Internet Engineering Task Force (IETF) D. Bider
Request for Comments: 8308 Bitvise Limited
Updates: 4251, 4252, 4253, 4254 March 2018
Category: Standards Track
Extension Negotiation in the Secure Shell (SSH) Protocol
This memo updates RFCs 4251, 4252, 4253, and 4254 by defining a
mechanism for Secure Shell (SSH) clients and servers to exchange
information about supported protocol extensions confidentially after
SSH key exchange.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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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
the process of updating SSH to support RSA signatures using SHA-256
and SHA-512 [RFC8332]. To avoid trial and error as well as
authentication penalties, a client must be able to discover public
key algorithms a server accepts. This extension mechanism permits
This memo updates RFCs 4251, 4252, 4253, and 4254.
1.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Wire Encoding Terminology
The wire encoding types in this document -- "byte", "uint32",
"string", "boolean", "name-list" -- have meanings as described in
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
o When acting as server: "ext-info-s"
o 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.
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
Implementations MUST NOT send an incorrect indicator name for their
role. Implementations MAY disconnect if the counterparty 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)
repeat the following 2 fields "nr-extensions" times:
string extension-value (binary)
Implementers should pay careful attention to Section 2.5, in
particular to the requirement to tolerate any sequence of bytes
(including null bytes at any position) in an unknown extension's
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
If a server sends SSH_MSG_EXT_INFO, it MAY send it at zero, one, or
both of the following opportunities:
o 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 the next packet after
SSH_MSG_NEWKEYS, but without delay.
Clients cannot rely on this because the server is not required to
send the message at this time; 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.
o 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.
When it is specified, an extension MAY dictate that, in order to take
effect, both parties must include it in their SSH_MSG_EXT_INFO or
that it is sufficient for only one party to include it. However,
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
3. Initially Defined Extensions
This extension is sent with the following extension name and value:
The name-list type is a strict subset of the string type and is thus
permissible as an extension-value. See [RFC4251] for more
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, early server
implementations that do not enumerate all accepted algorithms do
exist. For this reason, a client MAY send a user authentication
request using a public key algorithm not included in "server-sig-
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
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
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 they may trigger email alerts or auditing.
This extension MAY be sent by both parties as follows:
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, which puts new algorithms into effect immediately upon
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 "email@example.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
If this extension takes effect, the renegotiated compression
algorithm is activated for the very next SSH message after the
o Sent by the server, the trigger message is
o 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.
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
If the parties renegotiate compression using this extension in a
session where compression is already enabled and the renegotiated
algorithm is the same in one or both directions, then the internal
compression state MUST be reset for each direction at the time the
renegotiated 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:
o This extension was negotiated, and the party that's about to start
key re-exchange already sent its trigger message for compression.
o 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
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
3.2.3. Compatibility Note: OpenSSH up to Version 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
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.
This extension is sent with the following extension name and value:
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.
For this extension to take effect, the following must occur:
o This extension MUST be sent by both parties.
o 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 for, 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
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
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:
o 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].
o Implementations that cannot handle large channel window sizes
exist, and they can exhibit non-graceful behaviors, including
The terms "elevation" and "elevated" refer to an operating system
mechanism where an administrator'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 [WINADMIN] and [WINTOKEN].
This extension MAY be sent by the client as follows:
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:
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
4. IANA Considerations
4.1. Additions to Existing Registries
IANA has added the following entries to the "Message Numbers"
registry [IANA-M] under the "Secure Shell (SSH) Protocol Parameters"
Value Message ID Reference
7 SSH_MSG_EXT_INFO RFC 8308
8 SSH_MSG_NEWCOMPRESS RFC 8308
IANA has also added the following entries to the "Key Exchange Method
Names" registry [IANA-KE]:
Method Name Reference Note
ext-info-s RFC 8308 Section 2
ext-info-c RFC 8308 Section 24.2. New Registry: Extension Names
Also under the "Secure Shell (SSH) Protocol Parameters" registry,
IANA has created a new "Extension Names" registry, with the following
Extension Name Reference Note
server-sig-algs RFC 8308 Section 3.1
delay-compression RFC 8308 Section 3.2
no-flow-control RFC 8308 Section 3.3
elevation RFC 8308 Section 184.108.40.206. Future Assignments to Extension Names Registry
Names in the "Extension Names" registry 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" registry (i.e., names not including the '@' character) MUST be
done using the IETF Review policy, 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.