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Versions: 00 01 02 03 04 05 06 07 08 RFC 4314

IMAPEXT Working Group                                        A. Melnikov
Internet Draft                                                    Editor
Document: draft-ietf-imapext-2086upd-02.txt                December 2004
Updates: <<3501?>>
Obsoletes: 2086
Expires: June 2005

                          IMAP4 ACL extension


Status of this Memo

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   will be disclosed, and any of which I become aware will be disclosed,
   in accordance with RFC 3668.

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Abstract

   The ACL (Access Control List) extension (RFC 2086) of the Internet
   Message Access Protocol (IMAP4rev1) permits mailbox access control
   lists to be manipulated through the IMAP protocol.

   This document is a revision of the RFC 2086. It defines several new
   access control rights and clarifies which rights are required for
   different IMAP commands.


1.   Conventions Used in this Document

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

   In all examples "/" character is used as 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].


2.   Introduction and Overview

   The ACL (Access Control List) extension of the Internet Message Access
   Protocol [IMAP4] permits mailbox access control lists to be retrieved
   and manipulated through the IMAP protocol.

   This document is a revision of the RFC 2086. It tries to clarify
   different ambiguities in the RFC 2086, in particular use of UTF-8
   [UTF-8] in identifiers, which rights are required for different IMAP4
   commands; how READ-WRITE/READ-ONLY response codes are related to ACL.


3.   Access Control

   The ACL extension is present in any IMAP4 implementation which
   returns "ACL" as one of the supported capabilities to the CAPABILITY
   command.

   A server implementation conformant to this document MUST also return
   rights (see below) not defined in RFC 2086 in the "RIGHTS=" capability
   response.

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

   Identifier is a UTF-8 [UTF-8] string.  The identifier "anyone" is reserved
   to refer to the universal identity (all authentications, including
   anonymous). 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 starting with
   a dash ("-") are reserved for "negative rights", described below.
   All other identifier strings are interpreted in an implementation-
   defined manner.

   Rights is a string listing a (possibly empty) set of alphanumeric
   characters, each character listing a set of operations which is being
   controlled.  Lowercase letters are reserved for "standard" rights,
   listed in section 3.1.  (Note that for compatibility with deployed clients
   and servers uppercase rights are not allowed).  The set of
   standard rights may only be extended by a standards-track document.
   Digits are reserved for implementation or site defined rights.

   An implementation may tie rights together or 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, 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 in 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 may discover the set of rights
   which may be granted to a given identifier in the ACL for a given
   mailbox name by using the LISTRIGHTS command.

   It is possible for multiple identifiers in an access control list to
   apply to a given user (or other authentication identity).  For
   example, an ACL may include rights to be granted to the identifier
   matching the user, one or more implementation-defined identifiers
   matching groups which include the user, and/or the identifier
   "anyone".  How these rights are 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
   only use those rights granted to the most specific identifier present
   in the ACL. A client may determine the set of rights granted to the
   logged-in user for a given mailbox name by using the MYRIGHTS command.

   When an identifier in an ACL starts with a dash ("-"), that indicates
   that associated rights are to be removed from the identifier that is
   prefixed by the dash. This is referred to as a "negative right".
   This differs from DELETEACL in that a negative right is added to the
   ACL, and is a part of the calculation of the rights.

   Let's assume that an identifier "fred" refers to a user with login "fred".
   If the identifier "-fred" is granted the "w" right,
   that indicates that the "w" right is to be removed from users matching
   the identifier "fred", even though the user "fred" might have
   the "w" right as a consequence of some other identifier in the ACL.
   A DELETEACL of "fred" simply deletes the identifier "fred"
   from the ACL; it does not affect any rights that the user "fred"
   may get from another entry in the ACL.

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


3.1.   Standard rights

   The currently defined standard rights are (note, that the list below
   doesn't list all commands that use a particular right):

   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, APPEND or COPY)
   w - write (set or clear flags other than \SEEN and \DELETED via STORE,
       APPEND or 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)


3.1.1.   Obsolete rights

   Due to ambiguity in RFC 2086 some existing RFC 2086 server implementations
   use the "c" right to control the DELETE command. Others chose to use the
   "d" right to control the DELETE command.
   For the former group, let's define the "create" right as union of the "k"
   and "x" rights, and the "delete" right as union of the "e" and "t" rights.
   For the latter group, let's define the "create" rights as a synonym to the
   "k" right, and the "delete" right as union of the "e", "t" and "x" rights.

   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 is not an error for a client to specify both the "d" right and
   one or more member of the "delete" right, but the effect is no different
   than if just the "d" right or all members of the "delete" right had been
   specified). When all the "delete" member rights are set in a list of
   rights, the server MUST also include the "d" right when returning
   the list in a MYRIGHTS or ACL response.
   If member of the "delete" right are not tied together, the "d" right
   MUST NOT be returned in a LISTRIGHTS response.

   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 is not an error for a client to specify both the "c" right and
   one or more member of the "create" right, but the effect is no different
   than if just the "c" right or all members of the "create" right had been
   specified). When all the "create" member rights are set in a list of
   rights, the server MUST also include the "c" right when returning
   the list in a MYRIGHTS or ACL response.
   If member of the "create" right are not tied together, the "c" right
   MUST NOT be returned in a LISTRIGHTS response.


4.   Access control management commands and responses

   Servers, when processing a command that have an authentication identifier
   as a parameter (i.e. any of SETACL, DELETEACL and LISTRIGHTS commands),
   SHOULD first prepare the received identifier using "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.


4.1. SETACL command

   Arguments:  mailbox name
               authentication 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, server MUST NOT silently ignore
      unrecognized rights.

   Example:    C: A001 SETACL INBOX.Drafts Chris lrswicda
               S: A001 OK Setacl 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


4.2. DELETEACL command

   Arguments:  mailbox name
               authentication 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.


4.3. 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 mailbox in
      an untagged ACL response.

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

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


4.4. LISTRIGHTS command

   Arguments:  mailbox name
               authentication 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 may be granted to the identifier
      in the ACL for the mailbox.

      Some implementations may permit multiple forms of an authentication
      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 authentication identifier as specified
      by the client. This is to allow the client to correlate the response
      with the command.

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


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


4.5. 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 mailbox in an untagged MYRIGHTS reply.

   Example:    C: A003 MYRIGHTS INBOX
               S: * MYRIGHTS INBOX rwipslda
               S: A003 OK Myrights complete


4.6. 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 for which this ACL applies.  This is
      followed by zero or more pairs of strings, each pair contains the
      identifier for which the entry applies followed by the set of
      rights that the identifier has.


4.7. LISTRIGHTS response

   Data:       mailbox name
               identifier
               required rights
               list of optional rights

      The LISTRIGHTS response occurs as a result of a LISTRIGHTS
      command. The first two strings are the mailbox name and identifier
      for which this rights list applies.  Following the identifier is a
      string containing the (possibly empty) set of rights the
      identifier will always be granted in the mailbox.

      Following this are zero or more strings each containing a set of
      rights the identifier may be granted in the mailbox.  Rights
      mentioned in the same string are tied together--either all must be
      granted to the identifier in the mailbox or none may be granted.

      The same right may not be listed more than once in the LISTRIGHTS
      command.


4.8. MYRIGHTS response

   Data:       mailbox name
               rights

      The MYRIGHTS response occurs as a result of a MYRIGHTS command.
      The first string is the mailbox name for which these rights apply.
      The second string is the set of rights that the client has.


5.   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 groups command by functions
   they perform and list the rights required. It also gives the detailed
   description of any special processing required.

   For the purpose of this section the UID counterpart of a command is
   considered to be the same command, e.g. both UID COPY and COPY commands
   require the same set of rights.

   The table below summarizes different rights or their combinations that are
   required in order to perform different IMAP operations. As it is not always
   possible to express complex right checking and interactions, the description
   after the table should be used as the primary reference.

   +---------------------+---+---+---+---+---+---+---+---+---+---+-----+------+
   | Operations\Rights   | l | r | s | w | i | k | x | t | e | a | Any | None |
   +---------------------+---+---+---+---+---+---+---+---+---+---+-----+------+
   |       LIST          | + |   |   |   |   |   |   |   |   |   |     |      |
   |    SUBSCRIBE        | * |   |   |   |   |   |   |   |   |   |     |   *  |
   |   UNSUBSCRIBE       |   |   |   |   |   |   |   |   |   |   |     |   +  |
   |       LSUB          | * |   |   |   |   |   |   |   |   |   |     |   *  |
   | CREATE (for parent) |   |   |   |   |   | + |   |   |   |   |     |      |
   |      DELETE         |   |   |   |   |   |   | + | ? | ? |   |     |      |
   |      RENAME         |   |   |   |   |   | + | + |   |   |   |     |      |
   |   COPY/APPEND       |   |   | ? | ? | + |   |   | ? |   |   |     |      |
   |  EXPUNGE/CLOSE      |   |   |   |   |   |   |   |   | + |   |     |      |
   |SELECT/EXAMINE/STATUS|   | + |   |   |   |   |   |   |   |   |     |      |
   |       FETCH         |   |   | ? |   |   |   |   |   |   |   |     |      |
   |    STORE flags      |   |   | ? | ? |   |   |   | ? |   |   |     |      |
   |   SETACL/DELETEACL  |   |   |   |   |   |   |   |   |   | + |     |      |
   |  GETACL/LISTRIGHTS  |   |   |   |   |   |   |   |   |   | + |     |      |
   |      MYRIGHTS       |   |   |   |   |   |   |   |   |   |   |  +  |      |
   +---------------------+---+---+---+---+---+---+---+---+---+---+-----+------+

   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
    "None" - No rights required to perform the command


   Listing and subscribing/unsubscribing mailboxes:
    LIST - "l" right is required.

    Note, that if the user has "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 - "l" right is required only if the server checks for mailbox existence
           when performing SUBSCRIBE.

    UNSUBSCRIBE - no rights required to perform this operation.

    LSUB - "l" right is required only if the server checks for mailbox existence when
           performing SUBSCRIBE.

   Mailbox management:
    CREATE - "k" right on a 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 - "x" right on the mailbox. Note, that some servers don't allow
             to delete a non-empty mailbox. If this is the case, the user
             also need both "e" and "t" rights, in order to make the mailbox
             empty first.

             The DELETE command MUST delete the ACL associated with the
             deleted mailbox.

    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 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.

   Copying or appending messages:

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

   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 that 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

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

       If the MYRIGHTS command with the tag A003 would have returned:
               S: * MYRIGHTS TargetMailbox rsti

       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 the latter case \Answered, $Forwarded and \Draft flags were lost
       on COPY, as the user has no "w" right in the target mailbox.

   Expunging the selected mailbox:
    EXPUNGE - "e" right on the selected mailbox.

    CLOSE - "e" right on the selected mailbox. If the server is unable to
            expunge the mailbox because the user doesn't have the "e" right,
            the server MUST ignore expunge request, close the mailbox
            and return tagged OK response.

   Fetch information about a mailbox and its messages:
    SELECT/EXAMINE/STATUS - "r" right on the mailbox.

    FETCH - A FETCH request that implies setting \Seen flag MUST NOT set it,
            if the current user doesn't have "s" right.

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

   Changing ACLs:
    SETACL/DELETEACL - "a" right on the mailbox.

   Reading ACLs:
    GETACL - "a" right on the mailbox.

    MYRIGHTS - any of the following rights is required to perform
     the operation: "l", "r", "i", "k", "x", "a".

    LISTRIGHTS - "a" right on the mailbox.


6.  Other considerations

6.1. Additional requirements and Implementation notes

6.1.1. Servers

   This document defines an additional capability that is used to announce
   the list of extra rights (excluding the ones defined in the RFC 2086)
   supported by the server. The set of rights MUST include "t", "e", "x" and "k".
   Note, that the extra rights can appear in any order.

   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.

   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 \*)] Limited
               S: A142 OK [READ-WRITE] SELECT completed
               C: A143 MYRIGHTS INBOX
               S: * MYRIGHTS INBOX rwis
               S: A143 OK completed

   Servers MAY cache the rights a user has on a mailbox when the mailbox
   is selected, so that if a client's rights on a mailbox are changed with
   SETACL or DELETEACL, commands specific to the selected state (e.g., STORE,
   EXPUNGE) might not reflect the changed rights until the mailbox is
   re-selected. If the server checks the rights on each command, then it SHOULD
   send FLAGS and PERMANENTFLAGS responses if they have changed.

   An ACL server MAY modify one or more ACL for one or more identifier as a
   side effect of modifying the ACL specified in a SETACL/DELETEACL.
   If the server does that it MUST send untagged ACL response to notify the
   client about the changes made.

6.1.2.  Clients

   A client implementation that allows a user to read and update ACLs MUST
   preserve unrecognized rights that it doesn't allow the user to change
   when updating the rights. Otherwise the client may unintentionally remove
   permissions.


6.2. Mapping of ACL rights to READ-WRITE and READ-ONLY response codes

   A particular ACL server implementation may allow "shared multiuser
   access" to some mailboxes. "Shared multiuser access" to a mailbox means
   that multiple different users are able to access the same mailbox,
   if they have proper access rights. "Shared multiuser access" to the
   mailbox doesn't mean that the ACL for the mailbox is currently set
   to allow access by multiple users. Let's denote a "shared multiuser
   write access" as a "shared multiuser access" when a user may be
   granted flag modification rights (any of "w", "s" or "t").

   Section 5 describes which rights are required for modifying different flags.

   If the ACL server implements some flags as shared for a mailbox (i.e.,
   the ACL for the mailbox may be set up so that changes to those flags are
   visible to another user), let's call the set of rights associated with these
   flags (as described in Section 5) for that mailbox collectively as
   "shared flag rights". Note, that "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 the mailbox, "shared flag
   rights" for the mailbox is defined to be the empty set.


   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).

   The server MUST include a READ-ONLY response code in the tagged OK response to
   a SELECT command if none of the following rights is granted to the
   current user:
    "i", "e" and "shared flag rights"*.
   The server SHOULD include a READ-WRITE response code in the tagged OK response
   if at least one of the "i", "e" or "shared flag rights"* is granted to the
   current user.

   * - Note that a future extension to this document may extend the list of
       rights that causes the server to return the READ-WRITE response code.

   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.


7.   Security Considerations

   An implementation must make sure the ACL commands themselves do not
   give information about mailboxes with appropriately restricted ACL's.
   For example, a GETACL command on a mailbox for which the user has
   insufficient rights should not admit that the mailbox exists, much less
   return the mailbox's ACL.

   LISTRIGHTS command MUST NOT check that a particular identifier exists,
   however it SHOULD recognize special identifiers like "anyone".

   IMAP clients implementing ACL that are able to modify ACLs SHOULD
   warn a user that wants to give full access (or even just the "a" right)
   to the special identifier "anyone".


8.   Formal Syntax

   Formal syntax is defined using ABNF [ABNF] as modified by [IMAP4].
   Non-terminals referenced but not defined below are as defined by
   [IMAP4].

   Except as noted otherwise, all alphabetic characters are
   case-insensitive.  The use of upper or lower case characters to
   define token strings is for editorial clarity only.  Implementations
   MUST accept these strings in a case-insensitive fashion.

   LOWER_ALPHA     =  %x61-7A   ;; a-z

   acl_data        = "ACL" SP mailbox *(SP identifier SP
                        rights)

   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)

   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 / DIGIT)
                      ;; MUST include "t", "e", "x" and "k"

   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


9.  IANA Considerations

   IMAP4 capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  The registry is currently located
   at:

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

   This document defines the RIGHTS= IMAP capability.  IANA is requested
   to add this capability to the registry.


10.   References

10.1.   Normative References

   [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, Harvard University, March 1997.

   [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
   ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd,
   November 1997.

   [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
   4rev1", RFC 3501, University of Washington, March 2003.

   [UTF-8] Yergeau, F., "UTF-8, a transformation format of IS0 10646",
   RFC 3629, Alis Technologies, November 2003.

   [Stringprep] Hoffman, P., Blanchet, M., "Preparation of
   Internationalized Strings ("stringprep")", RFC 3454, December 2002.

   [SASLprep] Zeilenga, K., "SASLprep: Stringprep profile for user names
   and passwords", Work in progress, draft-ietf-sasl-saslprep-XX.txt.


10.2.   Informative References

   [RFC2086] Myers, J., "IMAP4 ACL extension", RFC 2086, Carnegie Mellon,
   January 1997.


11.  Editor's Address

   Alexey Melnikov
   email: alexey.melnikov@isode.com

   Isode Limited


12.  IPR Disclosure Acknowledgement

    By submitting this Internet-Draft, I certify that any applicable
    patent or other IPR claims of which I am aware have been disclosed,
    and any of which I become aware will be disclosed, in accordance with
    RFC 3668.


13.  Intellectual Property Statement

    The IETF takes no position regarding the validity or scope of any
    intellectual property or other rights that might be claimed to
    pertain to the implementation or use of the technology described in
    this document or the extent to which any license under such rights
    might or might not be available; neither does it represent that it
    has made any effort to identify any such rights. Information on the
    IETF's procedures with respect to rights in standards-track and
    standards-related documentation can be found in BCP-11. Copies of
    claims of rights made available for publication 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 implementors or users of this specification can
    be obtained from the IETF Secretariat.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights which may cover technology that may be required to practice
    this standard. Please address the information to the IETF Executive
    Director.


14.  Full Copyright Statement

    Copyright (C) The Internet Society (2004).  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.


Acknowledgement

    Funding for the RFC Editor function is currently provided by the
    Internet Society.


Appendix A. Changes since RFC 2086

   1. Changed the charset of "identifier" from US-ASCII to UTF-8.

   2. Specified that mailbox deletion is controled by the "x" right and
      EXPUNGE is controlled by the "e" right.

   3. Added the "t" right that controls STORE \Deleted. Redefined the "d"
      right to be a macro for "e", "t" and possibly "x".

   4. Added the "k" right that controls CREATE. Redefined the "c"
      right to be a macro for "k" and possibly "x".

   5. Specified that the "a" right also controls DELETEACL.

   6. Specified that the "r" right also controls STATUS.

   7. Removed the requirement to check the "r" right for CHECK, SEARCH and
      FETCH, as this is required for SELECT/EXAMINE to be successful.

   8. LISTRIGHTS requires the "a" right on the mailbox (same as SETACL).

   9. Deleted "PARTIAL", this is a deprecated feature of RFC 1730.

  10. 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.

  11. SUBSCRIBE is NOT allowed with the "r" right.

  12. Specified that the "l" right controls SUBSCRIBE.

  13. 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.

  14. Clarified that RENAME requires the "k" right for the new parent and
      the "x" right for the old name.

  15. Added new section that describes which rights are required and/or
      checked when performing various IMAP commands.

  16. Added mail client security considerations when dealing with special
      identifier "anyone".

  17. Clarified that negative rights are not the same as DELETEACL.

  18. Added "Compatibility with RFC 2086" section.

  19. Added section about mapping of ACL rights to READ-WRITE and READ-ONLY
      response codes.

  20. Changed BNF to ABNF.

  21. Added "Implementation Notes" section.

  22. Updated "References" section.


Appendix B. Compatibility with RFC 2086

   This section gives guidelines how an existing RFC 2086 server
   implementation may be updated to comply with this document.

   This document splits the "d" right into several new different rights: "t",
   "e" and possibly "x" (see section 3.1.1 for more details). The "d" right
   remains for backwards-compatibility but it is a virtual right.  The server
   should implement one of the following two approaches to handle the "d" right
   and the new rights that have replaced it.

    a). "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 all of the "t", "e" (and "x") are granted.

   In a similar manner this document splits the "c" right into several new
   different rights: "k" and possibly "x" (see section 3.1.1 for more details).
   The "c" right remains for backwards-compatibility but it is a virtual right.
   The server should implement one of the two approaches described above.

   Also check Sections 6.1 and 6.2, as well as the appendix A to see
   other changes required. Server implementors should check which rights
   are required to invoke different IMAP4 commands as described in
   Section 5.


Appendix C.  Known deficiencies

   This specification has some known deficiencies including:

   1. 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.

   2. Because this extension leaves the specific semantics of how rights are
      combined by the server and which rights are tied as implementation
      defined, the ability to build a user-friendly interface is limited.

   The work-in-progress "ACL2" extension is intended to redesign this
   extension to address these deficiencies without the constraint of
   backwards-compatibility and may eventually supercede this facility.
   However, RFC 2086 is deployed in multiple implementations so this
   intermediate step which fixes the straightforward deficiencies in a
   backwards compatible fashion is considered worthwhile.


Appendix D.  Acknowledgment

   This document is a revision of the RFC 2086 written by John G. Myers.

   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 and other participants of the IMAPEXT working group.


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