NNTP Extensions Working Group J. Vinocur
Internet Draft Cornell University
Updates: 2980 (if approved) K. Murchison
Expires: February 2006 Oceana Matrix Ltd.
C. Newman
Sun Microsystems
August 2005
NNTP Extension for Authentication
draft-ietf-nntpext-authinfo-10
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines an extension to the Network News Transport
Protocol (NNTP) which allows a client to indicate an authentication
mechanism to the server, perform an authentication protocol
exchange, and optionally negotiate a security layer for subsequent
protocol interactions during the remainder of an NNTP session.
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This document updates and formalizes the AUTHINFO USER/PASS
authentication method specified in RFC 2980 and deprecates the
AUTHINFO SIMPLE and AUTHINFO GENERIC authentication methods.
Additionally, this document defines a profile of the Simple
Authentication and Security Layer (SASL) for NNTP.
Note to the RFC Editor
The normative references to RFC 2234 and RFC 3454 may
be replaced by draft-crocker-abnf-rfc2234bis and
draft-hoffman-rfc3454bis respectively should one or both of those
documents reach RFC status before this one.
The normative references to [NNTP] and [NNTP-TLS] are documents
which are expected to be published simultaneously with this one
and so can be replaced by references to the resulting RFCs.
Table of Contents
1. Introduction ............................................. 3
1.1. Conventions Used in this Document ................... 3
2. The AUTHINFO Extension ................................... 4
2.1. Advertising the AUTHINFO Extension .................. 4
2.2. Authenticating with the AUTHINFO Extension .......... 5
2.3. AUTHINFO USER/PASS Command .......................... 6
2.3.1. Usage .......................................... 6
2.3.2. Description .................................... 7
2.3.3. Examples ....................................... 9
2.4. AUTHINFO SASL Command ............................... 9
2.4.1. Usage .......................................... 9
2.4.2. Description .................................... 10
2.4.3. Examples ....................................... 13
3. Augmented BNF Syntax for the AUTHINFO Extension .......... 16
3.1. Commands ............................................ 16
3.2. Command Continuation ................................ 17
3.3. Responses ........................................... 17
3.4. Capability entries .................................. 17
3.5. General non-terminals ............................... 17
4. Summary of Response Codes ................................ 17
5. Authentication Tracking/Logging .......................... 18
6. Security Considerations .................................. 19
7. IANA Considerations ...................................... 19
7.1. IANA Considerations for SASL/GSSAPI services ........ 19
7.2. IANA Considerations for NNTP extensions ............. 20
8. References ............................................... 21
8.1. Normative References ................................ 21
8.2. Informative References .............................. 22
9. Authors' Addresses ....................................... 22
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10. Acknowledgments ......................................... 23
11. Intellectual Property Rights ............................ 23
12. Copyright ............................................... 23
1. Introduction
Although NNTP [NNTP] has traditionally been used to provide public
access to newsgroups, authentication is often useful, for example
to control resource consumption, to allow abusers of the POST
command to be identified, and to restrict access to "local"
newsgroups.
The ad-hoc AUTHINFO USER and AUTHINFO PASS commands, documented in
[NNTP-COMMON], provide a very weak authentication mechanism in
widespread use by the installed base. Due to their ubiquity they
are formalized in this specification but, because of their
insecurity, only for use in combination with appropriate security
layers.
The ad-hoc AUTHINFO GENERIC command, also documented in
[NNTP-COMMON] but much less ubiquitous, provided an NNTP-specific
equivalent of the generic SASL [SASL] facility. This document
deprecates AUTHINFO GENERIC in favor of an AUTHINFO SASL
replacement so that NNTP can benefit from authentication mechanisms
developed for other SASL-enabled application protocols including
SMTP, POP, IMAP, LDAP, and BEEP.
This specification is to be read in conjunction with the NNTP base
specification [NNTP]. Except where specifically stated otherwise,
in the case of a conflict between these two documents [NNTP] takes
precedence over this one.
It is also recommended that this specification be read in
conjunction with the SASL base specification [SASL].
1.1. Conventions Used in this Document
The notational conventions used in this document are the same as
those in [NNTP] and any term not defined in this document has the
same meaning as in that one.
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].
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In the examples, commands from the client are indicated with [C],
and responses from the server are indicated with [S].
2. The AUTHINFO Extension
The AUTHINFO extension is used to authenticate a user. Note that
authorization is a matter of site policy, not network protocol, and
is therefore not discussed in this document. The server determines
authorization in whatever manner is defined by its implementation
as configured by the site administrator.
This extension provides three new commands: AUTHINFO USER, AUTHINFO
PASS, and AUTHINFO SASL. The capability label for this extension
is AUTHINFO.
2.1. Advertising the AUTHINFO Extension
A server MUST implement at least one of the AUTHINFO USER or
AUTHINFO SASL commands in order to advertise the AUTHINFO
capability in the response to the CAPABILITIES command. However,
this capability MUST NOT be advertised after successful
authentication (see section 2.2). This capability MAY be
advertised both before and after any use of MODE READER, with the
same semantics.
The AUTHINFO capability label contains an argument list detailing
which authentication commands are available.
The "USER" argument indicates that AUTHINFO USER/PASS is supported
as defined by Section 2.3 of this document. The "USER" argument
MUST NOT be advertised, and the AUTHINFO USER/PASS commands SHOULD
NOT be provided, unless a strong encryption layer (e.g. TLS
[NNTP-TLS]) is in use or backward compatibility dictates otherwise.
The "SASL" argument indicates that AUTHINFO SASL is supported as
defined by Section 2.4 of this document. If the server advertises
the "SASL" argument, then it MUST also advertise the "SASL"
capability in response to the CAPABILITIES command. The SASL
capability is followed by a whitespace-separated list of available
SASL mechanism names.
The server MAY list the AUTHINFO capability with no arguments,
which indicates that it complies with this specification and does
not permit any authentication commands in its current state. In
this case, the client MUST NOT attempt to utilize any AUTHINFO
commands, even if it contains logic to do so (e.g. for backward
compatibility with servers that are not compliant with this
specification).
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Future extensions may add additional arguments to this capability.
Unrecognized arguments MUST be ignored by the client.
As the AUTHINFO command is related to security, cached results of
CAPABILITIES from a previous session MUST NOT be relied on, as per
section 12.6 of [NNTP]. However, a client MAY use such cached
results in order to detect active down-negotiation attacks.
Example (here, the STARTTLS extension [NNTP-TLS] is also in use):
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] IHAVE
[S] STARTTLS
[S] AUTHINFO SASL
[S] SASL CRAM-MD5 DIGEST-MD5 GSSAPI
[S] LIST ACTIVE NEWSGROUPS
[S] .
[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation proceeds, further commands protected by TLS]
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] IHAVE
[S] AUTHINFO USER SASL
[S] SASL CRAM-MD5 DIGEST-MD5 GSSAPI PLAIN EXTERNAL
[S] LIST ACTIVE NEWSGROUPS
[S] .
2.2. Authenticating with the AUTHINFO Extension
An NNTP server responds to a client command with a 480 response to
indicate that the client MUST authenticate and/or authorize in
order to use that command or access the indicated resource. Use of
the AUTHINFO command as described below is one such way that a
client can authenticate/authorize to the server. The client MAY
therefore use an AUTHINFO command after receiving a 480 response.
A client intending to use an AUTHINFO command SHOULD issue the
CAPABILITIES command to obtain the available authentication
commands and mechanisms before attempting authentication.
If a server advertises the AUTHINFO capability, a client MAY
attempt the first step of authentication at any time during a
session to acquire additional privileges without having received a
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480 response. Servers SHOULD accept such unsolicited
authentication requests. A server MUST NOT under any circumstances
reply to an AUTHINFO command with a 480 response.
A client MUST NOT under any circumstances continue with any steps
of authentication beyond the first, unless the response code from
the server indicates that the authentication exchange is welcomed.
In particular, anything other than a 38x response code indicates
that the client MUST NOT continue the authentication exchange.
After a successful authentication, the client MUST NOT issue
another AUTHINFO command in the same session. A server MUST NOT
return the AUTHINFO capability in response to a CAPABILITIES
command and a server MUST reject any subsequent AUTHINFO commands
with a 502 response. Additionally, the client MUST NOT issue a
MODE READER command after authentication and a server MUST NOT
advertise the MODE-READER capability.
In agreement with [SASL], the server MUST continue to advertise the
SASL capability in response to a CAPABILITIES command with the same
list of SASL mechanisms as before authentication (thereby enabling
the client to detect a possible active down-negotiation attack).
Other capabilities returned in response to a CAPABILITIES command
received after authentication MAY be different than those returned
before authentication. For example, an NNTP server may not want to
advertise support for a specific extension unless a client has been
authenticated.
It should be noted that a server may perform a successful
authentication exchange with a client and yet still deny access to
some or all resources; the permanent 502 response indicates a
resource is unavailable even though authentication has been
performed (this is in contrast to the temporary 480 error
indicating that a resource is unavailable now but may become
available after authentication).
2.3. AUTHINFO USER/PASS Command
This section supersedes the definition of the AUTHINFO USER and
AUTHINFO PASS commands as documented in Section 3.1.1 of
[NNTP-COMMON].
2.3.1. Usage
These commands MUST NOT be pipelined.
Syntax
AUTHINFO USER username
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AUTHINFO PASS password
Responses
281 Authentication accepted
381 Password required [1]
481 Authentication failed/rejected
482 Authentication commands issued out of sequence
502 Command unavailable [2]
[1] Only valid for AUTHINFO USER. Note that unlike traditional 3xx
codes which indicate that the client may continue the current
command, the legacy 381 code means that the AUTHINFO PASS command
must be used to complete the authentication exchange.
[2] If authentication has already occurred, AUTHINFO USER/PASS are
not valid commands (see section 2.2).
NOTE: Notwithstanding section 3.2.1 of [NNTP], the server MUST NOT
return 480 in response to AUTHINFO USER/PASS.
Parameters
username = string identifying the user/client
password = string representing the user's password
2.3.2. Description
The AUTHINFO USER and AUTHINFO PASS commands are used to present
clear text credentials to the server. These credentials consist of
a username or a username plus a password (the distinction is that a
password is expected to be kept secret while a username is not;
this does not directly affect the protocol but may have an impact
on user interfaces). The username is supplied through the AUTHINFO
USER command, and the password through the AUTHINFO PASS command.
If the server requires only a username, it MUST NOT give a 381
response to AUTHINFO USER and MUST give a 482 response to AUTHINFO
PASS.
If the server requires both username and password, the former MUST
be sent before the latter. The server will need to cache the
username until the password is received; it MAY require the
password to be sent in the immediately next command (in other
words, only caching the username until the next command is sent).
The server:
- MUST return a 381 response to AUTHINFO USER;
- MUST return a 482 response to AUTHINFO PASS if there is no
cached username;
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- MUST use the argument of the most recent AUTHINFO USER for
authentication;
- MUST NOT return a 381 response to AUTHINFO PASS.
The server MAY determine whether or not a password is needed based
on the username. Thus the same server can respond with both 381
and other response codes to AUTHINFO USER.
Should the client successfully present proper credentials, the
server issues a 281 reply. If the server is unable to authenticate
the client, it MUST reject the AUTHINFO USER/PASS command with a
481 reply. If an AUTHINFO USER/PASS command fails, the client MAY
proceed without authentication. Alternatively, the client MAY try
another authentication mechanism, or present different credentials
by issuing another AUTHINFO command.
The AUTHINFO PASS command permits the client to use a clear-text
password to authenticate. A compliant implementation MUST NOT
implement this command without also implementing support for TLS
[NNTP-TLS]. Use of this command without an active strong
encryption layer is deprecated, as it exposes the user's password
to all parties on the network between the client and the server.
Any implementation of this command SHOULD be configurable to
disable it whenever a strong encryption layer such as that provided
by [NNTP-TLS] is not active, and this configuration SHOULD be the
default. The server will use the 483 response code to indicate
that the datastream is insufficiently secure for the command being
attempted (see section 3.2.1 of [NNTP]).
Note that a server MAY, but is not required to, allow white space
characters in usernames and passwords. A server implementation MAY
blindly split command arguments at white space and therefore not
preserve the exact sequence of white space characters in the
username or password. Therefore a client SHOULD scan the username
and password for whitespace, and if detected, warn the user of the
likelihood of problems. The SASL PLAIN [PLAIN] mechanism is
recommended as an alternative, as it does not suffer from these
issues.
Also note that historically the username is not canonicalized in
any way. Servers MAY use the [SASLprep] profile of the
[StringPrep] algorithm to prepare usernames for comparison, but
doing so may cause interoperability problems with legacy
implementations. If canonicalization is desired, the SASL PLAIN
[PLAIN] mechanism is recommended as an alternative.
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2.3.3. Examples
Example of successful AUTHINFO USER:
[C] AUTHINFO USER wilma
[S] 281 Authentication accepted
Example of successful AUTHINFO USER/PASS:
[C] AUTHINFO USER fred
[S] 381 Enter passphrase
[C] AUTHINFO PASS flintstone
[S] 281 Authentication accepted
Example of AUTHINFO USER/PASS requiring a security layer:
[C] AUTHINFO USER fred@stonecanyon.example
[S] 483 Encryption or stronger authentication required
Example of failed AUTHINFO USER/PASS:
[C] AUTHINFO USER barney
[S] 381 Enter passphrase
[C] AUTHINFO PASS flintstone
[S] 481 Authentication failed
Example of AUTHINFO PASS before AUTHINFO USER:
[C] AUTHINFO PASS flintstone
[S] 482 Authentication commands issued out of sequence
2.4. AUTHINFO SASL Command
This section defines a formal profile of the Simple Authentication
and Security Layer [SASL]. The use of the AUTHINFO GENERIC command
as documented in Section 3.1.3 of [NNTP-COMMON] as a way to perform
SASL authentication is deprecated in favor of the AUTHINFO SASL
command. A server SHOULD NOT advertise AUTHINFO GENERIC in the
list of capabilities returned by CAPABILITIES.
2.4.1. Usage
This command MUST NOT be pipelined.
Syntax
AUTHINFO SASL mechanism [initial-response]
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This command MAY exceed 512 octets. The maximum length of this
command is increased to that which can accommodate the largest
encoded initial response possible for any of the SASL mechanisms
supported by the implementation.
Responses
281 Authentication accepted
283 challenge Authentication accepted (with success data) [1]
383 challenge Continue with SASL exchange [1]
481 Authentication failed/rejected
482 SASL protocol error
502 Command unavailable [2]
[1] These responses MAY exceed 512 octets. The maximum length of
these responses is increased to that which can accommodate the
largest encoded challenge possible for any of the SASL mechanisms
supported by the implementation.
[2] If authentication has already occurred, AUTHINFO SASL is not a
valid command (see section 2.2).
NOTE: Notwithstanding section 3.2.1 of [NNTP], the server MUST NOT
return 480 in response to AUTHINFO SASL.
Parameters
mechanism = String identifying a [SASL] authentication
mechanism.
initial-response = Optional initial client response.
If present, the response MUST be encoded as
specified in Section 3 of [BASE64]. [3]
challenge = Server challenge.
The challenge MUST be encoded as specified
in Section 3 of [BASE64].
[3] This argument MAY exceed 497 octets. The maximum length of
this argument is increased to that which can accommodate the
largest encoded initial response possible for any of the SASL
mechanisms supported by the implementation.
2.4.2. Description
The AUTHINFO SASL command initiates a [SASL] authentication
exchange between the client and the server. The client identifies
the SASL mechanism to use with the first parameter of the AUTHINFO
SASL command. If the server supports the requested authentication
mechanism, it performs the SASL exchange to authenticate the user.
Optionally, it also negotiates a security layer for subsequent
protocol interactions during this session. If the requested
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authentication mechanism is invalid (e.g. is not supported), the
server rejects the AUTHINFO SASL command with a 503 reply (see
section 3.2.1 of [NNTP]). If the requested authentication
mechanism requires an encryption layer, the server rejects the
AUTHINFO SASL command with a 483 reply (see section 3.2.1 of
[NNTP]).
The service name specified by this protocol's profile of SASL is
"nntp".
The SASL authentication exchange consists of a series of server
challenges and client responses that are specific to the chosen
[SASL] mechanism.
A server challenge is sent as a 383 reply with a single argument
containing the [BASE64] encoded string supplied by the SASL
mechanism. A server challenge that has zero length MUST be sent as
a single equals sign ("=") and not omitted (in order to comply with
the [NNTP] requirement that responses always have the same number
of arguments).
A client response consists of a line containing a [BASE64] encoded
string. A client response that has zero length MUST be sent as a
single equals sign ("=") and not omitted (for consistency with the
server challenge format). If the client wishes to cancel the
authentication exchange, it issues a line with a single "*". If
the server receives such a response, it MUST reject the AUTHINFO
SASL command by sending a 481 reply.
Note that these [BASE64] strings can be much longer than normal
NNTP responses. Clients and servers MUST be able to handle the
maximum encoded size of challenges and responses generated by their
supported authentication mechanisms. This requirement is
independent of any line length limitations the client or server may
have in other parts of its protocol implementation.
The optional initial response argument to the AUTHINFO SASL command
is used to save a round trip when using authentication mechanisms
that support an initial client response. If the initial response
argument is omitted and the chosen mechanism requires an initial
client response, the server MUST proceed as defined in section 5.1
of [SASL]. In NNTP, a server challenge that contains no data is
equivalent to a zero length challenge and is encoded as a single
equals sign ("=").
Note that the [BASE64] encoded initial response argument can exceed
497 octets and therefore the AUTHINFO SASL command can exceed 512
octets. Clients SHOULD, and servers MUST be able to handle the
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maximum encoded size of initial responses possible for their
supported authentication mechanisms. This requirement is
independent of any command or argument length limitations the
client or server may have in other parts of its protocol
implementation.
If use of the initial response argument would cause the AUTHINFO
SASL command to exceed 512 octets, the client MAY choose to omit
the initial response parameter (and instead proceed as defined in
section 5.1 of [SASL]).
If the client is transmitting an initial response of zero length,
it MUST instead transmit the response as a single equals sign
("="). This indicates that the response is present, but contains
no data.
If the client uses an initial-response argument to the AUTHINFO
SASL command with a SASL mechanism that does not support an initial
client response, the server MUST reject the AUTHINFO SASL command
with a 482 reply.
If the server cannot [BASE64] decode any client response, it MUST
reject the AUTHINFO SASL command with a 504 reply (see section
3.2.1 of [NNTP]). If the client cannot BASE64 decode any of the
server's challenges, it MUST cancel the authentication using the
"*" response. In particular, servers and clients MUST reject (and
not ignore) any character not explicitly allowed by the BASE64
alphabet, and MUST reject any sequence of BASE64 characters that
contains the pad character ('=') anywhere other than the end of the
string (e.g. "=AAA" and "AAA=BBB" are not allowed).
The authorization identity generated by this [SASL] exchange is a
simple username, and both client and server MUST use the [SASLprep]
profile of the [StringPrep] algorithm to prepare these names for
transmission or comparison. If preparation of the authorization
identity fails or results in an empty string (unless it was
transmitted as the empty string), the server MUST fail the
authentication with a 481 reply.
Should the client successfully complete the exchange, the server
issues either a 281 or 283 reply. If the server is unable to
authenticate the client, it MUST reject the AUTHINFO SASL command
with a 481 reply. If an AUTHINFO SASL command fails, the client
MAY proceed without authentication. Alternatively, the client MAY
try another authentication mechanism, or present different
credentials by issuing another AUTHINFO command.
If the SASL mechanism returns additional data on success (e.g.
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server authentication), the NNTP server issues a 283 reply with a
single argument containing the [BASE64] encoded string supplied by
the SASL mechanism. If no additional data is returned on success,
the server issues a 281 reply.
If a security layer is negotiated during the SASL exchange, it
takes effect for the client on the octet immediately following the
CRLF that concludes the last response generated by the client. For
the server, it takes effect immediately following the CRLF of its
success reply.
When a security layer takes effect, the NNTP protocol is reset to
the state immediately after the initial greeting response (see 5.1
of [NNTP]) has been sent, with the exception that if a MODE READER
command has been issued, the effects of it (if any) are not
reversed. The server MUST discard any knowledge obtained from the
client, such as the current newsgroup and article number, that was
not obtained from the SASL negotiation itself. Likewise, the
client SHOULD discard and MUST NOT rely on any knowledge obtained
from the server, such as the capability list, that was not obtained
from the SASL negotiation itself. (Note that a client MAY compare
the advertised SASL mechanisms before and after authentication in
order to detect an active down-negotiation attack.)
When both TLS [NNTP-TLS] and SASL security layers are in effect,
the TLS encoding MUST be applied after the SASL encoding (the
cleartext data is always SASL encoded first and then the resultant
data is TLS encoded).
To ensure interoperability, client and server implementations of
this extension MUST implement the [DIGEST-MD5] SASL mechanism.
If AUTHINFO USER/PASS and AUTHINFO SASL are both implemented, the
SASL [PLAIN] mechanism SHOULD also be implemented, as the
functionality of DIGEST-MD5 is insufficient for some environments
(e.g. the server may need to pass the plaintext password off to an
external authentication service). The SASL PLAIN mechanism is
preferred over AUTHINFO USER, even if there is not a strong
encryption layer active, because it eliminates limitations that
AUTHINFO USER/PASS has with regards to white space characters being
used in usernames and passwords.
2.4.3. Examples
Example of the [PLAIN] SASL mechanism under a TLS layer, using an
initial client response:
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[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] STARTTLS
[S] AUTHINFO SASL
[S] SASL CRAM-MD5 DIGEST-MD5 GSSAPI
[S] LIST ACTIVE NEWSGROUPS
[S] .
[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation proceeds, further commands protected by TLS]
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] AUTHINFO USER SASL
[S] SASL CRAM-MD5 DIGEST-MD5 GSSAPI PLAIN EXTERNAL
[S] LIST ACTIVE NEWSGROUPS
[S] .
[C] AUTHINFO SASL PLAIN AHRlc3QAMTIzNA==
[S] 281 Authentication accepted
Example of the EXTERNAL SASL mechanism under a TLS layer, using the
authorization identity derived from the client TLS certificate, and
thus a zero-length initial client response (commands prior to
AUTHINFO SASL are the same as the previous example and have been
omitted):
[C] AUTHINFO SASL EXTERNAL =
[S] 281 Authentication accepted
Example of the [DIGEST-MD5] SASL mechanism, which includes a server
challenge and server success data (whitespace has been inserted for
clarity; base64-encoded data is actually sent as a single line with
no embedded whitespace):
[C] AUTHINFO SASL DIGEST-MD5
[S] 383 bm9uY2U9InNheUFPaENFS0dJZFBNSEMwd3RsZUxxT0ljT0kyd1FZSWU0
enplQXR1aVE9IixyZWFsbT0iZWFnbGUub2NlYW5hLmNvbSIscW9wPSJhdXRo
LGF1dGgtaW50LGF1dGgtY29uZiIsY2lwaGVyPSJyYzQtNDAscmM0LTU2LHJj
NCxkZXMsM2RlcyIsbWF4YnVmPTQwOTYsY2hhcnNldD11dGYtOCxhbGdvcml0
aG09bWQ1LXNlc3M=
[C] dXNlcm5hbWU9InRlc3QiLHJlYWxtPSJlYWdsZS5vY2VhbmEuY29tIixub25j
ZT0ic2F5QU9oQ0VLR0lkUE1IQzB3dGxlTHFPSWNPSTJ3UVlJZTR6emVBdHVp
UT0iLGNub25jZT0iMFkzSlFWMlRnOVNjRGlwK08xU1ZDMHJoVmcvLytkbk9J
aUd6LzdDZU5KOD0iLG5jPTAwMDAwMDAxLHFvcD1hdXRoLWNvbmYsY2lwaGVy
PXJjNCxtYXhidWY9MTAyNCxkaWdlc3QtdXJpPSJubnRwL2xvY2FsaG9zdCIs
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cmVzcG9uc2U9ZDQzY2Y2NmNmZmE5MDNmOWViMDM1NmMwOGEzZGIwZjI=
[S] 283 cnNwYXV0aD1kZTJlMTI3ZTVhODFjZGE1M2Q5N2FjZGEzNWNkZTgzYQ==
Example of a failed authentication due to bad [GSSAPI] credentials.
Note that while the mechanism can utilize the initial response, the
client chooses not to use it because of its length, resulting in a
zero-length server challenge (here whitespace has been inserted for
clarity; base64-encoded data is actually sent as a single line with
no embedded whitespace):
[C] AUTHINFO SASL GSSAPI
[S] 383 =
[C] YIICOAYJKoZIhvcSAQICAQBuggInMIICI6ADAgEFoQMCAQ6iBwMFACAAAACj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[S] 481 Authentication error
Example of a client aborting in the midst of an exchange:
[C] AUTHINFO SASL GSSAPI
[S] 383 =
[C] *
[S] 481 Authentication aborted as requested
Example of attempting to use a mechanism that is not supported by
the server:
[C] AUTHINFO SASL EXAMPLE
[S] 503 Mechanism not recognized
Example of attempting to use a mechanism that requires a security
layer:
[C] AUTHINFO SASL PLAIN
[S] 483 Encryption or stronger authentication required
Example of using an initial response with a mechanism that doesn't
support it (server must start the exchange):
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[C] AUTHINFO SASL CRAM-MD5 AHRlc3QAMTIzNA==
[S] 482 SASL protocol error
Example of a failed authentication due to an incorrectly encoded
response:
[C] AUTHINFO SASL CRAM-MD5
[S] 383 PDE1NDE2NzQ5My4zMjY4MzE3QHRlc3RAZXhhbXBsZS5jb20+
[C] abcd=efg
[S] 504 Base64 encoding error
3. Augmented BNF Syntax for the AUTHINFO Extension
This section describes the formal syntax of the AUTHINFO extension
using ABNF [ABNF]. It extends the syntax in section 9 of [NNTP],
and non-terminals not defined in this document are defined there.
The [NNTP] ABNF should be imported first before attempting to
validate these rules.
3.1. Commands
This syntax extends the non-terminal "command", which represents an
NNTP command.
command =/ authinfo-sasl-command /
authinfo-user-command /
authinfo-pass-command
authinfo-sasl-command = "AUTHINFO" WS "SASL" WS mechanism
[WS initial-response]
authinfo-user-command = "AUTHINFO" WS "USER" WS username
authinfo-pass-command = "AUTHINFO" WS "PASS" WS password
initial-response = base64-opt
username = 1*user-pass-char
password = 1*user-pass-char
user-pass-char = B-CHAR
NOTE: A server implementation MAY parse AUTHINFO USER and AUTHINFO
PASS specially as to allow white space to be used within the
username or password. Such implementations accept the additional
syntax (making these two items inconsistent with "token" in section
9.8 of [NNTP]):
user-pass-char =/ SP / TAB
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In doing so, the grammar can become ambiguous if the username or
password begins or ends with white space. To solve this ambiguity,
such implementations typically treat everything after the first
white space character following "USER"/"PASS", up to, but not
including, the CRLF as the username/password.
3.2. Command Continuation
This syntax extends the non-terminal "command-continuation", which
represents the further material sent by the client in the case of
multi-stage commands.
command-continuation =/ authinfo-sasl-383-continuation
authinfo-sasl-383-continuation = ("*" / base64-opt) CRLF
3.3. Responses
This syntax extends the non-terminal "initial-response-content",
which represents an initial response line sent by the server.
initial-response-content =/ response-283-content /
response-383-content
response-283-content = "283" SP base64
response-383-content = "383" SP base64-opt
3.4. Capability entries
This syntax extends the non-terminal "capability-entry", which rep-
resents a capability that may be advertised by the server.
capability-entry =/ authinfo-capability /
sasl-capability
authinfo-capability = "AUTHINFO" *(WS authinfo-variant)
authinfo-variant = "USER" / "SASL"
sasl-capability = "SASL" 1*(WS mechanism)
3.5. General non-terminals
base64-opt = "=" / base64
mechanism = 1*20mech-char
mech-char = UPPER / DIGIT / "-" / "_"
4. Summary of Response Codes
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This section contains a list of every new response code defined in
this document, whether it is multi-line, which commands can
generate it, what arguments it has, and what its meaning is.
Response code 281
Generated by: AUTHINFO USER, AUTHINFO PASS, AUTHINFO SASL
Meaning: authentication accepted
Response code 283
Generated by: AUTHINFO SASL
1 argument: challenge
Meaning: authentication accepted (with success data)
Response code 381
Generated by: AUTHINFO USER
Meaning: password required via AUTHINFO PASS command. Note
that this code is used for backwards compatibility and does
not conform to the traditional use of 3xx codes.
Response code 383
Generated by: AUTHINFO SASL
1 argument: challenge
Meaning: continue with SASL exchange
Response code 481
Generated by: AUTHINFO USER, AUTHINFO PASS, AUTHINFO SASL
Meaning: authentication failed/rejected
Response code 482
Generated by: AUTHINFO USER, AUTHINFO PASS, AUTHINFO SASL
Meaning: authentication commands issued out of sequence or
SASL protocol error
5. Authentication Tracking/Logging
This section contains implementation suggestions and notes of best
current practice, and does not specify further network protocol
requirements.
Once authenticated, the authorization identity presented in the
AUTHINFO exchange (username when using USER/PASS) SHOULD be
included in an audit trail associating the identity with any
articles supplied during a POST operation, and this configuration
SHOULD be the default. This may be accomplished, for example, by
inserting headers in the posted articles or by a server logging
mechanism. The server MAY provide a facility for disabling the
procedure described above, as some users or administrators may
consider it a violation of privacy.
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6. Security Considerations
Security issues are discussed throughout this memo.
In general, the security considerations of [SASL] and any
implemented SASL mechanisms are applicable here; only the most
important are highlighted specifically below. Also, this extension
is not intended to cure the security considerations described in
section 12 of [NNTP]; those considerations remain relevant to any
NNTP implementation.
Before the [SASL] negotiation has begun, any protocol interactions
may have been performed in the clear and may have been modified by
an active attacker. For this reason, clients and servers MUST
discard any sensitive knowledge obtained prior to the start of the
SASL negotiation upon the establishment of a security layer.
Furthermore, the CAPABILITIES command SHOULD be re-issued upon the
establishment of a security layer, and other protocol state SHOULD
be re-negotiated as well.
Servers MAY implement a policy whereby the connection is dropped
after a number of failed authentication attempts. If they do so,
they SHOULD NOT drop the connection until at least 3 attempts at
authentication have failed.
Implementations MUST support a configuration where authentication
mechanisms that are vulnerable to passive eavesdropping attacks
(such as AUTHINFO USER/PASS and SASL [PLAIN]) are not advertised or
used without the presence of an external security layer such as TLS
[NNTP-TLS].
When multiple authentication mechanisms are permitted by both
client and server, an active attacker can cause a down-negotiation
to the weakest mechanism. For this reason, both clients and
servers SHOULD be configurable to forbid use of weak mechanisms.
The minimum strength acceptable is a policy decision which is
outside the scope of this specification.
7. IANA Considerations
7.1. IANA Considerations for SASL/GSSAPI services
Please register the SASL/GSSAPI service name "nntp". This service
name refers to authenticated use of Usenet news service when
provided via the [NNTP] protocol.
o Published Specification: This document.
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o Contact for Further Information: Authors of this document.
o Change Controller: IESG <iesg@ietf.org>.
7.2. IANA Considerations for NNTP extensions
This section gives a formal definition of the AUTHINFO extension as
required by Section 3.3.3 of [NNTP] for the IANA registry.
o This extension provides an extensible mechanism for NNTP
authentication via a variety of methods.
o The capability label for this extension is "AUTHINFO".
o The "AUTHINFO" capability label has two possible optional
arguments "USER" and "SASL" (as defined in Section 2.1)
indicating which variants of the AUTHINFO command are supported.
o This extension also provides the "SASL" capability label whose
arguments list the available SASL mechanisms.
o This extension defines three new commands, AUTHINFO USER,
AUTHINFO PASS, and AUTHINFO SASL, whose behavior, arguments, and
responses are defined in Sections 2.3 and 2.4.
o This extension does not associate any new responses with pre-
existing NNTP commands.
o This extension may affect the overall behavior of both server
and client, in that the AUTHINFO SASL command may require that
subsequent communication be transmitted via an intermediary
security layer.
o The length of the AUTHINFO SASL command (as defined in this
document) may exceed 512 octets. The maximum length of this
command is increased to that which can accommodate the largest
initial response possible for any of the SASL mechanisms
supported by the implementation.
o This extension defines two new responses, 283 and 383, whose
lengths may exceed 512 octets. The maximum length of these
responses is increased to that which can accommodate the largest
challenge possible for any of the SASL mechanisms supported by
the implementation.
o This extension does not alter pipelining, but AUTHINFO commands
cannot be pipelined.
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o Use of this extension may alter the capabilities list; once the
AUTHINFO command has been used successfully, the AUTHINFO
capability can no longer be advertised by CAPABILITIES.
Additionally, the MODE-READER capability MUST NOT be advertised
after successful authentication.
o This extension does not cause any pre-existing command to
produce a 401, 480, or 483 response.
o This extension is unaffected by any use of the MODE READER
command, however the MODE READER command MUST NOT be used in the
same session following successful authentication.
o Published Specification: This document.
o Author, Change Controller, and Contact for Further Information:
Author of this document.
8. References
8.1. Normative References
[ABNF] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[AUTH] Haller, N., Atkinson, R., "On Internet Authentication",
RFC 1704, October 1994.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[DIGEST-MD5] Leach, P., Newman, C., Melnikov, A.,
"Using Digest Authentication as a SASL Mechanism",
draft-ietf-sasl-rfc2831bis-*, Work in Progress.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[NNTP] Feather, C., "Network News Transport Protocol",
draft-ietf-nntpext-base-*, Work in Progress.
[NNTP-TLS] Murchison, K., Vinocur, J., Newman, C.,
"Using TLS with NNTP", draft-ietf-nntpext-tls-nntp-*,
Work in Progress.
[SASL] Melnikov, A. and Zeilenga, K., "Simple Authentication and
Security Layer (SASL)", draft-ietf-sasl-rfc2222bis-*,
Work in Progress.
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[SASLprep] Zeilega, K., "SASLprep: Stringprep Profile for
User Names and Passwords", RFC 4013, February 2005.
[StringPrep] Hoffman, P. and Blanchet, M., "Preparation of
Internationalized Strings ("stringprep")",
RFC 3454, December 2002.
8.2. Informative References
[CRAM-MD5] Nerenberg, L., "The CRAM-MD5 SASL Mechanism", draft-
ietf-sasl-crammd5-*, Work in Progress.
[GSSAPI] Melnikov, A., "SASL GSSAPI mechanisms", draft-ietf-sasl-
gssapi-*, Work in Progress.
[NNTP-COMMON] Barber, S., "Common NNTP Extensions", RFC 2980,
October 2000.
[PLAIN] Zeilenga, K., "The PLAIN SASL Mechanism", draft-ietf-sasl-
plain-*, Work in Progress.
[SMTP] Klensin, J., "Simple Mail Transport Protocol", RFC 2821,
April 2001.
9. Authors' Addresses
Jeffrey M. Vinocur
Department of Computer Science
Upson Hall
Cornell University
Ithaca, NY 14853 USA
Email: vinocur@cs.cornell.edu
Kenneth Murchison
Oceana Matrix Ltd.
2495 Main St, Suite 401
Buffalo, NY 14214 USA
Email: ken@oceana.com
Chris Newman
Sun Microsystems
1050 Lakes Drive, Suite 250
West Covina, CA 91790 USA
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Email: Chris.Newman@sun.com
10. Acknowledgments
A significant amount of the authentication text was originally from
the NNTP revision or common authentication specs written by Stan
Barber. A significant amount of the SASL text was lifted from the
revisions to RFC 1734 and RFC 2554 by Rob Siemborski.
Special acknowledgment also goes to Russ Allbery, Clive Feather,
and others who commented privately on intermediate revisions of
this document, as well as the members of the IETF NNTP Working
Group for continual (yet sporadic) insight in discussion.
11. Intellectual Property Rights
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
12. Copyright
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 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
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REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET 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 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
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