Internet DRAFT - draft-ietf-curdle-ssh-ext-info
draft-ietf-curdle-ssh-ext-info
Internet-Draft D. Bider
Updates: 4252, 4253, 4254 (if approved) Bitvise Limited
Intended status: Standards Track September 23, 2017
Expires: March 23, 2018
Extension Negotiation in Secure Shell (SSH)
draft-ietf-curdle-ssh-ext-info-15.txt
Abstract
This memo updates RFC 4252, RFC 4253, and RFC 4254 to define a
mechanism for SSH clients and servers to exchange information about
supported protocol extensions confidentially after SSH key exchange.
Status
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Overview and Rationale ...........................................3
1.1. Requirements Terminology ...................................3
1.2. Wire Encoding Terminology ..................................3
2. Extension Negotiation Mechanism ..................................3
2.1. Signaling of Extension Negotiation in SSH_MSG_KEXINIT .......3
2.2. Enabling Criteria ...........................................4
2.3. SSH_MSG_EXT_INFO Message ....................................4
2.4. Message Order ...............................................4
2.5. Interpretation of Extension Names and Values ................5
3. Initially Defined Extensions .....................................6
3.1. "server-sig-algs" ...........................................6
3.2. "delay-compression" .........................................7
3.2.1. Awkwardly Timed Key Re-Exchange ......................8
3.2.2. Subsequent Re-Exchange ...............................8
3.2.3. Compatibility Note: OpenSSH up to 7.5 ................8
3.3. "no-flow-control" ...........................................9
3.3.1. Prior "No Flow Control" Practice .....................9
3.4. "elevation" ................................................10
4. IANA Considerations .............................................11
4.1. Additions to existing tables ...............................11
4.2. New table: Extension Names .................................11
4.2.1. Future Assignments to Extension Names ...............11
5. Security Considerations .........................................11
6. References ......................................................12
6.1. Normative References .......................................12
6.2. Informative References .....................................12
Author's Address ...................................................13
Acknowledgments ....................................................13
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1. Overview and Rationale
Secure Shell (SSH) is a common protocol for secure communication on
the Internet. The original design of the SSH transport layer [RFC4253]
lacks proper extension negotiation. Meanwhile, diverse implementations
take steps to ensure that known message types contain no unrecognized
information. This makes it difficult for implementations to signal
capabilities and negotiate extensions without risking disconnection.
This obstacle has been recognized in relationship with [SSH-RSA-SHA2],
where the need arises for a client to discover public key algorithms a
server accepts, to avoid authentication penalties and trial-and-error.
This memo updates RFC 4252, RFC 4253, and RFC 4254.
1.1. Requirements Terminology
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].
1.2. Wire Encoding Terminology
The wire encoding types in this document - "byte", "uint32", "string",
"boolean", "name-list" - have meanings as described in [RFC4251].
2. Extension Negotiation Mechanism
2.1. Signaling of Extension Negotiation in SSH_MSG_KEXINIT
Applications implementing this mechanism MUST add one of the following
indicator names to the field "kex_algorithms" in the SSH_MSG_KEXINIT
message sent by the application in the first key exchange:
- When acting as server: "ext-info-s"
- When acting as client: "ext-info-c"
The indicator name is added without quotes, and MAY be added at any
position in the name-list, subject to proper separation from other
names as per name-list conventions.
The names are added to the "kex_algorithms" field because this is one
of two name-list fields in SSH_MSG_KEXINIT that do not have a separate
copy for each data direction.
The indicator names inserted by the client and server are different to
ensure these names will not produce a match, and therefore not affect
the algorithm chosen in key exchange algorithm negotiation.
The inclusion of textual indicator names is intended to provide a clue
for implementers to discover this mechanism.
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2.2. Enabling Criteria
If a client or server offers "ext-info-c" or "ext-info-s"
respectively, it MUST be prepared to accept an SSH_MSG_EXT_INFO
message from the peer.
A server only needs to send "ext-info-s" if it intends to process
SSH_MSG_EXT_INFO from the client. A client only needs to send
"ext-info-c" if it plans to process SSH_MSG_EXT_INFO from the server.
If a server receives an "ext-info-c", or a client receives an
"ext-info-s", it MAY send an SSH_MSG_EXT_INFO message, but is not
required to do so.
Neither party needs to wait for the other's SSH_MSG_KEXINIT in order
to decide whether to send the appropriate indicator in its own
SSH_MSG_KEXINIT.
Implementations MUST NOT send an incorrect indicator name for their
role. Implementations MAY disconnect if the counter-party sends an
incorrect indicator. If "ext-info-c" or "ext-info-s" ends up being
negotiated as a key exchange method, the parties MUST disconnect.
2.3. SSH_MSG_EXT_INFO Message
A party that received the "ext-info-c" or "ext-info-s" indicator
MAY send the following message:
byte SSH_MSG_EXT_INFO (value 7)
uint32 nr-extensions
repeat the following 2 fields "nr-extensions" times:
string extension-name
string extension-value (binary)
Implementers' attention is called to Section 2.5., in particular the
requirement to tolerate any sequence of bytes - including null bytes
at any position - in an unknown extension's extension-value.
2.4. Message Order
If a client sends SSH_MSG_EXT_INFO, it MUST send it as the next packet
following the client's first SSH_MSG_NEWKEYS message to the server.
If a server sends SSH_MSG_EXT_INFO, it MAY send it at zero, one, or
both of the following opportunities:
- As the next packet following the server's first SSH_MSG_NEWKEYS.
Where clients need information in the server's SSH_MSG_EXT_INFO to
authenticate, it is helpful if the server sends its SSH_MSG_EXT_INFO
not only as next packet after SSH_MSG_NEWKEYS, but without delay.
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Clients cannot rely on this because the server is not required to
send the message at this time; and if sent, it may be delayed by
the network. However, if a timely SSH_MSG_EXT_INFO is received,
a client can pipeline an authentication request after its
SSH_MSG_SERVICE_REQUEST, even when it needs extension information.
- Immediately preceding the server's SSH_MSG_USERAUTH_SUCCESS, as
defined in [RFC4252].
The server MAY send SSH_MSG_EXT_INFO at this second opportunity,
whether or not it sent it at the first. A client that sent
"ext-info-c" MUST accept a server's SSH_MSG_EXT_INFO at both
opportunities, but MUST NOT require it.
This allows a server to reveal support for additional extensions
that it was unwilling to reveal to an unauthenticated client. If a
server sends a second SSH_MSG_EXT_INFO, this replaces any initial
one, and both the client and the server re-evaluate extensions in
effect. The server's second SSH_MSG_EXT_INFO is matched against the
client's original.
The timing of the second opportunity is chosen for the following
reasons. If the message was sent earlier, it would not allow the
server to withhold information until the client has authenticated.
If it was sent later, a client that needs information from the
second SSH_MSG_EXT_INFO immediately after it authenticates would
have no way to reliably know whether to expect the message.
2.5. Interpretation of Extension Names and Values
Each extension is identified by its extension-name, and defines the
conditions under which the extension is considered to be in effect.
Applications MUST ignore unrecognized extension-names.
An extension MAY dictate, where it is specified, that in order to take
effect, both parties must include it in their SSH_MSG_EXT_INFO; or it
can be sufficient that only one party includes it; or other rules MAY
be specified. The relative order in which extensions appear in an
SSH_MSG_EXT_INFO message MUST be ignored.
Extension-value fields are interpreted as defined by their respective
extension. This field MAY be empty if permitted by the extension.
Applications that do not implement or recognize an extension MUST
ignore its extension-value, regardless of its size or content.
Applications MUST tolerate any sequence of bytes - including null
bytes at any position - in an unknown extension's extension-value.
The cumulative size of an SSH_MSG_EXT_INFO message is limited only by
the maximum packet length that an implementation may apply in
accordance with [RFC4253]. Implementations MUST accept well-formed
SSH_MSG_EXT_INFO messages up to the maximum packet length they accept.
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3. Initially Defined Extensions
3.1. "server-sig-algs"
This extension is sent with the following extension name and value:
string "server-sig-algs"
name-list public-key-algorithms-accepted
The name-list type is a strict subset of the string type, and is thus
permissible as an extension-value. See [RFC4251] for more information.
This extension is sent by the server, and contains a list of public
key algorithms that the server is able to process as part of a
"publickey" authentication request. If a client sends this extension,
the server MAY ignore it, and MAY disconnect.
In this extension, a server MUST enumerate all public key algorithms
it might accept during user authentication. However, there exist early
server implementations which do not enumerate all accepted algorithms.
For this reason, a client MAY send a user authentication request using
a public key algorithm not included in "server-sig-algs".
A client that wishes to proceed with public key authentication MAY
wait for the server's SSH_MSG_EXT_INFO so it can send a "publickey"
authentication request with an appropriate public key algorithm,
rather than resorting to trial and error.
Servers that implement public key authentication SHOULD implement this
extension.
If a server does not send this extension, a client MUST NOT make any
assumptions about the server's public key algorithm support, and MAY
proceed with authentication requests using trial and error. Note that
implementations are known to exist that apply authentication
penalties (*) if the client attempts to use an unexpected public key
algorithm.
(*) Authentication penalties are applied by servers to deter brute
force password guessing, username enumeration, and other types of
behavior deemed suspicious by server administrators or implementers.
Penalties may include automatic IP address throttling or blocking,
and may trigger email alerts or auditing.
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3.2. "delay-compression"
This extension MAY be sent by both parties as follows:
string "delay-compression"
string:
name-list compression_algorithms_client_to_server
name-list compression_algorithms_server_to_client
The extension-value is a string that encodes two name-lists. The
name-lists themselves have the encoding of strings. For example: to
indicate a preference for algorithms "foo,bar" in the client-to-server
direction, and "bar,baz" in the server-to-client direction, a sender
encodes the extension-value as follows (including its length):
00000016 00000007 666f6f2c626172 00000007 6261722c62617a
This same encoding could be sent by either party - client or server.
This extension allows the server and client to renegotiate compression
algorithm support without having to conduct a key re-exchange, putting
new algorithms into effect immediately upon successful authentication.
This extension takes effect only if both parties send it. Name-lists
MAY include any compression algorithm that could have been negotiated
in SSH_MSG_KEXINIT, except algorithms that define their own delayed
compression semantics. This means "zlib,none" is a valid algorithm
list in this context; but "zlib@openssh.com" is not.
If both parties send this extension, but the name-lists do not contain
a common algorithm in either direction, the parties MUST disconnect in
the same way as if negotiation failed as part of SSH_MSG_KEXINIT.
If this extension takes effect, the renegotiated compression algorithm
is activated for the very next SSH message after the trigger message:
- Sent by the server, the trigger message is SSH_MSG_USERAUTH_SUCCESS.
- Sent by the client, the trigger message is SSH_MSG_NEWCOMPRESS.
If this extension takes effect, the client MUST send the following
message within a reasonable number of outgoing SSH messages after
receiving SSH_MSG_USERAUTH_SUCCESS - but not necessarily as the first
such outgoing message:
byte SSH_MSG_NEWCOMPRESS (value 8)
The purpose of SSH_MSG_NEWCOMPRESS is to avoid a race condition
where the server cannot reliably know whether a message sent by
the client was sent before or after receiving the server's
SSH_MSG_USERAUTH_SUCCESS. For example, clients may send keep-alive
messages during logon processing.
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As is the case for all extensions unless otherwise noted, the server
MAY delay including this extension until its secondary
SSH_MSG_EXT_INFO, sent before SSH_MSG_USERAUTH_SUCCESS. This allows
the server to avoid advertising compression until the client has
authenticated.
If the parties re-negotiate compression using this extension in a
session where compression is already enabled; and the re-negotiated
algorithm is the same in one or both directions; then the internal
compression state MUST be reset for each direction at the time the
re-negotiated algorithm takes effect.
3.2.1. Awkwardly Timed Key Re-Exchange
A party that has signaled, or intends to signal, support for this
extension in an SSH session, MUST NOT initiate key re-exchange in that
session until either of the following occurs:
- This extension was negotiated, and the party that's about to start
key re-exchange already sent its trigger message for compression.
- The party has sent (if server) or received (if client) the message
SSH_MSG_USERAUTH_SUCCESS, and this extension was not negotiated.
If a party violates this rule, the other party MAY disconnect.
In general, parties SHOULD NOT start key re-exchange before successful
user authentication, but MAY tolerate it if not using this extension.
3.2.2. Subsequent Re-Exchange
In subsequent key re-exchanges that unambiguously begin after the
compression trigger messages, the compression algorithms negotiated in
re-exchange override the algorithms negotiated with this extension.
3.2.3. Compatibility Note: OpenSSH up to 7.5
This extension uses a binary extension-value encoding. OpenSSH clients
up to and including version 7.5 advertise support to receive
SSH_MSG_EXT_INFO, but disconnect on receipt of an extension-value
containing null bytes. This is an error fixed in OpenSSH version 7.6.
Implementations that wish to interoperate with OpenSSH 7.5 and earlier
are advised to check the remote party's SSH version string, and omit
this extension if an affected version is detected. Affected versions
do not implement this extension, so there is no harm in omitting it.
The extension SHOULD NOT be omitted if the detected OpenSSH version is
7.6 or higher. This would make it harder for the OpenSSH project to
implement this extension in a higher version.
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3.3. "no-flow-control"
This extension is sent with the following extension name and value:
string "no-flow-control"
string choice of: "p" for preferred | "s" for supported
A party SHOULD send "s" if it supports "no-flow-control", but does not
prefer to enable it. A party SHOULD send "p" if it prefers to enable
the extension if the other party supports it. Parties MAY disconnect
if they receive a different extension value.
To take effect, this extension MUST be:
- Sent by both parties.
- At least one party MUST have sent the value "p" (preferred).
If this extension takes effect, the "initial window size" fields in
SSH_MSG_CHANNEL_OPEN and SSH_MSG_CHANNEL_OPEN_CONFIRMATION, as defined
in [RFC4254], become meaningless. The values of these fields MUST be
ignored, and a channel behaves as if all window sizes are infinite.
Neither side is required to send any SSH_MSG_CHANNEL_WINDOW_ADJUST
messages, and if received, such messages MUST be ignored.
This extension is intended, but not limited to, use by file transfer
applications that are only going to use one channel, and for which the
flow control provided by SSH is an impediment, rather than a feature.
Implementations MUST refuse to open more than one simultaneous channel
when this extension is in effect. Nevertheless, server implementations
SHOULD support clients opening more than one non-simultaneous channel.
3.3.1. Prior "No Flow Control" Practice
Before this extension, some applications would simply not implement
SSH flow control, sending an initial channel window size of 2^32 - 1.
Applications SHOULD NOT do this for the following reasons:
- It is plausible to transfer more than 2^32 bytes over a channel.
Such a channel will hang if the other party implements SSH flow
control according to [RFC4254].
- There exist implementations which cannot handle large channel window
sizes, and can exhibit non-graceful behaviors, including disconnect.
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3.4. "elevation"
The terms "elevation" and "elevated" refer to an operating system
mechanism where an administrator user's logon session is associated
with two security contexts: one limited, and one with administrative
rights. To "elevate" such a session is to activate the security
context with full administrative rights. For more information about
this mechanism on Windows, see also [WINADMIN] and [WINTOKEN].
This extension MAY be sent by the client as follows:
string "elevation"
string choice of: "y" | "n" | "d"
A client sends "y" to indicate its preference that the session should
be elevated; "n" to not be elevated; and "d" for the server to use its
default behavior. The server MAY disconnect if it receives a different
extension value. If a client does not send the "elevation" extension,
the server SHOULD act as if "d" was sent.
If a client has included this extension, then after authentication, a
server that supports this extension SHOULD indicate to the client
whether elevation was done by sending the following global request:
byte SSH_MSG_GLOBAL_REQUEST
string "elevation"
boolean want reply = false
boolean elevation performed
Clients that implement this extension help reduce attack surface for
Windows servers that handle administrative logins. Where clients do
not support this extension, servers must elevate sessions to allow
full access by administrative users always. Where clients support this
extension, sessions can be created without elevation unless requested.
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4. IANA Considerations
4.1. Additions to existing tables
IANA is requested to insert the following entries into the table
Message Numbers [IANA-M] under Secure Shell (SSH) Protocol Parameters
[RFC4250]:
Value Message ID Reference
7 SSH_MSG_EXT_INFO [this document]
8 SSH_MSG_NEWCOMPRESS [this document]
IANA is requested to insert the following entries into the table Key
Exchange Method Names [IANA-KE]:
Method Name Reference Note
ext-info-s [this document] Section 2.2
ext-info-c [this document] Section 2.2
4.2. New table: Extension Names
Also under Secure Shell (SSH) Protocol Parameters, IANA is requested
to create a new table, Extension Names, with initial content:
Extension Name Reference Note
server-sig-algs [this document] Section 3.1
delay-compression [this document] Section 3.2
no-flow-control [this document] Section 3.3
elevation [this document] Section 3.4
4.2.1. Future Assignments to Extension Names
Names in the Extension Names table MUST follow the Conventions for
Names defined in [RFC4250], Section 4.6.1.
Requests for assignments of new non-local names in the Extension Names
table (i.e. names not including the '@' character) MUST be done
through the IETF CONSENSUS method, as described in [RFC8126].
5. Security Considerations
Security considerations are discussed throughout this document. This
document updates the SSH protocol as defined in [RFC4251] and related
documents. The security considerations of [RFC4251] apply.
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6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, January 2006.
[RFC4251] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[RFC4254] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Connection Protocol", RFC 4254, January 2006.
[RFC8126] Cotton, M., Leiba, B. and Narten, T., "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, June 2017.
6.2. Informative References
[SSH-RSA-SHA2]
Bider, D., "Use of RSA Keys with SHA-2 256 and 512 in
Secure Shell (SSH)", draft-ietf-curdle-rsa-sha2-10.txt,
August 2017, <https://tools.ietf.org/html/
draft-ietf-curdle-rsa-sha2-10>.
[IANA-M] "Secure Shell (SSH) Protocol Parameters",
<https://www.iana.org/assignments/ssh-parameters/
ssh-parameters.xhtml#ssh-parameters-1>.
[IANA-KE] "Secure Shell (SSH) Protocol Parameters",
<https://www.iana.org/assignments/ssh-parameters/
ssh-parameters.xhtml#ssh-parameters-16>.
[WINADMIN] "How to launch a process as a Full Administrator when UAC
is enabled?", <https://blogs.msdn.microsoft.com/winsdk/
2013/03/22/how-to-launch-a-process-as-a-full-
administrator-when-uac-is-enabled/>.
[WINTOKEN] "TOKEN_ELEVATION_TYPE enumeration",
<https://msdn.microsoft.com/en-us/library/windows/desktop/
bb530718.aspx>.
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Author's Address
Denis Bider
Bitvise Limited
4105 Lombardy Court
Colleyville, Texas 76034
United States of America
EMail: ietf-ssh3@denisbider.com
URI: https://www.bitvise.com/
Acknowledgments
Thanks to Markus Friedl and Damien Miller for comments and initial
implementation. Thanks to Peter Gutmann, Roumen Petrov, Mark D.
Baushke, Daniel Migault, Eric Rescorla, Matthew A. Miller, Mirja
Kuehlewind, Adam Roach, Spencer Dawkins, Alexey Melnikov, and Ben
Campbell for reviews and feedback.
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