalendar SHOULD only process calendar data received from a known sender. The address and envelope [RFC5228] tests (optionally along with the extlists [RFC6134] extension) can be used to create an "allow list" and make processcalendar conditional on the sender being a member of that list.¶
• Processcalendar SHOULD NOT process calendar data received from an untrustworthy sender. Trustworthiness may depend on whether the message has a valid signature (see [RFC8551]) and/or on whether one or more of Sender Policy Framework (SPF) [RFC7208], DomainKeys Identified Mail (DKIM) Signatures [RFC6376], Domain-based Message Authentication, Reporting, and Conformance (DMARC) [RFC7489] passes or fails on the message. The mechanism by which a Sieve interpreter accesses the results of such checks is outside the scope of this document, but if the results are available in the message's header fields, the header [RFC5228] test can be used to make processcalendar conditional on the sender being trustworthy.¶

Additionally, if the calendar data has embedded (a.k.a. inline) attachments, implementations SHOULD:¶

• Decode the embedded attachment, if necessary.¶
• Scan the (decoded) attachment for malicious content.¶

If an attachment is found to be malicious, processcalendar MUST NOT process the calendar data.¶

7. Privacy Considerations

It is believed that this extension doesn't introduce any privacy considerations beyond those in [RFC5228].¶

Authors' Addresses