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.
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] 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.
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".
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.)
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.
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
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
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 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 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.
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