RFC 4314

IMAP4 Access Control List (ACL) Extension

📅 published December 2005, from ietf

RFC 4314: IMAP4 Access Control List (ACL) Extension 💖

Okay bestie, first things first 🎀 This is RFC 4314, called "IMAP4 Access Control List (ACL) Extension." It came from the Network Working Group and was written by A. Melnikov at Isode Ltd. It's dated December 2005 and it's a Standards Track document. It obsoletes RFC 2086, and that's the only entry in its header. ✨

Status of this Memo 📜

This is an Internet standards track protocol for the Internet community, and the authors are literally asking for discussion and suggestions for improvements 🫶 If you want the official standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited. 🌸

Copyright Notice 🦋

Abstract ✨

So the Access Control List (ACL) extension, defined in RFC 2086, is what lets the Internet Message Access Protocol (IMAP) fetch and edit mailbox access control lists through IMAP itself 🔥 This document is a revision of RFC 2086. It adds several new access control rights and clarifies which rights are needed for different IMAP commands. It's giving upgrade 💅

Table of Contents 🗂️

   1. Introduction and Overview .......................................3
      1.1. Conventions Used in This Document ..........................3
   2. Access Control ..................................................3
      2.1. Standard Rights ............................................5
           2.1.1. Obsolete Rights .....................................5
      2.2. Rights Defined in RFC 2086 .................................8
   3. Access control management commands and responses ................8
      3.1. SETACL Command .............................................8
      3.2. DELETEACL Command ..........................................9
      3.3. GETACL Command ............................................10
      3.4. LISTRIGHTS Command ........................................10
      3.5. MYRIGHTS Command ..........................................11
      3.6. ACL Response ..............................................11
      3.7. LISTRIGHTS Response .......................................12
      3.8. MYRIGHTS Response .........................................12
   4. Rights Required to Perform Different IMAP4rev1 Commands ........12
   5. Other Considerations ...........................................17
      5.1. Additional Requirements and Implementation Notes ..........17
           5.1.1. Servers ............................................17
           5.1.2. Clients ............................................18
      5.2. Mapping of ACL Rights to READ-WRITE and READ-ONLY
           Response Codes ............................................19
   6. Security Considerations ........................................20
   7. Formal Syntax ..................................................21
   8. IANA Considerations ............................................22
   9. Internationalization Considerations ............................22
   Appendix A. Changes since RFC 2086 ................................23
   Appendix B. Compatibility with RFC 2086 ...........................24
   Appendix C. Known Deficiencies ....................................24
   Appendix D. Acknowledgements ......................................25
   Normative References ..............................................25
   Informative References ............................................25

1. Introduction and Overview 🦋

The ACL (Access Control List) extension of the Internet Message Access Protocol [IMAP4] lets you retrieve and manipulate mailbox access control lists through IMAP, and it's honestly so useful 💅

This document is a revision of RFC 2086 [RFC2086]. It clears up a bunch of ambiguities in RFC 2086. The big ones are the use of UTF-8 [UTF-8] in access identifiers, which rights are required for different IMAP4 commands, and how the READ-WRITE and READ-ONLY response codes relate to ACL. Iconic 👑

1.1. Conventions Used in This Document 🎀

In examples, "C:" and "S:" show lines sent by the client and the server respectively. 🌈

In all examples, the "/" character is used as the hierarchy separator. 🍓

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 RFC 2119 [KEYWORDS]. These are the official requirement words, so they carry real weight in this doc 💀

The phrase "ACL server" is just a shortcut for "IMAP server that supports the ACL extension as defined in this document." 🧚

2. Access Control 🔐

The ACL extension is present in any IMAP4 implementation that returns "ACL" as one of its supported capabilities to the CAPABILITY command. So if you see ACL in the capability list, you're in the club 💖

A server implementation conformant to this document MUST also return the rights (see below) that are not defined in Section 2.2 in the RIGHTS= capability. Non-negotiable, bestie 🫶

An access control list is a set of <access identifier,rights> pairs. Each ACL applies to a mailbox name. ✨

An access identifier (or just "identifier") is a UTF-8 [UTF-8] string. The identifier "anyone" is reserved for the universal identity, meaning all authentications, including anonymous ones 🌸 All user name strings accepted by the LOGIN or AUTHENTICATE commands to authenticate to the IMAP server are reserved as identifiers for the corresponding users. Identifiers that start with a dash ("-") are reserved for "negative rights," which we'll get into below. Every other identifier string is interpreted in an implementation-defined manner 🌈

Rights is a string listing a (possibly empty) set of alphanumeric characters. Each character lists a set of operations being controlled. Lowercase letters are reserved for "standard" rights, which are listed in Section 2.1. (Note that uppercase rights are not allowed, for compatibility with deployed clients and servers 🙈) The set of standard rights can only be extended by a standards-track document. Digits are reserved for implementation- or site-defined rights. 🔥

An implementation MAY tie rights together, and it MAY force rights to always or never be granted to particular identifiers. For example, in an implementation that uses UNIX mode bits, the rights "swite" are tied together, and the "a" right is always granted to the owner of a mailbox and is never granted to another user. If rights are tied in an implementation, the implementation must be conservative when granting rights in response to SETACL commands. Unless all rights in a tied set are specified, none of that set should be included in the ACL entry for that identifier. A client can discover the set of rights that may be granted to a given identifier in the ACL for a given mailbox name by using the LISTRIGHTS command. 💅

It's possible for multiple identifiers in an access control list to apply to a single user. For example, an ACL may include rights granted to the identifier matching the user, one or more implementation-defined identifiers matching groups that include the user, and/or the identifier "anyone." How these rights get combined to determine the user's access is implementation defined. An implementation may choose, for example, to use the union of the rights granted to the applicable identifiers. An implementation may instead choose, for example, to use only the rights granted to the most specific identifier present in the ACL. A client can find out the set of rights granted to the logged-in user for a given mailbox name by using the MYRIGHTS command. It's a whole vibe 🦋

When an identifier in an ACL starts with a dash ("-"), that means the associated rights are to be removed from the identifier prefixed by the dash. This is called a "negative right." It's different from DELETEACL, because a negative right is added to the ACL and is part of the calculation of the rights. 🧚

Let's say an identifier "fred" refers to a user with the login "fred." If the identifier "-fred" is granted the "w" right, that means the "w" right is removed from users matching the identifier "fred," even if the user "fred" has the "w" right because of some other identifier in the ACL. A DELETEACL of "fred" just deletes the identifier "fred" from the ACL. It doesn't affect any rights that the user "fred" might get from another entry in the ACL, and in particular it doesn't affect rights granted to the identifier "-fred." Main character energy 👑

Server implementations are not required to support "negative right" identifiers. 🥺

2.1. Standard Rights 🎀

The currently defined standard rights are listed below. The list doesn't include every command that uses a particular right, so don't panic if a command isn't there 🍓

   l - lookup (mailbox is visible to LIST/LSUB commands, SUBSCRIBE
       mailbox)
   r - read (SELECT the mailbox, perform STATUS)
   s - keep seen/unseen information across sessions (set or clear
       \SEEN flag via STORE, also set \SEEN during APPEND/COPY/
       FETCH BODY[...])
   w - write (set or clear flags other than \SEEN and \DELETED via
       STORE, also set them during APPEND/COPY)
   i - insert (perform APPEND, COPY into mailbox)
   p - post (send mail to submission address for mailbox,
       not enforced by IMAP4 itself)
   k - create mailboxes (CREATE new sub-mailboxes in any
       implementation-defined hierarchy, parent mailbox for the new
       mailbox name in RENAME)
   x - delete mailbox (DELETE mailbox, old mailbox name in RENAME)
   t - delete messages (set or clear \DELETED flag via STORE, set
       \DELETED flag during APPEND/COPY)
   e - perform EXPUNGE and expunge as a part of CLOSE
   a - administer (perform SETACL/DELETEACL/GETACL/LISTRIGHTS)

2.1.1. Obsolete Rights 🦋

Okay bestie, so RFC 2086 was a little vague 🙈 and that caused drama. Some RFC 2086 servers used the c right to control DELETE, and others used d. So for the first group, the "create" right is the union of k and x, and the "delete" right is the union of e and t. For the second group, "create" is just a synonym for k, and "delete" is the union of e, t, and x. Iconic chaos 💀

For compatibility with RFC 2086, this section defines two virtual rights, d and c 🌸

If a client includes the d right in a rights list, then it MUST be treated as if the client had included every member of the "delete" right. It's not an error to specify d and one or more members of "delete", but the result is the same as specifying just d or all the "delete" members. When any "delete" member right is set in a list of rights, the server MUST also include d when it returns the list in a MYRIGHTS or ACL response. This is so older RFC 2086 clients can work with newer servers 🫶 (*)

Example:

C: A001 SeTacl INBOX/Drafts David lrswida
S: A001 OK Setacl complete

The client has specified the d right in the SETACL command above, and it expands to et on the server:

C: A002 getacl INBOX/Drafts
S: * ACL INBOX Fred rwipslxcetda David lrswideta
S: A002 OK Getacl complete

If the identifier in the LISTRIGHTS command can be granted any of the "delete" member rights on a mailbox, then the server MUST include d in the corresponding LISTRIGHTS response. (*) If the member rights aren't tied to non-member rights, then d is returned by itself in the LISTRIGHTS response. If any of the member rights needs to be tied to one (or more) non-member right, then d and all of the member rights need to be tied to the same non-member right(s). (**)

If a client includes the c right in a rights list, then it MUST be treated as if the client had included every member of the "create" right. It's not an error to specify c and one or more "create" members, but the result is the same as specifying just c or all the "create" members.

When any "create" member right is set in a list of rights, the server MUST also include c when it returns the list in a MYRIGHTS or ACL response. This is so older RFC 2086 clients can work with newer servers 💖 (*)

Example:

C: A003 Setacl INBOX/Drafts Byron lrswikda
S: A001 OK Setacl complete
C: A002 getAcl INBOX/Drafts
S: * ACL INBOX Fred rwipslxcetda Byron lrswikcdeta
S: A002 OK Getacl complete

The client has specified the d right in the SETACL command above, and it expands to et on the server. As the client has specified the k right (which is a member of c), the server also returns c.

If the identifier in the LISTRIGHTS command can be granted any of the "create" member rights on a mailbox, then the server MUST include c in the corresponding LISTRIGHTS response. (*) If the member rights aren't tied to non-member rights, then c is returned by itself in the LISTRIGHTS response. If any of the member rights needs to be tied to one (or more) non-member right, then c and all of the member rights need to be tied to the same non-member right(s). (**)

Example: The server that ties the rights as follows:

lr s w i p k x t

and c=k

will return:

S: * LISTRIGHTS archive/imap anyone ""
   lr s w i p k x t c d

Example: The server that ties the rights as follows:

lr s w i p k xte

and c=k

will return:

S: * LISTRIGHTS archive/imap anyone ""
   lr s w i p k xte c d

Example: The server that ties the rights as follows:

lr s w i p k x te

and c=k

will return:

S: * LISTRIGHTS archive/imap anyone ""
   lr s w i p k c x te d

Example: The server that ties the rights as follows:

lr swte i p k x

and c=kx

will return:

S: * LISTRIGHTS archive/imap anyone ""
   lr swted i p k x c

(*) Clients conforming to this document MUST ignore the virtual d and c rights in MYRIGHTS, ACL, and LISTRIGHTS responses.

(**) The IMAPEXT Working Group debated this issue at great length. After reviewing existing ACL implementations, they concluded that this is a reasonable restriction 🧚

2.2. Rights Defined in RFC 2086 💅

The RIGHTS= capability MUST NOT include any of the rights defined in RFC 2086: l, r, s, w, i, p, a, c, d, and the digits (0 .. 9).

3. Access control management commands and responses ✨

Servers, when processing a command that has an identifier as a parameter (that is, any of the SETACL, DELETEACL, and LISTRIGHTS commands), SHOULD first prepare the received identifier using the "SASLprep" profile [SASLprep] of the "stringprep" algorithm [Stringprep]. If the preparation of the identifier fails or results in an empty string, the server MUST refuse to perform the command with a BAD response. Note that Section 6 recommends additional identifier verification steps 🔥

3.1. SETACL Command 🎀

Arguments:  mailbox name
            identifier
            access right modification

Data:       no specific data for this command

Result:     OK - setacl completed
            NO - setacl failure: can't set acl
            BAD - arguments invalid

The SETACL command changes the access control list on the specified mailbox so that the specified identifier is granted permissions as specified in the third argument.

The third argument is a string containing an optional plus (+) or minus (-) prefix, followed by zero or more rights characters. If the string starts with a plus, the following rights are added to any existing rights for the identifier. If the string starts with a minus, the following rights are removed from any existing rights for the identifier. If the string does not start with a plus or minus, the rights replace any existing rights for the identifier.

Note that an unrecognized right MUST cause the command to return the BAD response. In particular, the server MUST NOT silently ignore unrecognized rights. 🙅‍♀️

Example:

C: A001 GETACL INBOX/Drafts
S: * ACL INBOX/Drafts Fred rwipslxetad Chris lrswi
S: A001 OK Getacl complete
C: A002 SETACL INBOX/Drafts Chris +cda
S: A002 OK Setacl complete
C: A003 GETACL INBOX/Drafts
S: * ACL INBOX/Drafts Fred rwipslxetad Chris lrswicdakxet
S: A003 OK Getacl complete


C: A035 SETACL INBOX/Drafts John lrQswicda
S: A035 BAD Uppercase rights are not allowed


C: A036 SETACL INBOX/Drafts John lrqswicda
S: A036 BAD The q right is not supported

3.2. DELETEACL Command 🌈

Arguments:  mailbox name
            identifier

Data:       no specific data for this command

Result:     OK - deleteacl completed
            NO - deleteacl failure: can't delete acl
           BAD - arguments invalid

The DELETEACL command removes any <identifier,rights> pair for the specified identifier from the access control list for the specified mailbox.

Example:

C: B001 getacl INBOX
S: * ACL INBOX Fred rwipslxetad -Fred wetd $team w
S: B001 OK Getacl complete
C: B002 DeleteAcl INBOX Fred
S: B002 OK Deleteacl complete
C: B003 GETACL INBOX
S: * ACL INBOX -Fred wetd $team w
S: B003 OK Getacl complete

3.3. GETACL Command 🧁

Arguments:  mailbox name

Data:       untagged responses: ACL

Result:     OK - getacl completed
            NO - getacl failure: can't get acl
           BAD - arguments invalid

The GETACL command returns the access control list for the mailbox in an untagged ACL response.

Some implementations MAY permit multiple forms of an identifier to reference the same IMAP account. Usually, such implementations will have a canonical form that is stored internally. An ACL response caused by a GETACL command MAY include a canonicalized form of the identifier that might be different from the one used in the corresponding SETACL command. 🤓

Example:

C: A002 GETACL INBOX
S: * ACL INBOX Fred rwipsldexta
S: A002 OK Getacl complete

3.4. LISTRIGHTS Command 🍓

Arguments:  mailbox name
            identifier

Data:       untagged responses: LISTRIGHTS

Result:     OK - listrights completed
            NO - listrights failure: can't get rights list
            BAD - arguments invalid

The LISTRIGHTS command takes a mailbox name and an identifier and returns information about what rights can be granted to the identifier in the ACL for the mailbox.

Some implementations MAY permit multiple forms of an identifier to reference the same IMAP account. Usually, such implementations will have a canonical form that is stored internally. A LISTRIGHTS response caused by a LISTRIGHTS command MUST always return the same form of an identifier as specified by the client. This is so the client can correlate the response with the command 🔗

Example:

C: a001 LISTRIGHTS ~/Mail/saved smith
S: * LISTRIGHTS ~/Mail/saved smith la r swicdkxte
S: a001 OK Listrights completed

Example:

C: a005 listrights archive/imap anyone
S: * LISTRIGHTS archive.imap anyone ""
   l r s w i p k x t e c d a 0 1 2 3 4 5 6 7 8 9
S: a005 Listrights successful

3.5. MYRIGHTS Command 💜

Arguments:  mailbox name

Data:       untagged responses: MYRIGHTS

Result:     OK - myrights completed
            NO - myrights failure: can't get rights
           BAD - arguments invalid

The MYRIGHTS command returns the set of rights that the user has to the mailbox in an untagged MYRIGHTS reply.

Example:

C: A003 MYRIGHTS INBOX
S: * MYRIGHTS INBOX rwiptsldaex
S: A003 OK Myrights complete

3.6. ACL Response 🦋

Data:       mailbox name
            zero or more identifier rights pairs

The ACL response occurs as a result of a GETACL command. The first string is the mailbox name the ACL applies to. This is followed by zero or more pairs of strings. Each pair contains the identifier the entry applies to, followed by the set of rights that identifier has.

Section 2.1.1 details additional server requirements related to handling of the virtual d and c rights. It's giving "read the fine print" energy ✨

3.7. LISTRIGHTS Response ✨

Data:       mailbox name
            identifier
            required rights
            list of optional rights

okay so the LISTRIGHTS response shows up whenever you send a LISTRIGHTS command 💅 The first two strings tell you which mailbox and which identifier this rights list is about. Right after the identifier comes a string with the rights that identifier will always get in that mailbox. That one can be empty, and that's totally valid 🫶

After that you get zero or more strings, and each one holds a set of rights the identifier can be granted in the mailbox. Rights in the same string are tied together, so the server MUST either grant all of the tied rights to the identifier in the mailbox or grant none of them 🔗 no splitting the squad. Section 2.1.1 goes into the extra server requirements for the virtual "d" and "c" rights, so go check that too 🧚

And the same right MUST NOT be listed more than once in the LISTRIGHTS command. one right, one spot, bestie 🌸

3.8. MYRIGHTS Response ✨

Data:       mailbox name
            rights

The MYRIGHTS response is what you get back from a MYRIGHTS command 🦋 The first string is the mailbox name these rights apply to, and the second string is the set of rights you (the client) currently have. Literally that's it, that's the whole vibe 💖

Section 2.1.1 also covers the extra server requirements for handling the virtual "d" and "c" rights here, so keep that section bookmarked 🔖

4. Rights Required to Perform Different IMAP4rev1 Commands ✨

Before executing a command, an ACL-compliant server MUST check which rights are required to perform it. 🔍 This section sorts the commands into groups by what they do, lists the rights they need, and gives the detailed description of any special processing that's required.

For the purpose of this section, the UID counterpart of a command counts as the same command. So UID COPY and COPY both need the same set of rights, no difference 👑

The table below summarizes the different rights (and combinations of rights) needed for different IMAP operations. Complex right checking and interactions aren't always easy to express in a table, so the description after the table is the primary reference. The table is the quick glance, the description is the real tea ☕

   +-------------------+---+---+---+---+---+---+---+---+---+---+---+---+
   |Operations\Rights  | l | r | s | w | i | k | x | t | e | a |Any|Non|
   +-------------------+---+---+---+---+---+---+---+---+---+---+---+---+
   |                  commands in authenticated state                  |
   +-------------------------------------------------------------------+
   |      LIST         | + |   |   |   |   |   |   |   |   |   |   |   |
   |   SUBSCRIBE       | * |   |   |   |   |   |   |   |   |   |   | * |
   |  UNSUBSCRIBE      |   |   |   |   |   |   |   |   |   |   |   | + |
   |      LSUB         | * |   |   |   |   |   |   |   |   |   |   | * |
   |CREATE (for parent)|   |   |   |   |   | + |   |   |   |   |   |   |
   |     DELETE        |   | ? |   |   |   |   | + | ? | ? |   |   |   |
   |     RENAME        |   |   |   |   |   | + | + |   |   |   |   |   |
   |  SELECT/EXAMINE   |   | + |   |   |   |   |   |   |   |   |   |   |
   |      STATUS       |   | + |   |   |   |   |   |   |   |   |   |   |
   |  SETACL/DELETEACL |   |   |   |   |   |   |   |   |   | + |   |   |
   | GETACL/LISTRIGHTS |   |   |   |   |   |   |   |   |   | + |   |   |
   |     MYRIGHTS      |   |   |   |   |   |   |   |   |   |   | + |   |
   |      APPEND       |   |   | ? | ? | + |   |   | ? |   |   |   |   |
   +-------------------------------------------------------------------+
   |                     commands in selected state                    |
   +-------------------------------------------------------------------+
   |       COPY        |   |   | ? | ? | + |   |   | ? |   |   |   |   |
   |     EXPUNGE       |   |   |   |   |   |   |   |   | + |   |   |   |
   |      CLOSE        |   |   |   |   |   |   |   |   | ? |   |   |   |
   |      FETCH        |   |   | ? |   |   |   |   |   |   |   |   |   |
   |   STORE flags     |   |   | ? | ? |   |   |   | ? |   |   |   |   |
   +-------------------+---+---+---+---+---+---+---+---+---+---+---+---+

Note: for all commands in the selected state, the "r" is implied, because it's required to SELECT or EXAMINE a mailbox 📬 Servers are not required to check for the presence of the "r" right once a mailbox has been successfully selected.

Legend 🗝️

 +     - The right is required
 *     - Only one of the rights marked with * is required
         (see description below)
 ?     - The right is OPTIONAL (see description below)
 "Any" - at least one of the "l", "r", "i", "k", "x", "a" rights is
         required
 "Non" - No rights required to perform the command

Listing and subscribing/unsubscribing mailboxes 📚

LIST: the "l" right is required. Unlike other commands (for example SELECT), though, the server MUST NOT return a NO response if it can't list a mailbox. no sad NO here 🙅‍♀️
Note that if the user has the "l" right to a mailbox "A/B", but not to its parent mailbox "A", the LIST command should behave as if the mailbox "A" doesn't exist. For example:

         C: A777 LIST "" *
         S: * LIST (\NoInferiors) "/" "A/B"
         S: * LIST () "/" "C"
         S: * LIST (\NoInferiors) "/" "C/D"
         S: A777 OK LIST completed

SUBSCRIBE: the "l" right is required only if the server checks for mailbox existence when performing SUBSCRIBE. it depends on the server, babe 🤷‍♀️
UNSUBSCRIBE: no rights are required to perform this operation. free real estate 🆓
LSUB: the "l" right is required only if the server checks for mailbox existence when performing SUBSCRIBE. Also, unlike other commands (for example SELECT), the server MUST NOT return a NO response if it can't list a subscribed mailbox. same energy as LIST, no NO 💕

Mailbox management 🏠

CREATE: the "k" right is needed on the nearest existing parent mailbox. When a new mailbox is created, it SHOULD inherit the ACL from the parent mailbox (if one exists) in the defined hierarchy 🌳
DELETE: the "x" right is needed on the mailbox. Some servers don't allow deleting a non-empty mailbox. If that's the case, the user would also need the "r", "e", and "t" rights, so they can open the mailbox and empty it 🧹
The DELETE command MUST delete the ACL associated with the deleted mailbox. gone means gone, the ACL goes with it 💀
RENAME: moving a mailbox from one parent to another requires the "x" right on the mailbox itself, and the "k" right for the new parent. For example, if the user wants to rename the mailbox named "A/B/C" to "D/E", the user must have the "x" right for the mailbox "A/B/C" and the "k" right for the mailbox "D". 🚚
The RENAME command SHOULD NOT change the ACLs on the renamed mailbox and submailboxes. the ACLs stay put 🫶

Copying or appending messages 📨

Before performing a COPY or APPEND command, the server MUST check whether the user has the "i" right for the target mailbox. If the user doesn't have the "i" right, the operation fails. 🚫
Otherwise, for each copied or appended message, the server MUST check whether the user has:
- the "t" right, when the message has the \Deleted flag set
- the "s" right, when the message has the \Seen flag set
- the "w" right, for all other message flags
Only when the user has a particular right are the corresponding flags stored for the newly created message. The server MUST NOT fail a COPY or APPEND if the user has no rights to set a particular flag. no drama, it just drops the flag 🫧

Here's an example, bestie 🎀 Let's say the user has rights on TargetMailbox:

   Example:    C: A003 MYRIGHTS TargetMailbox
               S: * MYRIGHTS TargetMailbox rwis
               S: A003 OK Myrights complete

               C: A004 FETCH 1:3 (FLAGS)
               S: * 1 FETCH (FLAGS (\Draft \Deleted)
               S: * 2 FETCH (FLAGS (\Answered)
               S: * 3 FETCH (FLAGS ($Forwarded \Seen)
               S: A004 OK Fetch Completed

               C: A005 COPY 1:3 TargetMailbox
               S: A005 OK Copy completed

               C: A006 SELECT TargetMailbox
                  ...
               S: A006 Select Completed

Let's assume the copied messages received message numbers 77:79.

               C: A007 FETCH 77:79 (FLAGS)
               S: * 77 FETCH (FLAGS (\Draft))
               S: * 78 FETCH (FLAGS (\Answered))
               S: * 79 FETCH (FLAGS ($Forwarded \Seen))
               S: A007 OK Fetch Completed

The \Deleted flag was lost on COPY, because the user has no "t" right in the target mailbox. 😭

If the MYRIGHTS command with the tag A003 had returned instead:

               S: * MYRIGHTS TargetMailbox rsti

then the response from the FETCH with the tag A007 would have been:

               C: A007 FETCH 77:79 (FLAGS)
               S: * 77 FETCH (FLAGS (\Deleted))
               S: * 78 FETCH (FLAGS ())
               S: * 79 FETCH (FLAGS (\Seen))
               S: A007 OK Fetch Completed

In this latter case, the \Answered, $Forwarded, and \Draft flags were lost on COPY, because the user has no "w" right in the target mailbox. see how the rights decide what sticks? iconic 🧚

Expunging the selected mailbox 🧽

EXPUNGE: the "e" right is needed on the selected mailbox.
CLOSE: the "e" right is needed on the selected mailbox. If the server can't expunge the mailbox because the user doesn't have the "e" right, the server MUST ignore the expunge request, close the mailbox, and return the tagged OK response. it closes quietly and still says OK, no fuss 🙈

Fetch information about a mailbox and its messages 👀

SELECT/EXAMINE/STATUS: the "r" right is needed on the mailbox.
FETCH: a FETCH request that implies setting the \Seen flag MUST NOT set it if the current user doesn't have the "s" right. no sneaky reads without the right 👑

Changing flags 🚩

STORE: the server MUST check whether the user has:
- the "t" right, when the user modifies the \Deleted flag
- the "s" right, when the user modifies the \Seen flag
- the "w" right, for all other message flags
The STORE operation SHOULD NOT fail if the user has rights to modify at least one flag specified in the STORE. The reason given is that the tagged NO response to a STORE command is not handled very well by deployed clients 🥺

Changing ACLs 🔐

SETACL/DELETEACL: the "a" right is needed on the mailbox.

Reading ACLs 📖

GETACL: the "a" right is needed on the mailbox.
MYRIGHTS: any of the following rights is required to perform the operation: "l", "r", "i", "k", "x", "a". 💅
LISTRIGHTS: the "a" right is needed on the mailbox.

5. Other Considerations ✨

5.1. Additional Requirements and Implementation Notes 🧠

5.1.1. Servers ✨

okay so this bit defines an extra capability that servers use to announce their bonus list of rights 🎀 (not counting the ones from RFC 2086). The set of rights MUST include "t", "e", "x", and "k". The extra rights can show up in any order, so don't stress about it 💅

Example:

C: 1 capability
S: * CAPABILITY IMAP4REV1 STARTTLS LITERAL+
   ACL RIGHTS=texk
S: 1 OK completed

Any server implementing an ACL extension MUST accurately reflect the current user's rights in FLAGS and PERMANENTFLAGS responses. This is a must-have, bestie 😭🫶

Example:

C: A142 SELECT INBOX
S: * 172 EXISTS
S: * 1 RECENT
S: * OK [UNSEEN 12] Message 12 is first unseen
S: * OK [UIDVALIDITY 3857529045] UIDs valid
S: * OK [UIDNEXT 4392] Predicted next UID
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Seen \Answered \Flagged \*)] L
S: A142 OK [READ-WRITE] SELECT completed
C: A143 MYRIGHTS INBOX
S: * MYRIGHTS INBOX lrwis
S: A143 OK completed

Note that in order to get better performance, the client MAY pipeline SELECT and MYRIGHTS commands. It's giving speed 🚀🦋

C: A142 SELECT INBOX
C: A143 MYRIGHTS INBOX
S: * 172 EXISTS
S: * 1 RECENT
S: * OK [UNSEEN 12] Message 12 is first unseen
S: * OK [UIDVALIDITY 3857529045] UIDs valid
S: * OK [UIDNEXT 4392] Predicted next UID
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Seen \Answered \Flagged \*)] L
S: A142 OK [READ-WRITE] SELECT completed
S: * MYRIGHTS INBOX lrwis
S: A143 OK completed

Now the plot twist 🙈 Servers MAY cache the rights a user has on a mailbox when that mailbox gets selected. So if someone's rights get changed with SETACL or DELETEACL, state-specific commands (like STORE or EXPUNGE) might not reflect the new rights until the mailbox is re-selected. If the server checks rights on every command, it SHOULD send FLAGS and PERMANENTFLAGS responses whenever they've changed. If the server notices the user no longer has read access to the mailbox, it MAY send an untagged BYE response and close the connection. It MAY also refuse to run any commands specific to the selected state until the mailbox is closed. Server implementors, please note: most clients don't handle NO responses very well 💀

An ACL server MAY modify one or more ACLs for one or more identifiers as a side effect of changing the ACL in a SETACL or DELETEACL. If it does that, it MUST send untagged ACL response(s) to tell the client about the changes it made. No sneaky edits allowed ✨

An ACL server implementation MUST treat received ACL modification commands as a possible ambiguity for any later commands affected by the ACL, as described in Section 5.5 of [IMAP4]. So a pipeline of SETACL + MYRIGHTS is an ambiguity for the server, which means the server has to finish executing SETACL before it starts on MYRIGHTS. Clients are still allowed to send that pipeline though 🌸

5.1.2. Clients

This one is for clients, and it's all about future-proofing 🔮 The following requirement is put on clients so that future extensions can happen. A client implementation that lets a user read and update ACLs MUST preserve unrecognized rights that it doesn't let the user change. In other words, if the client:

1) can read ACLs
 and
2) can update ACLs
 but
3) doesn't allow the user to change the rights the client doesn't
recognize, then it MUST preserve unrecognized rights.

Otherwise, the client could accidentally strip permissions it doesn't even understand 😭 That's a whole vibe-killer, so please don't do it 🫶

5.2. Mapping of ACL Rights to READ-WRITE and READ-ONLY Response Codes

Okay, this section is where the READ-WRITE and READ-ONLY codes get their logic 🧚 A particular ACL server implementation MAY allow "shared multiuser access" to some mailboxes. "Shared multiuser access" means multiple different users can get to the same mailbox, as long as they have the right permissions. Just because a mailbox allows shared access doesn't mean its ACL is currently set up for several users. Let's call "shared multiuser write access" the case where a user can be granted flag modification rights (any of "w", "s", or "t") 🍓

Section 4 explains which rights you need to modify different flags, so go check that out if you're curious 👑

If the ACL server treats some flags as shared for a mailbox (meaning the ACL for the mailbox MAY be set up so that changes to those flags are visible to another user), then the set of rights tied to those flags (as described in Section 4) for that mailbox gets a collective name: "shared flag rights". The "shared flag rights" set MAY be different for different mailboxes 🌈

If the server doesn't support "shared multiuser write access" to a mailbox, or doesn't implement shared flags on it, then the "shared flag rights" for that mailbox are defined to be the empty set. Empty set, no drama ✨

Example 1: Mailbox "banan" allows "shared multiuser write access" and implements flags \Deleted, \Answered, and $MDNSent as shared flags. "Shared flag rights" for the mailbox "banan" is a set containing flags "t" (because system flag \Deleted requires "t" right) and "w" (because both \Answered and $MDNSent require "w" right).

Example 2: Mailbox "apple" allows "shared multiuser write access" and implements \Seen system flag as shared flag. "Shared flag rights" for the mailbox "apple" contains "s" right because system flag \Seen requires "s" right.

Example 3: Mailbox "pear" allows "shared multiuser write access" and implements flags \Seen, \Draft as shared flags. "Shared flag rights" for the mailbox "apple" is a set containing flags "s" (because system flag \Seen requires "s" right) and "w" (because system flag \Draft requires "w" right).

Now the READ-ONLY code 🙈 The server MUST include a READ-ONLY response code in the tagged OK response to a SELECT command if the current user has none of the following rights:

"i", "e", and "shared flag rights"(***).

The server SHOULD include a READ-WRITE response code in the tagged OK response if the current user has at least one of the "i", "e", or "shared flag rights"(***).

(***) Note that a future extension to this document can extend the list of rights that causes the server to return the READ-WRITE response code. Future-proofing is everything 🦋

Example 1 (continued): The user that has "lrs" rights for the mailbox "banan". The server returns READ-ONLY response code on SELECT, as none of "iewt" rights is granted to the user.

Example 2 (continued): The user that has "rit" rights for the mailbox "apple". The server returns READ-WRITE response code on SELECT, as the user has "i" right.

Example 3 (continued): The user that has "rset" rights for the mailbox "pear". The server returns READ-WRITE response code on SELECT, as the user has "e" and "s" rights.

Lowkey the most fun part of this section is watching the codes flip based on the rights 🔥

6. Security Considerations 🔐

Okay, this part is SERIOUS, so pay attention 🚨 An implementation MUST make sure the ACL commands themselves don't leak information about mailboxes with properly restricted ACLs. For example, when a user agent runs a GETACL command on a mailbox that the user has no permission to LIST, the server has to respond with the same error it would use if the mailbox didn't exist. That way, nothing about the existence of the mailbox gets revealed, let alone its ACL 🙈 Hiding in plain sight, iconic.

IMAP clients that implement ACL and can modify ACLs SHOULD warn a user who wants to give full access (or even just the "a" right) to the special identifier "anyone". Giving "anyone" the keys is NOT it, bestie 💀

This document relies on [SASLprep] to describe the steps required to perform identifier canonicalization (preparation). The preparation algorithm in SASLprep was specifically designed so that its output is canonical and well-formed. However, due to an anomaly [PR29] in the specification of Unicode normalization, canonical equivalence is not guaranteed for a select few character sequences. Identifiers prepared with SASLprep can be stored and returned by an ACL server. The anomaly affects ACL manipulation and evaluation of identifiers containing those selected character sequences. These sequences, however, don't show up in well-formed text. To deal with this problem, an ACL server MAY reject identifiers containing sequences described in [PR29] by sending the tagged BAD response. This comes on top of the requirement to reject identifiers that fail SASLprep preparation, as described in Section 3. Double-layered protection, we love to see it 🛡️

Other security considerations described in [IMAP4] are relevant to this document too. Most importantly, ACL information is sent in the clear over the network unless confidentiality protection is negotiated. That's a big "ngl, this is dangerous" moment 😱

This can be accomplished either by using STARTTLS, negotiating privacy protection in the AUTHENTICATE command, or with some other protection mechanism. Lock it down, babe 🔒

7. Formal Syntax 📜

Formal syntax is defined using ABNF [ABNF], extending the ABNF rules in Section 9 of [IMAP4]. Anything not defined here can be found in [ABNF] and [IMAP4] 🧚

Except as noted otherwise, all alphabetic characters are case insensitive. Using uppercase or lowercase letters to define token strings is only for editorial clarity. Implementations MUST accept these strings in a case-insensitive fashion. Yay for not having to care about capitalization 💖

LOWER-ALPHA     =  %x61-7A   ;; a-z

acl-data        = "ACL" SP mailbox *(SP identifier SP
                    rights)

capability      =/ rights-capa
                    ;;capability is defined in [IMAP4]

command-auth    =/ setacl / deleteacl / getacl /
                    listrights / myrights
                    ;;command-auth is defined in [IMAP4]

deleteacl       = "DELETEACL" SP mailbox SP identifier

getacl          = "GETACL" SP mailbox

identifier      = astring

listrights      = "LISTRIGHTS" SP mailbox SP identifier

listrights-data = "LISTRIGHTS" SP mailbox SP identifier
                        SP rights *(SP rights)

mailbox-data    =/ acl-data / listrights-data / myrights-data
                    ;;mailbox-data is defined in [IMAP4]

mod-rights      = astring
                    ;; +rights to add, -rights to remove
                    ;; rights to replace

myrights        = "MYRIGHTS" SP mailbox

myrights-data   = "MYRIGHTS" SP mailbox SP rights

new-rights      = 1*LOWER-ALPHA
                    ;; MUST include "t", "e", "x", and "k".
                    ;; MUST NOT include standard rights listed
                    ;; in section 2.2

rights          = astring
                    ;; only lowercase ASCII letters and digits
                    ;; are allowed.

rights-capa     = "RIGHTS=" new-rights
                    ;; RIGHTS=... capability

setacl          = "SETACL" SP mailbox SP identifier
                    SP mod-rights

8. IANA Considerations 🌷

IMAP4 capabilities are registered by publishing a standards-track or IESG-approved experimental RFC. The registry lives at:

   http://www.iana.org/assignments/imap4-capabilities

This document defines the RIGHTS= IMAP capability. IANA has added this capability to the registry, so it's officially in the club now 👑✨

9. Internationalization Considerations 🌍✨

okay so bestie, this part is short and sweet 🫶 Section 3 is where the server rules for internationalization of identifiers live, so go check there if you want the details 💅

Appendix A. Changes since RFC 2086 🦋

here's everything that changed since RFC 2086, bestie, numbered so you can track it 💖

Changed the charset of identifier from US-ASCII to UTF-8.
Specified that mailbox deletion is controlled by the x right and EXPUNGE is controlled by the e right.
Added the t right that controls STORE \Deleted. Redefined the d right to be a macro for e, t, and possibly x.
Added the k right that controls CREATE. Redefined the c right to be a macro for k and possibly x.
Specified that the a right also controls DELETEACL.
Specified that the r right also controls STATUS.
Removed the requirement to check the r right for CHECK, SEARCH and FETCH, as this is required for SELECT/EXAMINE to be successful.
LISTRIGHTS requires the a right on the mailbox (same as SETACL).
Deleted "PARTIAL", this is a deprecated feature of RFC 1730.
Specified that the w right controls setting flags other than \Seen and \Deleted on APPEND. Also specified that the s right controls the \Seen flag and that the t right controls the \Deleted flag.
Specified that SUBSCRIBE is NOT allowed with the r right.
Specified that the l right controls SUBSCRIBE.
GETACL is NOT allowed with the r right, even though there are several implementations that allows that. If a user only has r right, GETACL can disclose information about identifiers existing on the mail system.
Clarified that RENAME requires the k right for the new parent and the x right for the old name.
Added new section that describes which rights are required and/or checked when performing various IMAP commands.
Added mail client security considerations when dealing with special identifier "anyone".
Clarified that negative rights are not the same as DELETEACL.
Added "Compatibility with RFC 2086" section.
Added section about mapping of ACL rights to READ-WRITE and READ-ONLY response codes.
Changed BNF to ABNF.
Added "Implementation Notes" section.
Updated "References" section.
Added more examples.
Clarified when the virtual c and d rights are returned in ACL, MYRIGHTS, and LISTRIGHTS responses.

Appendix B. Compatibility with RFC 2086 🎀

this one's non-normative, so it's just friendly guidelines 🌸 for how an existing RFC 2086 server can be updated to line up with this document 💅

This document splits the "d" right into several new different rights: "t", "e", and possibly "x" (see Section 2.1.1 for more details). The "d" right remains for backward-compatibility, but it is a virtual right. There are two approaches for RFC 2086 server implementors to handle the "d" right and the new rights that have replaced it:

a. Tie "t", "e" (and possibly "x) together - almost no changes.
b. Implement separate "x", "t" and "e". Return the "d" right in a MYRIGHTS response or an ACL response containing ACL information when any of the "t", "e" (and "x") is granted.

In a similar manner this document splits the "c" right into several new different rights: "k" and possibly "x" (see Section 2.1.1 for more details). The "c" right remains for backwards-compatibility but it is a virtual right. Again, RFC 2086 server implementors can choose to tie rights or to implement separate rights, as described above.

Also check Sections 5.1.1 and 5.1.2, as well as Appendix A, to see other changes required. Server implementors should check which rights are required to invoke different IMAP4 commands as described in Section 4.

Appendix C. Known Deficiencies 🔥

okay so this spec has a few known deficiencies, and we're being honest about them 🫶

This is inadequate to provide complete read-write access to mailboxes protected by Unix-style rights bits because there is no equivalent to "chown" and "chgrp" commands nor is there a good way to discover such limitations are present.
Because this extension leaves the specific semantics of how rights are combined by the server as implementation defined, the ability to build a user-friendly interface is limited.
Users, groups, and special identifiers (e.g., anyone) exist in the same namespace.

the work-in-progress "ACL2" extension is meant to redesign this extension to fix these deficiencies without the constraint of backward-compatibility, and it may eventually supercede this facility ✨

however, RFC 2086 is already deployed in multiple implementations, so this in-between step, which fixes the straightforward deficiencies in a backward-compatible way, is considered worth it 🌈

Appendix D. Acknowledgements 🧚

this document is a revision of RFC 2086, written by John G. Myers 💜

the editor appreciates comments received from Mark Crispin, Chris Newman, Cyrus Daboo, John G. Myers, Dave Cridland, Ken Murchison, Steve Hole, Vladimir Butenko, Larry Greenfield, Robert Siemborski, Harrie Hazewinkel, Philip Guenther, Brian Candler, Curtis King, Lyndon Nerenberg, Lisa Dusseault, Arnt Gulbrandsen, and other participants of the IMAPEXT working group 🫶🌸

Normative References 📚

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005.
[IMAP4] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
[Stringprep] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002.
[SASLprep] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005.

Informative References 🦋

[RFC2086] Myers, J., "IMAP4 ACL extension", RFC 2086, January 1997.
[PR29] "Public Review Issue #29: Normalization Issue", February 2004, <http://www.unicode.org/review/pr-29.html>.

Author's Address 💌

Alexey Melnikov
Isode Ltd.
5 Castle Business Village
36 Station Road
Hampton, Middlesex  TW12 2BX
GB

EMail: alexey.melnikov@isode.com

Full Copyright Statement ⚖️

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

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Acknowledgement 🌷

funding for the RFC Editor function is currently provided by the Internet Society 💖

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