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rfc:rfc4314

Network Working Group A. Melnikov Request for Comments: 4314 Isode Ltd. Obsoletes: 2086 December 2005 Category: Standards Track

             IMAP4 Access Control List (ACL) Extension

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 The Access Control List (ACL) extension (RFC 2086) of the Internet
 Message Access Protocol (IMAP) permits mailbox access control lists
 to be retrieved and manipulated through the IMAP protocol.
 This document is a revision of RFC 2086.  It defines several new
 access control rights and clarifies which rights are required for
 different IMAP commands.

Melnikov Standards Track [Page 1] RFC 4314 IMAP ACL December 2005

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

Melnikov Standards Track [Page 2] RFC 4314 IMAP ACL December 2005

1. 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 RFC 2086 [RFC2086].  It tries to
 clarify different ambiguities in RFC 2086, in particular, the use of
 UTF-8 [UTF-8] in access identifiers, which rights are required for
 different IMAP4 commands, and how READ-WRITE/READ-ONLY response codes
 are related to ACL.

1.1. 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].
 The phrase "ACL server" is just a shortcut for saying "IMAP server
 that supports 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 the supported capabilities to the CAPABILITY command.
 A server implementation conformant to this document MUST also return
 rights (see below) not defined in Section 2.2 in the "RIGHTS="
 capability.
 An access control list is a set of <access identifier,rights> pairs.
 An ACL applies to a mailbox name.
 Access identifier (or just "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.

Melnikov Standards Track [Page 3] RFC 4314 IMAP ACL December 2005

 Rights is a string listing a (possibly empty) set of alphanumeric
 characters, each character listing a set of operations that is being
 controlled.  Lowercase letters are reserved for "standard" rights,
 listed in Section 2.1.  (Note that for compatibility with deployed
 clients and servers uppercase rights are not allowed.)  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 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 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 is possible for multiple identifiers in an access control list to
 apply to a given user.  For example, an ACL may include rights to be
 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 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 use only those rights granted to the
 most specific identifier present in the ACL.  A client can 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 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, in particular it doesn't affect rights
 granted to the identifier "-fred".

Melnikov Standards Track [Page 4] RFC 4314 IMAP ACL December 2005

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

2.1. 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, 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

 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 members 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.)

Melnikov Standards Track [Page 5] RFC 4314 IMAP ACL December 2005

 When any of the "delete" member rights is set in a list of rights,
 the server MUST also include the "d" right when returning the list in
 a MYRIGHTS or ACL response.  This is to enable older clients
 conforming to RFC 2086 to 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 specified in the LISTRIGHTS command can be granted
 any of the "delete" member rights on a mailbox, then the server MUST
 include the "d" right in the corresponding LISTRIGHTS response. (*)
 If the member rights aren't tied to non-member rights, then the "d"
 right 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 the "d" right 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 is not an error for a client to specify both the "c"
 right and one or more members 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 any of the "create" member rights is set in a list of rights,
 the server MUST also include the "c" right when returning the list in
 a MYRIGHTS or ACL response.  This is to enable older clients
 conforming to RFC 2086 to 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 the "c" right), the server also
 returns the "c" right.

Melnikov Standards Track [Page 6] RFC 4314 IMAP ACL December 2005

 If the identifier specified in the LISTRIGHTS command can be granted
 any of the "create" member rights on a mailbox, then the server MUST
 include the "c" right in the corresponding LISTRIGHTS response. (*)
 If the member rights aren't tied to non-member rights, then the "c"
 right 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 the "c" right 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

Melnikov Standards Track [Page 7] RFC 4314 IMAP ACL December 2005

          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 has debated this issue in great length
      and after reviewing existing ACL implementations 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 (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.
 Note that Section 6 recommends additional identifier's 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

Melnikov Standards Track [Page 8] RFC 4314 IMAP ACL December 2005

 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

Melnikov Standards Track [Page 9] RFC 4314 IMAP ACL December 2005

             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 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

Melnikov Standards Track [Page 10] RFC 4314 IMAP ACL December 2005

 response caused by a LISTRIGHTS command MUST always return the same
 form of an 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 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
 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 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.
 Section 2.1.1 details additional server requirements related to
 handling of the virtual "d" and "c" rights.

Melnikov Standards Track [Page 11] RFC 4314 IMAP ACL December 2005

3.7. 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 can be granted in the mailbox.  Rights
 mentioned in the same string are tied together.  The server MUST
 either grant all tied rights to the identifier in the mailbox or
 grant none.  Section 2.1.1 details additional server requirements
 related to handling of the virtual "d" and "c" rights.
 The same right MUST NOT be listed more than once in the LISTRIGHTS
 command.

3.8. 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.
 Section 2.1.1 details additional server requirements related to
 handling of the virtual "d" and "c" rights.

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 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.

Melnikov Standards Track [Page 12] RFC 4314 IMAP ACL December 2005

 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|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 is required to SELECT/EXAMINE a mailbox.  Servers are not
 required to check presence of the "r" right once a mailbox is
 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

Melnikov Standards Track [Page 13] RFC 4314 IMAP ACL December 2005

 Listing and subscribing/unsubscribing mailboxes:
    LIST - "l" right is required.  However, unlike other commands
    (e.g., SELECT) the server MUST NOT return a NO response if it
    can't list a mailbox.
    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.  However, unlike other
    commands (e.g., SELECT) the server MUST NOT return a NO response
    if it can't list a subscribed mailbox.
 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 would also need "r", "e", and "t" rights, in order to open
    the mailbox and empty it.
    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 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.

Melnikov Standards Track [Page 14] RFC 4314 IMAP ACL December 2005

 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 are the corresponding
    flags 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)

Melnikov Standards Track [Page 15] RFC 4314 IMAP ACL December 2005

             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 the expunge request, close the
    mailbox, and return the 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.

Melnikov Standards Track [Page 16] RFC 4314 IMAP ACL December 2005

5. Other Considerations

5.1. Additional Requirements and Implementation Notes

5.1.1. Servers

 This document defines an additional capability that is used to
 announce the list of extra rights (excluding the ones defined in 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 \*)] 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:
             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

Melnikov Standards Track [Page 17] RFC 4314 IMAP ACL December 2005

 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.  If such server detects that the user no longer
 has read access to the mailbox, it MAY send an untagged BYE response
 and close connection.  It MAY also refuse to execute all commands
 specific to the selected state until the mailbox is closed; however,
 server implementors should note that 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 modifying the ACL specified in a
 SETACL/DELETEACL.  If the server does that, it MUST send untagged ACL
 response(s) to notify the client about the changes made.
 An ACL server implementation MUST treat received ACL modification
 commands as a possible ambiguity with respect to subsequent commands
 affected by the ACL, as described in Section 5.5 of [IMAP4].  Hence a
 pipeline SETACL + MYRIGHTS is an ambiguity with respect to the
 server, meaning that the server must execute the SETACL command to
 completion before the MYRIGHTS.  However, clients are permitted to
 send such a pipeline.

5.1.2. Clients

 The following requirement is put on clients in order to allow for
 future extensibility.  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.  That is, 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 risk unintentionally removing permissions
 it doesn't understand.

Melnikov Standards Track [Page 18] RFC 4314 IMAP ACL December 2005

5.2. 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 can be granted flag modification rights (any of "w", "s", or
 "t").
 Section 4 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 4) for that
 mailbox collectively as "shared flag rights".  Note that 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 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:

Melnikov Standards Track [Page 19] RFC 4314 IMAP ACL December 2005

  "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 can 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.

6. Security Considerations

 An implementation MUST make sure the ACL commands themselves do not
 give information about mailboxes with appropriately restricted ACLs.
 For example, when a user agent executes a GETACL command on a mailbox
 that the user has no permission to LIST, the server would respond to
 that request with the same error that would be used if the mailbox
 did not exist, thus revealing no existence information, much less the
 mailbox's ACL.
 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".
 This document relies on [SASLprep] to describe steps required to
 perform identifier canonicalization (preparation).  The preparation
 algorithm in SASLprep was specifically designed such that its output
 is canonical, and it is 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 the selected character sequences.  These

Melnikov Standards Track [Page 20] RFC 4314 IMAP ACL December 2005

 sequences, however, do not appear in well-formed text.  In order to
 address this problem, an ACL server MAY reject identifiers containing
 sequences described in [PR29] by sending the tagged BAD response.
 This is in addition to the requirement to reject identifiers that
 fail SASLprep preparation as described in Section 3.
 Other security considerations described in [IMAP4] are relevant to
 this document.  In particular, ACL information is sent in the clear
 over the network unless confidentiality protection is negotiated.
 This can be accomplished either by the use of STARTTLS, negotiated
 privacy protection in the AUTHENTICATE command, or some other
 protection mechanism.

7. Formal Syntax

 Formal syntax is defined using ABNF [ABNF], extending the ABNF rules
 in Section 9 of [IMAP4].  Elements not defined here can be found in
 [ABNF] and [IMAP4].
 Except as noted otherwise, all alphabetic characters are case
 insensitive.  The use of uppercase or lowercase 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)
 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)

Melnikov Standards Track [Page 21] RFC 4314 IMAP ACL December 2005

 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 is currently located
 at:
    http://www.iana.org/assignments/imap4-capabilities
 This document defines the RIGHTS= IMAP capability.  IANA has added
 this capability to the registry.

9. Internationalization Considerations

 Section 3 states requirements on servers regarding
 internationalization of identifiers.

Melnikov Standards Track [Page 22] RFC 4314 IMAP ACL December 2005

Appendix A. Changes since RFC 2086

 1.   Changed the charset of "identifier" from US-ASCII to UTF-8.
 2.   Specified that mailbox deletion is controlled 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.  Specified that 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.
 23.  Added more examples.
 24.  Clarified when the virtual "c" and "d" rights are returned in
      ACL, MYRIGHTS, and LISTRIGHTS responses.

Melnikov Standards Track [Page 23] RFC 4314 IMAP ACL December 2005

Appendix B. Compatibility with RFC 2086

 This non-normative section gives guidelines as to 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 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

 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 as implementation defined, the
     ability to build a user-friendly interface is limited.
 3.  Users, groups, and special identifiers (e.g., anyone) exist in
     the same namespace.
 The work-in-progress "ACL2" extension is intended to redesign this
 extension to address these deficiencies without the constraint of
 backward-compatibility and may eventually supercede this facility.

Melnikov Standards Track [Page 24] RFC 4314 IMAP ACL December 2005

 However, RFC 2086 is deployed in multiple implementations so this
 intermediate step, which fixes the straightforward deficiencies in a
 backward-compatible fashion, is considered worthwhile.

Appendix D. Acknowledgements

 This document is a revision of 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, 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>.

Melnikov Standards Track [Page 25] RFC 4314 IMAP ACL December 2005

Author's Address

 Alexey Melnikov
 Isode Ltd.
 5 Castle Business Village
 36 Station Road
 Hampton, Middlesex  TW12 2BX
 GB
 EMail: alexey.melnikov@isode.com

Melnikov Standards Track [Page 26] RFC 4314 IMAP ACL December 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 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
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 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
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 Copies of IPR disclosures made to the IETF Secretariat and any
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Acknowledgement

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

Melnikov Standards Track [Page 27]

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