Sieve Email Filtering: Extension for Processing Calendar Attachments

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 25 April 2024.¶

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

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 ]
                       [ :outcome <variablename: string> ]
                       [ :errstr <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 "variables" [RFC5229] extension. When the "variables" extension is enabled in a script using <require "variables">, the script can use the :outcome (Section 4.6) and :errstr (Section 4.7) arguments to the "processcalendar" action described below. When the "variables" extension is not enabled, the :outcome and :errstr arguments MUST NOT be used and MUST cause an error according to [RFC5228].¶

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_error" "";

processcalendar :outcome "processcal_outcome"
                :errstr "processcal_error";

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

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 make processcalendar conditional on the sender not being "untrustworthy".¶
• 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 make processcalendar conditional on the sender being "trustworthy".¶

7. Privacy Considerations

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

Authors' Addresses