Last was 9.5
Changes since OpenSSH 9.5
=========================
This release contains a number of security fixes, some small features
and bugfixes.
Security
========
This release contains fixes for a newly-discovered weakness in the
SSH transport protocol, a logic error relating to constrained PKCS#11
keys in ssh-agent(1) and countermeasures for programs that invoke
ssh(1) with user or hostnames containing invalid characters.
* ssh(1), sshd(8): implement protocol extensions to thwart the
so-called "Terrapin attack" discovered by Fabian Bäumer, Marcus
Brinkmann and Jörg Schwenk. This attack allows a MITM to effect a
limited break of the integrity of the early encrypted SSH transport
protocol by sending extra messages prior to the commencement of
encryption, and deleting an equal number of consecutive messages
immediately after encryption starts. A peer SSH client/server
would not be able to detect that messages were deleted.
While cryptographically novel, the security impact of this attack
is fortunately very limited as it only allows deletion of
consecutive messages, and deleting most messages at this stage of
the protocol prevents user user authentication from proceeding and
results in a stuck connection.
The most serious identified impact is that it lets a MITM to
delete the SSH2_MSG_EXT_INFO message sent before authentication
starts, allowing the attacker to disable a subset of the keystroke
timing obfuscation features introduced in OpenSSH 9.5. There is no
other discernable impact to session secrecy or session integrity.
OpenSSH 9.6 addresses this protocol weakness through a new "strict
KEX" protocol extension that will be automatically enabled when
both the client and server support it. This extension makes
two changes to the SSH transport protocol to improve the integrity
of the initial key exchange.
Firstly, it requires endpoints to terminate the connection if any
unnecessary or unexpected message is received during key exchange
(including messages that were previously legal but not strictly
required like SSH2_MSG_DEBUG). This removes most malleability from
the early protocol.
Secondly, it resets the Message Authentication Code counter at the
conclusion of each key exchange, preventing previously inserted
messages from being able to make persistent changes to the
sequence number across completion of a key exchange. Either of
these changes should be sufficient to thwart the Terrapin Attack.
More details of these changes are in the PROTOCOL file in the
OpenSSH source distribition.
* ssh-agent(1): when adding PKCS#11-hosted private keys while
specifying destination constraints, if the PKCS#11 token returned
multiple keys then only the first key had the constraints applied.
Use of regular private keys, FIDO tokens and unconstrained keys
are unaffected.
* ssh(1): if an invalid user or hostname that contained shell
metacharacters was passed to ssh(1), and a ProxyCommand,
LocalCommand directive or "match exec" predicate referenced the
user or hostname via %u, %h or similar expansion token, then
an attacker who could supply arbitrary user/hostnames to ssh(1)
could potentially perform command injection depending on what
quoting was present in the user-supplied ssh_config(5) directive.
This situation could arise in the case of git submodules, where
a repository could contain a submodule with shell characters in
its user/hostname. Git does not ban shell metacharacters in user
or host names when checking out repositories from untrusted
sources.
Although we believe it is the user's responsibility to ensure
validity of arguments passed to ssh(1), especially across a
security boundary such as the git example above, OpenSSH 9.6 now
bans most shell metacharacters from user and hostnames supplied
via the command-line. This countermeasure is not guaranteed to be
effective in all situations, as it is infeasible for ssh(1) to
universally filter shell metacharacters potentially relevant to
user-supplied commands.
User/hostnames provided via ssh_config(5) are not subject to these
restrictions, allowing configurations that use strange names to
continue to be used, under the assumption that the user knows what
they are doing in their own configuration files.
Potentially incompatible changes
--------------------------------
* ssh(1), sshd(8): the RFC4254 connection/channels protocol provides
a TCP-like window mechanism that limits the amount of data that
can be sent without acceptance from the peer. In cases where this
limit was exceeded by a non-conforming peer SSH implementation,
ssh(1)/sshd(8) previously discarded the extra data. From OpenSSH
9.6, ssh(1)/sshd(8) will now terminate the connection if a peer
exceeds the window limit by more than a small grace factor. This
change should have no effect of SSH implementations that follow
the specification.
New features
------------
* ssh(1): add a %j token that expands to the configured ProxyJump
hostname (or the empty string if this option is not being used)
that can be used in a number of ssh_config(5) keywords. bz3610
* ssh(1): add ChannelTimeout support to the client, mirroring the
same option in the server and allowing ssh(1) to terminate
quiescent channels.
* ssh(1), sshd(8), ssh-add(1), ssh-keygen(1): add support for
reading ED25519 private keys in PEM PKCS8 format. Previously
only the OpenSSH private key format was supported.
* ssh(1), sshd(8): introduce a protocol extension to allow
renegotiation of acceptable signature algorithms for public key
authentication after the server has learned the username being
used for authentication. This allows varying sshd_config(5)
PubkeyAcceptedAlgorithms in a "Match user" block.
* ssh-add(1), ssh-agent(1): add an agent protocol extension to allow
specifying certificates when loading PKCS#11 keys. This allows the
use of certificates backed by PKCS#11 private keys in all OpenSSH
tools that support ssh-agent(1). Previously only ssh(1) supported
this use-case.
Bugfixes
--------
* ssh(1): when deciding whether to enable the keystroke timing
obfuscation, enable it only if a channel with a TTY is active.
* ssh(1): switch mainloop from poll(3) to ppoll(3) and mask signals
before checking flags set in signal handler. Avoids potential
race condition between signaling ssh to exit and polling. bz3531
* ssh(1): when connecting to a destination with both the
AddressFamily and CanonicalizeHostname directives in use,
the AddressFamily directive could be ignored. bz5326
* sftp(1): correct handling of the limits@openssh.com option when
the server returned an unexpected message.
* A number of fixes to the PuTTY and Dropbear regress/integration
tests.
* ssh(1): release GSS OIDs only at end of authentication, avoiding
unnecessary init/cleanup cycles. bz2982
* ssh_config(5): mention "none" is a valid argument to IdentityFile
in the manual. bz3080
* scp(1): improved debugging for paths from the server rejected for
not matching the client's glob(3) pattern in old SCP/RCP protocol
mode.
* ssh-agent(1): refuse signing operations on destination-constrained
keys if a previous session-bind operation has failed. This may
prevent a fail-open situation in future if a user uses a mismatched
ssh(1) client and ssh-agent(1) where the client supports a key type
that the agent does not support.
Portability
-----------
* Better identify unsupported and unstable compiler flags, such as
-fzero-call-used-regs which has been unstable across a several
clang releases.
* A number of fixes to regression test reliability and log
collection.
* Update the OpenSSL dependency in the RPM specification.
* sshd(8): for OpenSolaris systems that support privilege limitation
via the getpflags() interface, prefer using the newer PRIV_XPOLICY
to PRIV_LIMIT. bz2833
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