Internet Engineering Task Force (IETF) K. Murchison
Request for Comments: 9671 R. Signes
Category: Standards Track M. Horsfall
ISSN: 2070-1721 Fastmail
October 2024
Sieve Email Filtering: Extension for Processing Calendar Attachments
Abstract
This document describes the "processcalendar" extension to the Sieve
email filtering language. The "processcalendar" extension gives
Sieve the ability to process machine-readable calendar data that is
encapsulated in an email message using Multipurpose Internet Mail
Extensions (MIME).
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9671.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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in the Revised BSD License.
Table of Contents
1. Introduction
2. Conventions Used in This Document
3. Capability Identifier
4. Process Calendar Action
4.1. Allow Public Argument
4.2. Addresses Argument
4.3. Updates Only Argument
4.4. Calendar ID Argument
4.5. Delete Cancelled Argument
4.6. Organizers Argument
4.7. Outcome Argument
4.8. Reason Argument
4.9. Interaction with Other Sieve Actions
4.10. Examples
5. Security Considerations
6. Privacy Considerations
7. IANA Considerations
7.1. Registration of Sieve Extension
7.2. Registration of Sieve Action
8. References
8.1. Normative References
8.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
Users frequently receive invites, replies, and cancellations for
events, tasks, etc. via Internet mail messages. It is sometimes
desirable to have such messages automatically parsed and the enclosed
calendar data added to, updated on, or deleted from the user's
calendars.
Typically, such messages are based on the iCalendar Message-Based
Interoperability Protocol (iMIP) [RFC6047]. However, sometimes the
enclosed iCalendar [RFC5545] data does not include an iCalendar
Transport-Independent Interoperability Protocol (iTIP) method
property (see [RFC5546], Section 1.4), or the enclosed data may be in
some other machine-readable format (e.g., JSCalendar [RFC8984]).
This document defines an extension to the Sieve language [RFC5228]
that enables scripts to process machine-readable calendar data that
is encapsulated in an email message using MIME [RFC2045].
Specifically, this extension provides the ability to alter items on a
user's calendars that are referenced in the encapsulated calendar
data.
2. Conventions Used in This Document
Conventions for notations are as in Section 1.1 of [RFC5228],
including use of the "Usage:" label for the definition of action and
tagged arguments syntax.
This document uses terminology and concepts from iCalendar [RFC5545]
and iTIP [RFC5546] to describe the processing of calendar data, but
this extension can be used with any machine-readable calendar data
format that can express similar concepts.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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 may be created,
updated, or deleted from a given calendar.
This action can be used with or without the "extlists" extension
[RFC6134]. When the "extlists" extension is enabled in a script
using <require "extlists">, the script can use the :organizers
argument (Section 4.6) in 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" extension
[RFC5229]. 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 in 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 message 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. Failure to do so could result in unwelcome
notifications being triggered for the recipient.
4.1. Allow Public Argument
The optional :allowpublic argument is used to tell the implementation
that it can process calendar data that does not contain any ATTENDEE
properties, such as iTIP messages where the METHOD is PUBLISH or non-
iTIP messages where the calendar data does not contain METHOD and/or
ORGANIZER properties.
If used in conjunction with the :organizers argument (Section 4.6),
the implementation MUST NOT process non-iTIP messages.
If :allowpublic is omitted, the implementation MUST NOT process
calendar data unless is it is a well-formed iTIP message and one of
the recipient user's email addresses matches the Calendar User
Address (see Section 3.3.3 of [RFC5545]) of the intended target of
the message, as determined by the iTIP method (see Section 1.4 of
[RFC5546]) of the message:
* "REPLY": Value of the ORGANIZER property (see Section 3.8.4.3 of
[RFC5545])
* "REQUEST", "CANCEL", "ADD": Value of one of the ATTENDEE
properties (see Section 3.8.4.1 of [RFC5545])
The recipient user's email address matches the Calendar User Address
of the target if the Calendar User Address is in the form of a mailto
URI and the email address matches the "addr-spec" of the URI.
An email address is considered to belong to the recipient if it is
one of the following:
* an email address known by the implementation to be associated with
the recipient,
* the final envelope recipient address if it's available to the
implementation, or
* an address specified by the script writer via the :addresses
argument (Section 4.2).
4.2. Addresses Argument
The optional :addresses argument is used to specify email addresses
that belong to the recipient in addition to the addresses known to
the implementation.
4.3. Updates Only Argument
The optional :updatesonly argument is used to limit the messages
processed to those targeting existing calendar objects only. If the
message contains a new calendar object (its unique identifier does
not exist on any of the user's calendars), the implementation MUST
NOT add the object to a calendar.
If :updatesonly is omitted, new calendar objects may be added to one
of the user's calendars.
The :updatesonly and :calendarid (Section 4.4) arguments are
incompatible with each other. It is an error if both arguments are
used in the same "processcalendar" action.
4.4. Calendar ID Argument
The optional :calendarid argument specifies the identifier of the
calendar onto which new calendar objects should be placed.
If :calendarid is omitted, new calendar objects will be placed on the
user's "default" calendar as determined by the implementation.
The :updatesonly (Section 4.3) and :calendarid arguments are
incompatible with each other. It is an error if both arguments are
used in the same "processcalendar" action.
4.5. Delete Cancelled Argument
The optional :deletecancelled argument is used to tell the
implementation that if it receives a cancellation message, it SHOULD
remove the associated calendar object from the calendar.
If :deletecancelled is omitted, the status of the associated calendar
object will be set to cancelled and will remain on the calendar.
4.6. Organizers Argument
The optional :organizers argument is used to specify an external list
of email addresses from which the recipient is willing to accept
public events, invites, updates, and cancellations. Implementations
MUST NOT process calendar data unless is it is a well-formed iTIP
message and one of the addresses in the external list matches the
Calendar User Address of the ORGANIZER property. An email address in
the external list matches the Calendar User Address of the ORGANIZER
property if it is in the form of a mailto URI and the email address
matches the "addr-spec" of the URI.
If :organizers is omitted, no validation of the ORGANIZER property is
performed.
4.7. Outcome Argument
The optional :outcome argument specifies the name of a variable into
which one of the following strings specifying the outcome of the
action will be stored:
"no_action": No action was performed (e.g., the message didn't
contain calendar data, or the set of provided options prevented
the message from being processed).
"added": A new calendar object was added to a calendar.
"updated": A calendar resource object was updated, cancelled, or removed from
the calendar.
"error": The message would have been processed but encountered an
error in doing so.
4.8. Reason Argument
The optional :reason argument specifies the name of a variable into
which a string describing the reason for the outcome will be stored.
If no reason for the outcome is available, implementations MUST set
the variable to the empty string.
For example, an outcome of "no_action" may have a reason of "only
processing updates", or an outcome of "error" may have a reason of
"missing unique identifier".
4.9. Interaction with Other Sieve Actions
The "processcalendar" action does not cancel Sieve's implicit keep
action.
The "processcalendar" action can only be executed once per script. A
script MUST fail with an appropriate error if it attempts to execute
two or more "processcalendar" actions.
The "processcalendar" action is incompatible with the Sieve "reject"
and "ereject" actions [RFC5429].
4.10. Examples
The following example specifies email addresses belonging to the user
and the identifier of the calendar onto which to place new calendar
objects:
require [ "processcalendar" ];
processcalendar :addresses [ "me@example.com", "alsome@example.com" ]
:calendarid "1ea6d86b-6c7f-48a2-bed3-2a4c40ec281a";
The following example tells the interpreter to process flight
itineraries from a particular airline:
require [ "processcalendar" ];
if allof (address ["from", "sender"] "airline@example.com",
header :contains "subject" "itinerary") {
processcalendar :allowpublic;
}
The following example adds headers to the message if calendar data
isn't processed :
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. 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:
* The "processcalendar" extension MUST NOT process any calendar data
enclosed in a message flagged as spam and/or malicious. The
"spamtest" and "virustest" extensions [RFC5235] (or the header
test [RFC5228] if messages are scanned outside of the Sieve
interpreter) can be used to make "processcalendar" conditional on
"safe" content.
* The "processcalendar" extension SHOULD NOT process calendar data
received from a potentially malicious sender. The address and
envelope tests [RFC5228] (optionally along with the "extlists"
extension [RFC6134]) can be used to create a "deny list" and make
"processcalendar" conditional on the sender not being a member of
that list.
* Similarly, the "processcalendar" extension SHOULD only process
calendar data received from a known sender. The address and
envelope tests [RFC5228] (optionally along with the "extlists"
extension [RFC6134]) can be used to create an "allow list" and
make "processcalendar" conditional on the sender being a member of
that list.
* The "processcalendar" extension 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 the following passes or fails on the
message: Sender Policy Framework (SPF) [RFC7208], DomainKeys
Identified Mail (DKIM) Signatures [RFC6376], and Domain-based
Message Authentication, Reporting, and Conformance (DMARC)
[RFC7489]. 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 test [RFC5228] 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.
6. Privacy Considerations
It is believed that this extension doesn't introduce any privacy
considerations beyond those in [RFC5228].
7. IANA Considerations
7.1. Registration of Sieve Extension
This document defines the following new Sieve extension, which IANA
has added to the "Sieve Extensions" registry
(https://www.iana.org/assignments/sieve-extensions). The registry is
defined in Section 6.2 of [RFC5228].
Capability name: processcalendar
Description: Adds the "processcalendar" action command to add and
update items on a user's calendars.
RFC number: RFC 9671
Contact address: Sieve discussion list <sieve@ietf.org>
7.2. Registration of Sieve Action
This document defines the following new Sieve action, which IANA has
added to the "Sieve Actions" registry
(https://www.iana.org/assignments/sieve-extensions). The registry is
defined in Section 2.1 of [RFC9122].
Name: processcalendar
Description: Add and update items on a user's calendars
References: RFC 9671 [RFC5229] [RFC6134]
Capabilities: "processcalendar", "variables", "extlists"
Action Interactions: This action is incompatible with the "reject"
and "ereject" actions.
Cancels Implicit Keep? No
Can Use with IMAP Events? No
8. References
8.1. Normative References
[CALSPAM] The Calendaring and Scheduling Consortium, "Calendar
operator practices - Guidelines to protect against
calendar abuse", CC/R 18003:2019, 2019,
<https://standards.calconnect.org/csd/cc-18003.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5228] Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
Filtering Language", RFC 5228, DOI 10.17487/RFC5228,
January 2008, <https://www.rfc-editor.org/info/rfc5228>.
[RFC5229] Homme, K., "Sieve Email Filtering: Variables Extension",
RFC 5229, DOI 10.17487/RFC5229, January 2008,
<https://www.rfc-editor.org/info/rfc5229>.
[RFC6047] Melnikov, A., Ed., "iCalendar Message-Based
Interoperability Protocol (iMIP)", RFC 6047,
DOI 10.17487/RFC6047, December 2010,
<https://www.rfc-editor.org/info/rfc6047>.
[RFC6134] Melnikov, A. and B. Leiba, "Sieve Extension: Externally
Stored Lists", RFC 6134, DOI 10.17487/RFC6134, July 2011,
<https://www.rfc-editor.org/info/rfc6134>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9122] Melnikov, A. and K. Murchison, "IANA Registry for Sieve
Actions", RFC 9122, DOI 10.17487/RFC9122, June 2023,
<https://www.rfc-editor.org/info/rfc9122>.
8.2. Informative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC5235] Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
Extensions", RFC 5235, DOI 10.17487/RFC5235, January 2008,
<https://www.rfc-editor.org/info/rfc5235>.
[RFC5429] Stone, A., Ed., "Sieve Email Filtering: Reject and
Extended Reject Extensions", RFC 5429,
DOI 10.17487/RFC5429, March 2009,
<https://www.rfc-editor.org/info/rfc5429>.
[RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and
Scheduling Core Object Specification (iCalendar)",
RFC 5545, DOI 10.17487/RFC5545, September 2009,
<https://www.rfc-editor.org/info/rfc5545>.
[RFC5546] Daboo, C., Ed., "iCalendar Transport-Independent
Interoperability Protocol (iTIP)", RFC 5546,
DOI 10.17487/RFC5546, December 2009,
<https://www.rfc-editor.org/info/rfc5546>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/info/rfc8551>.
[RFC8984] Jenkins, N. and R. Stepanek, "JSCalendar: A JSON
Representation of Calendar Data", RFC 8984,
DOI 10.17487/RFC8984, July 2021,
<https://www.rfc-editor.org/info/rfc8984>.
Acknowledgments
The authors would like to thank the following individuals for
contributing their ideas and support for writing this specification:
Ned Freed and Alexey Melnikov.
Authors' Addresses
Kenneth Murchison
Fastmail US LLC
1429 Walnut Street, Suite 1201
Philadelphia, PA 19102
United States of America
Email: murch@fastmailteam.com
Ricardo Signes
Fastmail US LLC
1429 Walnut Street, Suite 1201
Philadelphia, PA 19102
United States of America
Email: rjbs@fastmailteam.com
Matthew Horsfall
Fastmail US LLC
1429 Walnut Street, Suite 1201
Philadelphia, PA 19102
United States of America
Email: alh@fastmailteam.com