Network Working Group C. Newman
Internet Draft: Using TLS with IMAP, POP3 and ACAP Innosoft
Document: draft-newman-tls-imappop-09.txt April 1999
Using TLS with IMAP, POP3 and ACAP
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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Abstract
This specification defines extensions to IMAP [IMAP], POP [POP3]
and ACAP [ACAP] which activate TLS [TLS]. This also defines a
simple PLAIN SASL [SASL] mechanism for use underneath strong TLS
encryption with ACAP or other protocols lacking a clear-text login
command.
1. Motivation
The TLS protocol (formerly known as SSL) provides a way to secure
an application protocol from tampering and eavesdropping. The
option of using such security is desirable for IMAP, POP and ACAP
due to common connection eavesdropping and hijacking attacks
[AUTH]. Although advanced SASL authentication mechanisms can
provide a lightweight version of this service, TLS is complimentary
to simple authentication-only SASL mechanisms or deployed
clear-text password login commands.
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Many sites have a high investment in authentication infrastructure
(e.g., a large database of a one-way-function applied to user
passwords), so a privacy layer which is not tightly bound to user
authentication can protect against network eavesdropping attacks
without requiring a new authentication infrastructure and/or
forcing all users to change their password. Recognizing that such
sites will desire simple password authentication in combination
with TLS encryption, this specification defines the PLAIN SASL
mechanism for use with protocols which lack a simple password
authentication command such as ACAP and SMTP. (Note there is a
separate RFC for the STARTTLS command in SMTP [SMTPTLS].)
There is a strong desire in the IETF to eliminate the transmission
of clear-text passwords over unencrypted channels. While SASL can
be used for this purpose, TLS provides an additional tool with
different deployability characteristics. A server supporting both
TLS with simple passwords and a challenge/response SASL mechanism
is likely to interoperate with a wide variety of clients without
resorting to unencrypted clear-text passwords.
The STARTTLS command rectifies a number of the problems with using
a separate port for a "secure" protocol variant. Some of these are
mentioned in section 7.
1.1. Conventions Used in this Document
The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD
NOT", "MAY", and "OPTIONAL" in this document are to be interpreted
as described in "Key words for use in RFCs to Indicate Requirement
Levels" [KEYWORDS].
Terms related to authentication are defined in "On Internet
Authentication" [AUTH].
Formal syntax is defined using ABNF [ABNF].
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
2. Basic Interoperability and Security Requirements
The following requirements apply to all implementations of the
STARTTLS extension for IMAP, POP3 and ACAP.
2.1. Cipher Suite Requirements
Implementation of the TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA [TLS]
cipher suite is REQUIRED. This is important as it assures that any
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two compliant implementations can be configured to interoperate.
All other cipher suites are OPTIONAL.
2.2. Privacy Operational Mode Security Requirements
Both clients and servers SHOULD have a privacy operational mode
which refuses authentication unless successful activation of an
encryption layer (such as that provided by TLS) occurs prior to or
at the time of authentication and which will terminate the
connection if that encryption layer is deactivated.
Implementations are encouraged to have flexability with respect to
the minimal encryption strength or cipher suites permitted. A
minimalist approach to this recommendation would be an operational
mode where the TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA cipher suite is
mandatory prior to permitting authentication.
Clients MAY have an operational mode which uses encryption only
when it is advertised by the server, but authentication continues
regardless. For backwards compatibility, servers SHOULD have an
operational mode where only the authentication mechanisms required
by the relevant base protocol specification are needed to
successfully authenticate.
2.3. Clear-Text Password Requirements
Clients and servers which implement STARTTLS MUST be configurable
to refuse all clear-text login commands or mechanisms (including
both standards-track and nonstandard mechanisms) unless an
encryption layer of adequate strength is active. Servers which
allow unencrypted clear-text logins SHOULD be configurable to
refuse clear-text logins both for the entire server, and on a
per-user basis.
2.4. Server Identity Check
During the TLS negotiation, the client MUST check its understanding
of the server hostname against the server's identity as presented
in the server Certificate message, in order to prevent
man-in-the-middle attacks. Matching is performed according to
these rules:
o The client MUST use the server hostname it used to open the
connection as the value to compare against the server name as
expressed in the server certificate. The client MUST NOT use
any form of the server hostname derived from an insecure remote
source (e.g., insecure DNS lookup). CNAME canonicalization is
not done.
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o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the left-most name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc. but would not match
example.com.
o If the certificate contains multiple names (e.g. more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client SHOULD either ask for explicit user
confirmation, or terminate the connection and indicate the server's
identity is suspect.
2.5. TLS Security Policy Check
Both the client and server MUST check the result of the STARTTLS
command and subsequent TLS negotiation to see whether acceptable
authentication or privacy was achieved. Ignoring this step
completely invalidates using TLS for security. The decision about
whether acceptable authentication or privacy was achieved is made
locally, is implementation-dependent, and is beyond the scope of
this document.
3. IMAP STARTTLS extension
When the TLS extension is present in IMAP, "STARTTLS" is listed as
a capability in response to the CAPABILITY command. This extension
adds a single command, "STARTTLS" to the IMAP protocol which is
used to begin a TLS negotiation.
3.1. STARTTLS Command
Arguments: none
Responses: no specific responses for this command
Result: OK - begin TLS negotiation
BAD - command unknown or arguments invalid
A TLS negotiation begins immediately after the CRLF at the end of
the tagged OK response from the server. Once a client issues a
STARTTLS command, it MUST NOT issue further commands until a
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server response is seen and the TLS negotiation is complete.
The STARTTLS command is only valid in non-authenticated state.
The server remains in non-authenticated state, even if client
credentials are supplied during the TLS negotiation. The SASL
[SASL] EXTERNAL mechanism MAY be used to authenticate once TLS
client credentials are successfully exchanged, but servers
supporting the STARTTLS command are not required to support the
EXTERNAL mechanism.
Once TLS has been started, the client MUST discard cached
information about server capabilities and SHOULD re-issue the
CAPABILITY command. This is necessary to protect against
man-in-the-middle attacks which alter the capabilities list prior
to STARTTLS. The server MAY advertise different capabilities
after STARTTLS.
The formal syntax for IMAP is amended as follows:
command_any =/ "STARTTLS"
Example: C: a001 CAPABILITY
S: * CAPABILITY IMAP4rev1 STARTTLS LOGINDISABLED
S: a001 OK CAPABILITY completed
C: a002 STARTTLS
S: a002 OK Begin TLS negotiation now
<TLS negotiation, further commands are under TLS layer>
C: a003 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=EXTERNAL
S: a003 OK CAPABILITY completed
C: a004 LOGIN joe password
S: a004 OK LOGIN completed
3.2. IMAP LOGINDISABLED capability
The current IMAP protocol specification (RFC 2060) requires the
implementation of the LOGIN command which uses clear-text
passwords. Many sites may choose to disable this command unless
encryption is active for security reasons. An IMAP server MAY
advertise that the LOGIN command is disabled by including the
LOGINDISABLED capability in the capability response. Such a server
will respond with a tagged "NO" response to any attempt to use the
LOGIN command.
An IMAP server which implements STARTTLS MUST implement support for
the LOGINDISABLED capability on unencrypted connections.
An IMAP client which complies with this specification MUST NOT
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issue the LOGIN command if this capability is present.
This capability is useful to prevent clients compliant with this
specification from sending an unencrypted password in an
environment subject to passive attacks. It has no impact on an
environment subject to active attacks as a man-in-the-middle
attacker can remove this capability. Therefore this does not
relieve clients of the need to follow the privacy mode
recommendation in section 2.2.
Servers advertising this capability will fail to interoperate with
many existing compliant IMAP clients and will be unable to prevent
those clients from disclosing the user's password.
4. POP3 STARTTLS extension
The POP3 STARTTLS extension adds the STLS command to POP3 servers.
If this is implemented, the POP3 extension mechanism [POP3EXT] MUST
also be implemented to avoid the need for client probing of multiple
commands. The capability name "STLS" indicates this command is
present and permitted in the current state.
STLS
Arguments: none
Restrictions:
Only permitted in AUTHORIZATION state.
Discussion:
A TLS negotiation begins immediately after the CRLF at the
end of the +OK response from the server. A -ERR response
MAY result if a security layer is already active. Once a
client issues a STLS command, it MUST NOT issue further
commands until a server response is seen and the TLS
negotiation is complete.
The STLS command is only permitted in AUTHORIZATION state
and the server remains in AUTHORIZATION state, even if
client credentials are supplied during the TLS negotiation.
The AUTH command [POP-AUTH] with the EXTERNAL mechanism
[SASL] MAY be used to authenticate once TLS client
credentials are successfully exchanged, but servers
supporting the STLS command are not required to support the
EXTERNAL mechanism.
Once TLS has been started, the client MUST discard cached
information about server capabilities and SHOULD re-issue
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the CAPA command. This is necessary to protect against
man-in-the-middle attacks which alter the capabilities list
prior to STLS. The server MAY advertise different
capabilities after STLS.
Possible Responses:
+OK -ERR
Examples:
C: STLS
S: +OK Begin TLS negotiation
<TLS negotiation, further commands are under TLS layer>
...
C: STLS
S: -ERR Command not permitted when TLS active
5. ACAP STARTTLS extension
When the TLS extension is present in ACAP, "STARTTLS" is listed as
a capability in the ACAP greeting. No arguments to this capability
are defined at this time. This extension adds a single command,
"STARTTLS" to the ACAP protocol which is used to begin a TLS
negotiation.
5.1. STARTTLS Command
Arguments: none
Responses: no specific responses for this command
Result: OK - begin TLS negotiation
BAD - command unknown or arguments invalid
A TLS negotiation begins immediately after the CRLF at the end of
the tagged OK response from the server. Once a client issues a
STARTTLS command, it MUST NOT issue further commands until a
server response is seen and the TLS negotiation is complete.
The STARTTLS command is only valid in non-authenticated state.
The server remains in non-authenticated state, even if client
credentials are supplied during the TLS negotiation. The SASL
[SASL] EXTERNAL mechanism MAY be used to authenticate once TLS
client credentials are successfully exchanged, but servers
supporting the STARTTLS command are not required to support the
EXTERNAL mechanism.
After the TLS layer is established, the server MUST re-issue an
untagged ACAP greeting. This is necessary to protect against
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man-in-the-middle attacks which alter the capabilities list prior
to STARTTLS. The client MUST discard cached capability
information and replace it with the information from the new ACAP
greeting. The server MAY advertise different capabilities after
STARTTLS.
The formal syntax for ACAP is amended as follows:
command_any =/ "STARTTLS"
Example: S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
C: a002 STARTTLS
S: a002 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer>
S: * ACAP (SASL "CRAM-MD5" "PLAIN" "EXTERNAL")
6. PLAIN SASL mechanism
Clear-text passwords are simple, interoperate with almost all
existing operating system authentication databases, and are useful
for a smooth transition to a more secure password-based
authentication mechanism. The drawback is that they are
unacceptable for use over an unencrypted network connection.
This defines the "PLAIN" SASL mechanism for use with ACAP and other
protocols with no clear-text login command. The PLAIN SASL
mechanism MUST NOT be advertised or used unless a strong encryption
layer (such as the provided by TLS) is active or backwards
compatibility dictates otherwise.
The mechanism consists of a single message from the client to the
server. The client sends the authorization identity (identity to
login as), followed by a US-ASCII NUL character, followed by the
authentication identity (identity whose password will be used),
followed by a US-ASCII NUL character, followed by the clear-text
password. The client may leave the authorization identity empty to
indicate that it is the same as the authentication identity.
The server will verify the authentication identity and password
with the system authentication database and verify that the
authentication credentials permit the client to login as the
authorization identity. If both steps succeed, the user is logged
in.
The server MAY also use the password to initialize any new
authentication database, such as one suitable for CRAM-MD5
[CRAM-MD5].
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Non-US-ASCII characters are permitted as long as they are
represented in UTF-8 [UTF-8]. Use of non-visible characters or
characters which a user may be unable to enter on some keyboards is
discouraged.
The formal grammar for the client message using Augmented BNF
[ABNF] follows.
message = [authorize-id] NUL authenticate-id NUL password
authenticate-id = 1*UTF8-SAFE ; MUST accept up to 255 octets
authorize-id = 1*UTF8-SAFE ; MUST accept up to 255 octets
password = 1*UTF8-SAFE ; MUST accept up to 255 octets
NUL = %x00
UTF8-SAFE = %x01-09 / %x0B-0C / %x0E-7F / UTF8-2 /
UTF8-3 / UTF8-4 / UTF8-5 / UTF8-6
UTF8-1 = %x80-BF
UTF8-2 = %xC0-DF UTF8-1
UTF8-3 = %xE0-EF 2UTF8-1
UTF8-4 = %xF0-F7 3UTF8-1
UTF8-5 = %xF8-FB 4UTF8-1
UTF8-6 = %xFC-FD 5UTF8-1
Here is an example of how this might be used to initialize a
CRAM-MD5 authentication database for ACAP:
Example: S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
C: a001 AUTHENTICATE "CRAM-MD5"
S: + "<1896.697170952@postoffice.reston.mci.net>"
C: "tim b913a602c7eda7a495b4e6e7334d3890"
S: a001 NO (TRANSITION-NEEDED)
"Please change your password, or use TLS to login"
C: a002 STARTTLS
S: a002 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer>
S: * ACAP (SASL "CRAM-MD5" "PLAIN" "EXTERNAL")
C: a003 AUTHENTICATE "PLAIN" {21+}
C: <NUL>tim<NUL>tanstaaftanstaaf
S: a003 OK CRAM-MD5 password initialized
Note: In this example, <NUL> represents a single ASCII NUL octet.
7. imaps and pop3s ports
Separate "imaps" and "pop3s" ports were registered for use with
SSL. Use of these ports is discouraged in favor of the STARTTLS or
STLS commands.
A number of problems have been observed with separate ports for
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"secure" variants of protocols. This is an attempt to enumerate
some of those problems.
o Separate ports lead to a separate URL scheme which intrudes into
the user interface in inappropriate ways. For example, many web
pages use language like "click here if your browser supports
SSL." This is a decision the browser is often more capable of
making than the user.
o Separate ports imply a model of either "secure" or "not secure."
This can be misleading in a number of ways. First, the "secure"
port may not in fact be acceptably secure as an export-crippled
cipher suite might be in use. This can mislead users into a
false sense of security. Second, the normal port might in fact
be secured by using a SASL mechanism which includes a security
layer. Thus the separate port distinction makes the complex
topic of security policy even more confusing. One common result
of this confusion is that firewall administrators are often
misled into permitting the "secure" port and blocking the
standard port. This could be a poor choice given the common use
of SSL with a 40-bit key encryption layer and plain-text
password authentication is less secure than strong SASL
mechanisms such as GSSAPI with Kerberos 5.
o Use of separate ports for SSL has caused clients to implement
only two security policies: use SSL or don't use SSL. The
desirable security policy "use TLS when available" would be
cumbersome with the separate port model, but is simple with
STARTTLS.
o Port numbers are a limited resource. While they are not yet in
short supply, it is unwise to set a precedent that could double
(or worse) the speed of their consumption.
8. IANA Considerations
This constitutes registration of the "STARTTLS" and "LOGINDISABLED"
IMAP capabilities as required by section 7.2.1 of RFC 2060 [IMAP].
The registration for the POP3 "STLS" capability follows:
CAPA tag: STLS
Arguments: none
Added commands: STLS
Standard commands affected: May enable USER/PASS as a side-effect.
CAPA command SHOULD be re-issued after successful completion.
Announced states/Valid states: AUTHORIZATION state only.
Specification reference: this memo
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The registration for the ACAP "STARTTLS" capability follows:
Capability name: STARTTLS
Capability keyword: STARTTLS
Capability arguments: none
Published Specification(s): this memo
Person and email address for further information:
see author's address section below
The registration for the PLAIN SASL mechanism follows:
SASL mechanism name: PLAIN
Security Considerations: See section 9 of this memo
Published specification: this memo
Person & email address to contact for further information:
see author's address section below
Intended usage: COMMON
Author/Change controller: see author's address section below
9. Security Considerations
TLS only provides protection for data sent over a network
connection. Messages transferred over IMAP or POP3 are still
available to server administrators and usually subject to
eavesdropping, tampering and forgery when transmitted through SMTP
or NNTP. TLS is no substitute for an end-to-end message security
mechanism using MIME security multiparts [MIME-SEC].
A man-in-the-middle attacker can remove STARTTLS from the
capability list or generate a failure response to the STARTTLS
command. In order to detect such an attack, clients SHOULD warn
the user when session privacy is not active and/or be configurable
to refuse to proceed without an acceptable level of security.
A man-in-the-middle attacker can always cause a down-negotiation to
the weakest authentication mechanism or cipher suite available.
For this reason, implementations SHOULD be configurable to refuse
weak mechanisms or cipher suites.
Any protocol interactions prior to the TLS handshake are performed
in the clear and can be modified by a man-in-the-middle attacker.
For this reason, clients MUST discard cached information about
server capabilities advertised prior to the start of the TLS
handshake.
Clients are encouraged to clearly indicate when the level of
encryption active is known to be vulnerable to attack using modern
hardware (such as encryption keys with 56 bits of entropy or less).
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The LOGINDISABLED IMAP capability (discussed in section 3.2) only
reduces the potential for passive attacks, it provides no
protection against active attacks. The responsibility remains with
the client to avoid sending a password over a vulnerable channel.
The PLAIN mechanism relies on the TLS encryption layer for
security. When used without TLS, it is vulnerable to a common
network eavesdropping attack. Therefore PLAIN MUST NOT be
advertised or used unless a suitable TLS encryption layer is active
or backwards compatibility dictates otherwise.
When the PLAIN mechanism is used, the server gains the ability to
impersonate the user to all services with the same password
regardless of any encryption provided by TLS or other network
privacy mechanisms. While many other authentication mechanisms
have similar weaknesses, stronger SASL mechanisms such as Kerberos
address this issue. Clients are encouraged to have an operational
mode where all mechanisms which are likely to reveal the user's
password to the server are disabled.
The security considerations for TLS apply to STARTTLS and the
security considerations for SASL apply to the PLAIN mechanism.
Additional security requirements are discussed in section 2.
10. References
[ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd,
November 1997.
[ACAP] Newman, Myers, "ACAP -- Application Configuration Access
Protocol", RFC 2244, Innosoft, Netscape, November 1997.
[AUTH] Haller, N., Atkinson, R., "On Internet Authentication", RFC
1704, Bell Communications Research, October 1994.
[CRAM-MD5] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension
for Simple Challenge/Response", RFC 2195, MCI, September 1997.
[IMAP] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, University of Washington, December 1996.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997.
[MIME-SEC] Galvin, Murphy, Crocker, Freed, "Security Multiparts for
MIME: Multipart/Signed and Multipart/Encrypted", RFC 1847, Trusted
Information Systems, CyberCash, Innosoft International, October
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1995.
[POP3] Myers, J., Rose, M., "Post Office Protocol - Version 3", RFC
1939, Carnegie Mellon, Dover Beach Consulting, Inc., May 1996.
[POP3EXT] Gellens, R., Newman, C., Lundblade, L., "POP3 Extension
Mechanism", RFC 2449, November 1998.
[POP-AUTH] Myers, J., "POP3 AUTHentication command", RFC 1734,
Carnegie Mellon, December 1994.
[SASL] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, Netscape Communications, October 1997.
[SMTPTLS] Hoffman, P., "SMTP Service Extension for Secure SMTP over
TLS", RFC 2487, Internet Mail Consortium, January 1999.
[TLS] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC
2246, Certicom, January 1999.
[UTF-8] Yergeau, F. "UTF-8, a transformation format of ISO 10646",
RFC 2279, Alis Technologies, January 1998.
11. Author's Address
Chris Newman
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790 USA
Email: chris.newman@innosoft.com
A. Appendix -- Compliance Checklist
An implementation is not compliant if it fails to satisfy one or
more of the MUST requirements for the protocols it implements. An
implementation that satisfies all the MUST and all the SHOULD
requirements for its protocols is said to be "unconditionally
compliant"; one that satisfies all the MUST requirements but not
all the SHOULD requirements for its protocols is said to be
"conditionally compliant".
Rules Section
----- -------
Mandatory-to-implement Cipher Suite 2.1
SHOULD have mode where encryption required 2.2
server SHOULD have mode where TLS not required 2.2
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MUST be configurable to refuse all clear-text login
commands or mechanisms 2.3
server SHOULD be configurable to refuse clear-text
login commands on entire server and on per-user basis 2.3
client MUST check server identity 2.4
client MUST use hostname used to open connection 2.4
client MUST NOT use hostname from insecure remote lookup 2.4
client SHOULD support subjectAltName of dNSName type 2.4
client SHOULD ask for confirmation or terminate on fail 2.4
MUST check result of STARTTLS for acceptable privacy 2.5
client MUST NOT issue commands after STARTTLS
until server response and negotiation done 3.1,4,5.1
client MUST discard cached information 3.1,4,5.1,9
client SHOULD re-issue CAPABILITY/CAPA command 3.1,4
IMAP server with STARTTLS MUST implement LOGINDISABLED 3.2
IMAP client MUST NOT issue LOGIN if LOGINDISABLED 3.2
POP server MUST implement POP3 extensions 4
ACAP server MUST re-issue ACAP greeting 5.1
client SHOULD warn when session privacy not active and/or
refuse to proceed without acceptable security level 9
SHOULD be configurable to refuse weak mechanisms or
cipher suites 9
The PLAIN mechanism is an optional part of this specification.
However if it is implemented the following rules apply:
Rules Section
----- -------
MUST NOT use PLAIN unless strong encryption active
or backwards compatibility dictates otherwise 6,9
MUST use UTF-8 encoding for characters in PLAIN 6
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