Internet DRAFT - draft-newman-imap-unauth
draft-newman-imap-unauth
Network Working Group C. Newman
Internet-Draft Oracle
Intended status: Standards Track March 21, 2018
Expires: September 22, 2018
IMAP UNAUTHENTICATE for Connection Reuse
draft-newman-imap-unauth-02
Abstract
This specification extends the Internet Message Access Protocol
(IMAP) to allow an administrative client to reuse the same IMAP
connection on behalf of multiple IMAP user identities.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 22, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. UNAUTHENTICATE Command . . . . . . . . . . . . . . . . . . . 3
4. Interactions . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Stateful Extensions . . . . . . . . . . . . . . . . . . . 4
4.2. Client Certificates, SASL EXTERNAL and imaps . . . . . . 5
5. Revised State Machine . . . . . . . . . . . . . . . . . . . . 5
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Design Considerations . . . . . . . . . . . . . . . 10
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Modern IMAP [RFC3501] server deployments often have peer systems with
administrative privilege that perform actions on behalf of IMAP end-
users. For example, a voice mail gateway can use IMAP to store a
user's voicemail and mark that voicemail as \Seen when the user
listens to it via the phone interface. These systems can issue the
IMAP AUTHENTICATE command with administrative credentials to act on
behalf of other users. However, with the IMAP base specification,
these specialized IMAP clients must close the connection and create a
new connection for each user. For efficiency reasons, it is
desirable for these clients to reuse the same connection,
particularly if SSL has been negotiated. This specification proposes
the UNAUTHENTICATE command to achieve this goal.
The IMAP state machine described in section 3 of RFC 3501 does not
have a transition from authenticated or selected state to not
authenticated state. The UNAUTHENTICATE command adds a transition
from authenticated or selected state to not authenticated state.
2. Conventions Used in This Document
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 [RFC2119].
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3. UNAUTHENTICATE Command
Arguments: None
Responses: no specific response for this command
Result: OK - completed, now in not authenticated state
BAD - command unknown or arguments invalid
This command directs the server to reset all connection state, except
for state at the TLS [RFC5465] layer. Upon completion, the server
connection is placed in not authenticated state. This represents
transition 7 in the State Machine Diagram (Section 5).
If a mailbox was selected, the mailbox ceases to be selected but no
expunge event is generated. If a SASL [RFC4422] security layer was
active, it terminates immediately after the server sends the CRLF
following the OK response. For the client, it terminates immediately
after the CRLF following the UNAUTHENTICATE command.
After sending this command, the client is free to issue a new
AUTHENTICATE or LOGIN command as permitted based on the server's
capabilities. If no SASL security layer was active, the client is
permitted to pipeline the UNAUTHENTICATE command with a subsequent
AUTHENTICATE command. If the IMAP server also advertises SASL-IR
[RFC4959], this permits an administrative client to re-authenticate
in one round trip. Because of this pipelining optimization, a server
advertising UNAUTHENTICATE is not permitted to respond to the
UNAUTHENTICATE command with a NO response if it is unable to reset
state associated with the connection. Servers MAY close the
connection with an untagged BYE response if this preferably rare
situation occurs.
Servers MAY choose to advertise the UNAUTHENTICATE capability only
after authentication has completed. As a result, clients need to
issue an IMAP CAPABILITY command after authentication in order to
determine the availability of UNAUTHENTICATE.
The IMAP ID [RFC2971] command provides properties about the client
primarily for use in server log or audit files. Because IMAP ID is
not related to application authentication or user identity in any way
and caching it for the duration of the client connection can be
useful, the interaction between IMAP ID and the UNAUTHENTICATE
command is implementation defined.
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4. Interactions
This section describes interactions between this extension and other
IMAP extensions or usage models.
4.1. Stateful Extensions
This lists some IMAP extensions that have connection state that MUST
be reset if both the specified extension is advertised and the
UNAUTHENTICATE command is advertised and used. This list may not be
complete; the requirement to reset connection state applies to all
current and future extensions except STARTTLS and ID.
o Cached identity information, such a group memberships, that are
used to evaluate access control lists [RFC4314] MUST be reset.
o CONDSTORE servers [RFC4551] MUST behave as if no CONDSTORE
enabling command had been issued after an UNAUTHENTICATE command
is issued.
o If IMAP COMPRESS [RFC4978] is active, the compression layer
terminates after the server sends the CRLF following the OK
response and after the client sends the CRLF following the
UNAUTHENTICATE command. In the event it matters, the compression
layer terminates before a SASL layer terminates.
o Any extensions enabled by the IMAP ENABLE [RFC5161] command cease
to be enabled when the UNAUTHENTICATE command is issued. This
includes, but is not limited to, CONDSTORE [RFC4551], QRESYNC
[RFC5162], METADATA [RFC5464], METADATA-SERVER [RFC5464] and
UTF8=ACCEPT [RFC6855].
o A server advertising SEARCHRES [RFC5182] discards any saved search
results so that '$' subsequently represents the empty set.
o A server advertising LANGUAGE [RFC5255] will revert to the
"i-default" language.
o When a server advertises CONTEXT=SEARCH or CONTEXT=SORT [RFC5267],
the UNAUTHENTICATE command includes an implicit CANCELUPDATE for
all server contexts.
o When a server advertises NOTIFY [RFC5465], the UNAUTHENTICATE
command cancels server state related to the NOTIFY command and
reverts to default IMAP base-specification behavior for
notifications.
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4.2. Client Certificates, SASL EXTERNAL and imaps
When a TLS [RFC5246] security layer is negotiated either via the
STARTTLS command or use of the imaps port [RFC6186], IMAP servers MAY
be configured to request a client certificate and IMAP clients MAY
provide one. Client credentials at the TLS layer do not normally
impact the application layer without use of the SASL EXTERNAL
mechanism [RFC4422] in an IMAP AUTHENTICATE command that directs the
server to use the provided client certificate to authenticate as the
specified IMAP user. The UNAUTHENTICATE command breaks any
application-level binding of the TLS client credentials but does not
discard the client credentials. As a result, an administrative
client may use a client certificate with administrative privilege to
act on behalf of multiple IMAP users in the same connection via the
EXTERNAL mechanism and the UNAUTHENTICATE command.
Some server implementations using the imaps port will request and use
a TLS client certificate to authenticate immediately as the default
IMAP identity associated with that certificate. These
implementations indicate this behavior by using the PREAUTH greeting
as indicated by transition 2 in the State Machine Diagram
(Section 5). As a result, TLS client certificates can not be used
for administrative proxy authentication with the imaps port unless
the UNAUTHENTICATE command is also advertised. In that case, an
administrative client can respond to the PREAUTH greeting with an
UNAUTHENTICATE command and then issue an AUTHENTICATE EXTERNAL
command.
5. Revised State Machine
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+----------------------+
|connection established|
+----------------------+
||
\/
+--------------------------------------+
| server greeting |
+--------------------------------------+
|| (1) || (2) || (3)
\/ || ||
+-----------------+ || ||
|Not Authenticated|<===||=========++ ||
+-----------------+ || (7) || ||
|| (8) || (4) || || ||
|| \/ \/ || ||
|| +----------------+ || ||
|| | |========++ ||
|| | Authenticated |<=++ || ||
|| +----------------+ || || ||
|| || (8) || (5) ||(6) || ||
|| || \/ || || ||
|| || +--------+ || || ||
|| || |Selected|==++ || ||
|| || | |========++ ||
|| || +--------+ ||
|| || || (8) ||
\/ \/ \/ \/
+--------------------------------------+
| Logout |
+--------------------------------------+
||
\/
+-------------------------------+
|both sides close the connection|
+-------------------------------+
Revised IMAP state machine transitions:
1. connection without pre-authentication (OK greeting)
2. pre-authenticated connection (PREAUTH greeting)
3. rejected connection (BYE greeting)
4. successful LOGIN or AUTHENTICATE command
5. successful SELECT or EXAMINE command
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6. CLOSE, UNSELECT [RFC3691] or failed SELECT or EXAMINE command
7. UNAUTHENTICATE command
8. LOGOUT command, server shutdown, or connection closed
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) as described in [RFC5234]. Amended terms are defined in
[RFC3501].
capability =/ "UNAUTHENTICATE"
command-auth =/ "UNAUTHENTICATE"
command-select =/ "UNAUTHENTICATE"
7. IANA Considerations
The IANA shall add the UNAUTHENTICATE capability to the IMAP4
Capabilities Registry.
8. Security Considerations
The original IMAP state machine was designed to allow a server
implementation approach where each IMAP authentication identity
matches an operating system identity and the server revokes all
administrative privilege onces authentication completes. This
extension is not compatible with that implementation approach.
However, that approach has significant performance costs on Unix
systems, and this extension is designed for environments where
efficiency is a relatively high priority deployment goal. So this
extension is appropriate for some deployments but may not be
appropriate for the most security sensitive environments.
IMAP server implementations are complicated and can retain a lot of
state related to an authenticated user. Server implementers need to
take care to reset all server state such that authentication as a
subsequent user does not inherit any data or privileges from the
previous user. State data associated with a user can include cached
identity information such as group membership used to evaluate access
control lists [RFC4314], active notifications [RFC5465], access to
per-user data such as flags, etc.
IMAP server systems are often deployed in a two-tier model where a
server-side IMAP proxy routes to an IMAP back-end which handles all
connections for a subset of possible users. Some IMAP proxies enter
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a pass-through mode after authentication. The UNAUTHENTICATE
command, if enabled, would allow a client to bypass any security
restrictions present in the proxy layer but not in the back-end
server layer on a subsequent authentication. As a result, IMAP
server implementations of this extension MUST provide a way to
disable it when it is not needed. Use of an IMAP proxy that
processes the UNAUTHENTICATE command at the proxy layer eliminates
this concern. Another option to mitigate this concern is for servers
to only enable the UNAUTHENTICATE extension if the supplied
authentication credentials were associated with an administrative
identity.
9. Privacy Considerations
For the most part, this extension will have no impact on the privacy
considerations already present in an IMAP implementation. However,
if this extension were used between data centers, it could improve
end-user privacy by increasing the difficultly of traffic analysis
due to connection re-use.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
10.2. Informative References
[RFC2971] Showalter, T., "IMAP4 ID extension", RFC 2971,
DOI 10.17487/RFC2971, October 2000,
<https://www.rfc-editor.org/info/rfc2971>.
[RFC3691] Melnikov, A., "Internet Message Access Protocol (IMAP)
UNSELECT command", RFC 3691, DOI 10.17487/RFC3691,
February 2004, <https://www.rfc-editor.org/info/rfc3691>.
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[RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
RFC 4314, DOI 10.17487/RFC4314, December 2005,
<https://www.rfc-editor.org/info/rfc4314>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<https://www.rfc-editor.org/info/rfc4422>.
[RFC4551] Melnikov, A. and S. Hole, "IMAP Extension for Conditional
STORE Operation or Quick Flag Changes Resynchronization",
RFC 4551, DOI 10.17487/RFC4551, June 2006,
<https://www.rfc-editor.org/info/rfc4551>.
[RFC4959] Siemborski, R. and A. Gulbrandsen, "IMAP Extension for
Simple Authentication and Security Layer (SASL) Initial
Client Response", RFC 4959, DOI 10.17487/RFC4959,
September 2007, <https://www.rfc-editor.org/info/rfc4959>.
[RFC4978] Gulbrandsen, A., "The IMAP COMPRESS Extension", RFC 4978,
DOI 10.17487/RFC4978, August 2007,
<https://www.rfc-editor.org/info/rfc4978>.
[RFC5161] Gulbrandsen, A., Ed. and A. Melnikov, Ed., "The IMAP
ENABLE Extension", RFC 5161, DOI 10.17487/RFC5161, March
2008, <https://www.rfc-editor.org/info/rfc5161>.
[RFC5162] Melnikov, A., Cridland, D., and C. Wilson, "IMAP4
Extensions for Quick Mailbox Resynchronization", RFC 5162,
DOI 10.17487/RFC5162, March 2008,
<https://www.rfc-editor.org/info/rfc5162>.
[RFC5182] Melnikov, A., "IMAP Extension for Referencing the Last
SEARCH Result", RFC 5182, DOI 10.17487/RFC5182, March
2008, <https://www.rfc-editor.org/info/rfc5182>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5255] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Message Access Protocol Internationalization", RFC 5255,
DOI 10.17487/RFC5255, June 2008,
<https://www.rfc-editor.org/info/rfc5255>.
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[RFC5267] Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267,
DOI 10.17487/RFC5267, July 2008,
<https://www.rfc-editor.org/info/rfc5267>.
[RFC5464] Daboo, C., "The IMAP METADATA Extension", RFC 5464,
DOI 10.17487/RFC5464, February 2009,
<https://www.rfc-editor.org/info/rfc5464>.
[RFC5465] Gulbrandsen, A., King, C., and A. Melnikov, "The IMAP
NOTIFY Extension", RFC 5465, DOI 10.17487/RFC5465,
February 2009, <https://www.rfc-editor.org/info/rfc5465>.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<https://www.rfc-editor.org/info/rfc6186>.
[RFC6855] Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP
Support for UTF-8", RFC 6855, DOI 10.17487/RFC6855, March
2013, <https://www.rfc-editor.org/info/rfc6855>.
Appendix A. Design Considerations
The author deliberately choose to add a separate UNAUTHENTICATE
command instead of allowing the LOGIN or AUTHENTICATE commands to be
issued when the connection is in a state other than unauthenticated
state. The primary reason for this choice is because the code that
transitions from not authenticated state to authenticated state in a
server is often the most security sensitive code as it needs to
assume and handle unconditionally hostile attackers. That sensitive
code is simpler if it only handles a single server state
(unauthenticated) and the state transition is as simple as possible.
Smaller and simpler code is easier to audit and write in a secure
way.
A secondary reason to have a separate command is that it is simpler
to enable or disable the feature with that design. See the
discussion in the security considerations section recommending
implementations provide a way to disable this extension.
Appendix B. Acknowledgements
Thanks to Fred Batty for implementing UNAUTHENTICATE, and to Cyrus
Daboo for constructive suggestions to improve this draft.
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Author's Address
Chris Newman
Oracle
440 E. Huntington Dr., Suite 400
Arcadia, CA 91006
US
Email: chris.newman@oracle.com
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