IMAPEXT Working Group A. Melnikov Internet Draft Editor Document: draft-ietf-imapext-acl-04.txt June 2002 IMAP4 ACL extension Status of this Memo This document is an Internet Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as ``work in progress''. The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or munnari.oz.au. A revised version of this draft document will be submitted to the RFC editor as a Proposed Standard for the Internet Community. Discussion and suggestions for improvement are requested. Distribution of this draft is unlimited. 0. Open issues This section will be removed when the draft will be published as RFC. It is intended to simplify discussion. 1). Should the document define exact syntax for identifiers starting with '$'? For example, the following was proposed by Mark Crispin: Definable identifiers. Server implementations are *NOT* required to use any of these, but if they do, these are their semantics. Note that definable identifiers may expand to other definable identifiers. $G$xxx global identifier xxx. Valid everywhere $U$xxx per-user identifier xxx. Valid only for the logged-in user $M$xxx per-mailbox identifer xxx. Valid only for this mailbox $xxxxx reserved for future definition Non-definable identifiers. Server implementations SHOULD support $WORLD, and MUST support anyone and user names. However, a server can respond with a NO. $WORLD all authorized users but not anonymous $ANONYMOUS anonymous users $DISABLE disabled rights 2). Do we need a mechanism to encode login names if they start with $, -? 3). Should the annotations (ANNOTATE) be controlled by "w" right? 4). Other editorial comments/questions are marked in the text with << and >>. 1. Abstract The ACL extension of the Internet Message Access Protocol [IMAP4] permits access control lists to be manipulated through the IMAP protocol. 2. Conventions Used in this Document In examples, "C:" and "S:" indicate lines sent by the client and server respectively. In all examples "/" character is used as hierarchy separator. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [KEYWORDS]. 3. Introduction and Overview The ACL extension is present in any IMAP4 implementation which returns a capability starting with "ACL2=" as one of the supported capabilities to the CAPABILITY command. An access control list is a set of <identifier,rights> pairs. Identifier is a UTF-8 string. All user name strings accepted by the LOGIN or AUTHENTICATE commands to authenticate to the IMAP server are reserved as identifiers for the corresponding user. Identifiers starting with a dash ("-") are reserved for "negative rights", described below. Several special identifiers are defined by this document: a). The identifier "$anyone" is reserved to refer to the universal identity (all authentications, including anonymous). b). The identifier "$authuser" is reserved to refer to all authenticated users, but not anonymous. c). The identifier "$owner" refers to the owner of a mailbox (if any). d). The identifier "$administrators" refers to all users with administrative rights. Note that a server is not required to implement any special identifier mentioned above. However if it allows a user to perform ACL operations on any one of them, server MUST use the semantic as described above. All other identifiers starting with a dollar sign ("$") are reserved for groups and implementation defined aliases. (As a consequence, user name strings accepted by LOGIN or AUTHENTICATE MUST NOT start with "$".) 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. Letters are reserved for ''standard'' rights, listed below. The set of standard rights may only be extended by a standards-track document. Digits are reserved for implementation or site defined rights. The currently defined standard rights are: l - lookup (mailbox is visible to LIST/LSUB commands, SUBSCRIBE mailbox) r - read (SELECT the mailbox, perform STATUS, CHECK, FETCH, SEARCH, COPY from mailbox) 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) c - create mailboxes (CREATE new sub-mailboxes in any implementation-defined hierarchy, parent mailbox for the new mailbox name in RENAME). When a new mailbox is created it SHOULD inherit rights from the parent mailbox (if one exists) in the defined hierarchy. 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 d - This right is defined for backward compatibility with ACL extension (RFC 2086). If a client sets "d" right, the server MUST set "x", "e" and "t" rights. When the client clears the "d" right, the server MUST clear "x", "e" and "t" rights. When all three of "x", "e" and "t" are set, the server MUST return "d" right in response to a GETACL command. If "x", "e" and "t" rights are not tied together, "d" right MUST NOT be returned in a LISTRIGHTS response. a - administer (perform SETACL and DELETEACL) 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 "wisd" 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 by using the LISTRIGHTS 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". For example, 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". Server implementations are not required to support "negative right" identifiers. 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. The set of rules that describes how access is calculated is defined by a rule identifier (rule-ID). A client may determine the set of rights granted to the logged-in user for a given mailbox by using the MYRIGHTS command. If a server implementing ACL2 uses the union of the rights granted to the applicable identifiers minus the union of the negative rights in order to calculate access, it MUST report "ACL2=UNION" in the server's capability list. An implementation may instead choose to only use those rights granted to the most specific identifier present in the ACL. In this case the server MUST report "ACL2=MOST-SPECIFIC" in the server's capability list. If the server implements any other policy for rights calculation, it MUST be either registered with IANA using the template provided in 7.1 or start with "X-". The server MUST report "ACL2=<rule-ID>" capability. 3.1. Rights required to perform different IMAP4rev1 commands Before executing a command an ACL2 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. 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. <<Allow if the user has "r" right?>> 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 - "c" right on a nearest existing parent mailbox. When a new mailbox is created it SHOULD inherit rights from the parent mailbox (if one exists) in the defined hierarchy. DELETE - "x" right on the mailbox. RENAME - Moving a mailbox from one parent to another requires "x" right on the mailbox itself and "c" 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 "x" right for the mailbox "A/B/C" and "c" right for the mailbox "D". 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. <<Add an example here?>> Expunging the selected mailbox: EXPUNGE - "e" right on the selected mailbox. CLOSE - "e" right on the selected mailbox. Server MUST NOT fail CLOSE operation if it is unable to expunge the mailbox. Fetch information about a mailbox and its messages: SELECT/EXAMINE/STATUS - "r" right on the mailbox. CHECK - "r" right on the selected mailbox *. SEARCH - "r" right on the selected mailbox *. FETCH - "r" right on the selected mailbox *, "s" right is required when FETCH implicitly sets \Seen flag. FETCH operation MUST NOT fail if the server is required, but unable to set \Seen flag. * - no need to check explicitly, as "r" is required to open a mailbox in the first place. 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. Changing ACLs: SETACL/DELETEACL/REPLACEACL - "a" right on the mailbox. Reading ACLs: GETACL - "a" right on the mailbox. <<Several implementations allow GETACL with "r" right. If this is allowed, GETACL can disclose some identifiers existing on a mail system>> LISTRIGHTS - <<Same rights as for GETACL?>> MYRIGHTS - any of the following rights is required to perform the operation: "l", "r", "i", "c", "x", "e", "a". 4. Commands 4.1. SETACL 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 - command unknown or 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. 4.2. DELETEACL Arguments: mailbox name authentication identifier Data: no specific data for this command Result: OK - deleteacl completed NO - deleteacl failure: can't delete acl BAD - command unknown or 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. REPLACEACL Arguments: mailbox name list of (authentication identifier, access rights) pairs Data: no specific data for this command Result: OK - replaceacl completed NO - replaceacl failure: can't replace acl BAD - command unknown or arguments invalid The REPLACEACL command replaces the access control list of the specified mailbox with the one provided as the second parameter to REPLACEACL. This command is semantically equivalent to the following sequence of commands: 1). GETACL <mailbox name> 2). For each (authentication identifier AID, access rights RD) pair returned in the untagged ACL response that was caused by GETACL, perform DELETEACL <mailbox name> AID 3). For each (authentication identifier AIA, access rights RA) pair from the second parameter of REPLACEACL perform SETACL <mailbox name> AIA RA 4.4. GETACL Arguments: mailbox name Data: untagged responses: ACL Result: OK - getacl completed NO - getacl failure: can't get acl BAD - command unknown or arguments invalid The GETACL command returns the access control list for mailbox in an untagged ACL reply. Example: C: A002 GETACL INBOX S: * ACL INBOX Fred rwipslextda S: A002 OK Getacl complete 4.5. LISTRIGHTS Arguments: mailbox name authentication identifier Data: untagged responses: LISTRIGHTS Result: OK - listrights completed NO - listrights failure: can't get rights list BAD - command unknown or 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. Example: C: a001 LISTRIGHTS ~/Mail/saved smith S: * LISTRIGHTS ~/Mail/saved smith la r swi cdext S: a001 OK Listrights completed C: a005 LISTRIGHTS archive.imap anyone S: * LISTRIGHTS archive.imap anyone "" l r s w i p c dtex a 0 1 2 3 4 5 6 7 8 9 S: a005 OK Listrights completed 4.6. MYRIGHTS Arguments: mailbox name Data: untagged responses: MYRIGHTS Result: OK - myrights completed NO - myrights failure: can't get rights BAD - command unknown or arguments invalid The MYRIGHTS command returns the set of rights that the user has to mailbox in an untagged MYRIGHTS reply. The user must have any of the following rights to perform this operation: "l", "r", "i", "c", "x", "e", "a". If the user doesn't have any right from the above list, the server MUST behave as if the mailbox doesn't exist. Example: C: A003 MYRIGHTS INBOX S: * MYRIGHTS INBOX rwipsldexta S: A003 OK Myrights complete 5. Responses 5.1. ACL 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. 5.2. LISTRIGHTS 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. 5.3. MYRIGHTS untagged 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. 5.4. MYRIGHTS response code Data: rights The MYRIGHTS response code is sent in an untagged OK response that results from SELECT/EXAMINE. Also this response code can be sent at any time after opening a mailbox when the server detects that the set of rights allowed for the logged in user has changed. The MYRIGHT response code is equivalent to MYRIGHTS untagged response for the selected mailbox. 6. 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. acl_data = "ACL" SPACE mailbox *(SPACE identifier SPACE rights) deleteacl = "DELETEACL" SPACE mailbox SPACE identifier getacl = "GETACL" SPACE mailbox identifier = astring ;; UTF-8 listrights = "LISTRIGHTS" SPACE mailbox SPACE identifier listrights_data = "LISTRIGHTS" SPACE mailbox SPACE identifier SPACE rights *(SPACE rights) mod_rights = astring ;; +rights to add, -rights to remove myrights = "MYRIGHTS" SPACE mailbox myrights_data = "MYRIGHTS" SPACE mailbox SPACE rights myrights_rspcod = "MYRIGHTS" SPACE rights replaceacl = "REPLACEACL" SPACE mailbox *(SPACE identifier SPACE rights) resp-text-code =/ myrights_data rights = astring setacl = "SETACL" SPACE mailbox SPACE identifier SPACE mod_rights 7. IANA considerations 7.1. ACL access calculation rule Registration Template When an access calculation rule for ACL2 extension is registered, the following information is supplied: Rule Identification: specify a string that identifies this rule. Unless the rule is registered with the IANA, the rule's identification must start with "X-". The server supporting a particular rule <rule-ID> MUST report "ACL2=<rule-ID>" in the capability list. Rule Semantics: specify how access rights for a mailbox are calculated. Negative rights allowed: specify whether "negative right" identifiers are allowed. Groups allowed: specify whether group identifiers are allowed. Special Identifiers: describe whether any implementation defined aliases are allowed and define their meaning. Contact Information: specify the postal and electronic contact information for the author of the feature. 8. Initial Registrations 8.1. Registration: UNION access calculation rule Rule Identification: UNION Rule Semantics: the union of the rights granted to the applicable identifiers minus the union of the negative rights. Negative rights allowed: Yes. Groups allowed: Yes, but not required. Special Identifiers: No. Contact Information: c.f., the "Editor's Address" section of this memo 8.2. Registration: MOST-SPECIFIC access calculation rule Rule Identification: MOST-SPECIFIC Rule Semantics: the rights granted to the most specific identifier present in the ACL are used, i.e. if the user identifier is present, its ACL is used. If no user identifier is present, but a group that includes this user as a member is present, the group ACL is used. If neither user, nor group identifier is present, but an ACL for a special group "anyone" is present, the ACL for "anyone" is used. Negative rights allowed: No. Groups allowed: Yes, but not required. Special Identifiers: No. Contact Information: c.f., the "Editor's Address" section of this memo 9. 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 the mailbox exists, much less return the mailbox's ACL. 10. Other considerations 10.1. Compatibility with RFC 2086 Any server that prohibits a user name in LOGIN/AUTHENTICATE that starts with "$" MUST report "ACL" capability in addition to a "ACL=..." capability. 10.2. ACL2 interaction with QUOTA extension Server that implement both ACL2 and QUOTA extensions MUST use the following table to determine if a quote operation should be allowed for the user: GETQUOTAROOT - "r" right GETQUOTA - no rights required SETQUOTA - "a" right << Some implementations allows to set quota when the user has "r" right as well. Should we allow that? >> 10.3. Additional requirements and Implementation notes Any server implementing an ACL2 extension MUST accurately reflect the current user's rights in FLAGS and PERMANENTFLAGS responses. The server SHOULD issue a MYRIGHTS response code in an untagged OK response as a result of a SELECT or EXAMINE command. 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 (\Deleted \Seen \*)] Limited S: * OK [MYRIGHTS rwiste] Allowed rights S: A142 OK [READ-WRITE] SELECT completed An ACL2 server is allowed to add/remove some rights automatically when executing any of SETACL/DELETEACL/REPLACEACL. If the server does that it MUST send untagged ACL response to notify the client about the changes made. 11. 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 2060, University of Washington, December 1996. [UTF-8] Yergeau, F., "UTF-8, a transformation format of IS0 10646", RFC 2279, Alis Technologies, January 1998. [QUOTA] Myers, J., "IMAP4 QUOTA extension", RFC 2087, Carnegie Mellon University, January 1997. 12. Aknowledgement This document is a revision of the RFC 2086 written by John G. Myers. Editor also appreciate comments received from Mark Crispin, Chris Newman, Cyrus Daboo, Curtis King, Lyndon Nerenberg and other members of IMAPEXT working group. 13. Editor's Address Alexey Melnikov mailto: firstname.lastname@example.org ACI WorldWide/MessagingDirect 22 The Quadrant, Richmond, Surrey, United Kingdom, TW9 1BP 14. Full Copyright Statement Copyright (C) The Internet Society 2002. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Appendix A. Changes since RFC 2086 1. Changed the charset of "identifier" from US-ASCII to UTF-8. 2. Specified that identifiers starting with a dollar sign ("$") are reserved for groups and implementation defined aliases. 3. Specified that mailbox deletion is controled by the "x" right and EXPUNGE is controlled by "e" right. 4. Clarified that RENAME requires "c" right for the new parent and "x" right for the old name. 5. Changed capability name from "ACL" to "ACL2" because changes 2 and 3 are not backward compatible with ACL RFC. 6. Added "t" right that controls STORE \Deleted. Redefined "d" to be a macro for "e", "x" and "t". 7. Added "ACL2=UNION" and "ACL2=MOST-SPECIFIC" capabilities and IANA registration template. 8. Specified that "a" right also controls DELETEACL 9. Specified that "r" right also controls STATUS 10. Specified that "w" controls setting flags other than \Seen and \Deleted on APPEND. Same for "s" and "t" flags. 11. Specified that "l" controls SUBSCRIBE/UNSUBSCRIBE. 12. Added note about compatibility with RFC 2086 13. Added "Implementation Notes" section. 14. Updated "References" section. 15. Deleted "PARTIAL", this is a depricated feature of RFC1730. 16. Added MYRIGHT response code as per Cyrus suggestion. 17. Added REPLACEACL as per Mark and Cyrus request. 18. Added new section that describes which rights are required and/or checked when performing various IMAP commands. 19. Added section about interaction of ACL2 with the QUOTA extension.