Announce: OpenSSH 10.6 released

[Damien Miller <djm () cvs openbsd org>]

OpenSSH 10.6 has just been released. It will be available from the
mirrors listed at https://www.openssh.com/ shortly.

OpenSSH is a 100% complete SSH protocol 2.0 implementation and
includes sftp client and server support.

Recently the OpenSSH team have received a large number of security
bug reports, many of which are findings from AI models or made with
AI assistance. While many AI reports are determined not to have
security impact when considered in the context of a realistic
threat model, we very much welcome these reports, especially when
combined with human triage, analysis, test-cases and particularly
when accompanied by proposed fixes.

** We have seen a number of cases where a security bug identified
** by AI tools is subsequently independently discovered by a
** different researcher. This suggests that adversaries who do not
** report bugs to OSS projects are likely to be able to discover
** these bugs too. Given this, the OpenSSH team will, for now, be
** making more frequent releases to get bugfixes into users' hands
** more quickly rather than batching them until the next planned
** release.

Once again, we would like to thank the OpenSSH community for their
continued support of the project, especially those who contributed
code or patches, reported bugs, tested snapshots or donated to the
project. More information on donations may be found at:
https://www.openssh.com/donations.html

Future deprecation notice
-------------------------

 * scp(1): begin deprecating the -R flag, which is used to perform a
   remote-to-remote copy by executing scp on a remote host. This
   option is a fragile optimisation that is difficult to use because
   it requires credentials on the remote host. It also creates
   security risks if the shell quoting rules on the remote system
   where the copy is performed differ from the client's expectations.

   From OpenSSH 10.6, this option will continue to work but will
   cause a deprecation warning to be emitted to standard error. In
   a future release, the option will be ignored and will leave in
   place the default remote-to-remote copy behaviour (copy via the
   host running scp).

 * sshd(8): support for platforms that do not allow file descriptor
   passing and that also require root privilege for PTY allocation
   will be removed in a future release. Affected platforms are known
   to include SCO OpenServer 5 and QNX 6 but may include other
   similarly old operating systems. This deprecation can be avoided
   if the user community for these platforms is able to assist us in
   building alternatives, such as avoiding the need for root in PTY
   allocation.

Potentially-incompatible changes
--------------------------------

 * ssh(1), sshd(8): compression will be less effective as a result
   of the change noted below in the "Security" section

 * ssh(1): destination usernames entered on the commandline are now
   more stricly checked and will refuse usernames that include
   backslash and dollar symbols. Usernames that are specified by
   the "User" directive in configuration files are no subject to
   these restrictions. This motivation for this change is mentioned
   below in the "Security" section.

 * sshd(8): on platforms that do not support file descriptor passing
   and that require root for PTY allocation, the GatewayPorts and
   StreamLocalForwarding options are forcibly disabled. The
   motivation for this changes is discussed below in the "Security"
   section.

Changes since OpenSSH 10.5
==========================

This release contains a number of security fixes, several new
features and some small bugfixes.

Security
========

 * sftp(1): more strictly validate paths returned from the server to
   avoid some cases where a server could return paths that could
   manipulate a recursive copy operation into writing outside its
   target directory. Report and patch from Junghoon Cho.

 * sshd(8): when GSSAPIAuthentication is in use, only store GSSAPI
   credentials when authentication has succeeded. Avoids a situation
   where credentials from a failed GSSAPIAuthentication attempt may
   persist and be made inappropriately available if another
   authentication subsequently succeeds. Issue report and patch from
   Moritz Theile.

 * sshd(8): reset GSSAPIAuthentication before authentication, avoiding
   state from one authentication attempt being confused with that of
   a later attempt. Report and feedback from Moritz Theile.

 * sshd(8), ssh(1): disable LZ77 dictionary coder to mitigate the
   side-channel leaks described in "Crossing the Streams: SSH
   Plaintext Recovery via a Common Compression Context in
   Multiplexed Channels" by Fabian Bäumer and Marcus Brinkmann,
   preprint https://arxiv.org/abs/2609.07709 (2026)

   A chosen-plaintext attack method exists which makes use of
   dictionary-based compression to recover secrets from one channel
   by interacting with the SSH session's shared compression
   dictionary through another channel.

   Attacker-controlled input can recognizably reflect into the
   total length of transmitted ciphertexts by virtue of LZ77
   replacing repeated strings with back-references into the SSH
   session's encoder search buffer, which is shared across all
   channels. For this reason, the documentation already recommended
   against enabling compression for connections that share trusted
   and untrusted traffic.

   This change will reduce the effectiveness of the Compression
   option. Users are encouraged to use application-level compression
   over the SSH protocol where possible, as this will typically be
   more effective and will be completely immune to this type of
   attack.

 * ssh(1): disallow '$' and '\' characters in usernames entered on
   the command-line to avoid usernames from untrusted sources
   yielding injection in shell context via ProxyCommand, Match exec,
   etc. Usernames specified via the configuration files are not
   subject this this control. Reported by SecBuddyF KeenLab Tencent
   (CodeBuddy Security)

   We continue to recommend against directly exposing ssh(1) and
   other tools' command-lines to untrusted input. Mitigations such
   as this can not be absolute given the variety of shells and user
   configurations in use.

 * ssh-keygen(1): correct handling of Daylight Saving Time when
   converting dates. Previous handling could cause errors of
   up to +/- 1 hour (unless you are in the Antarctica/Troll
   timezone, where the error could be +/- 2 hours). These errors
   could result in creation of certificates with incorrect expiry
   times. bz4004; from Khush Patel

 * sshd(8), ssh(1): ensure that compressed payloads don't inflate
   past the maximum supported packet length. Reported by Oleh Konko.

 * sshd(8): fully honor the authorized_keys "restrict" keyword,
   which was not being properly applied to tunnel forwarding
   (PermitTunnel, disabled by default). This is a separate problem
   to the one fixed in openssh-10.5.

 * sshd(8): correctly handle some options that accept "none". Some
   options, including AuthorizedPrincipalsFile, were documented as
   accepting "none" as a way to disable them; however, when
   overridden by an sshd_config(5) Match keyword, this argument was
   being incorrectly interpreted as a literal file.
   With Chris Rohlf in collaboration with Claude and Anthropic Research

 * sshd(8): On OS X SDK >= 27, sandboxing is no longer supported
   as the API we depended upon has been removed and no obvious
   alternative provided.

 * sshd(8): on platforms that do not support file descriptor passing
   and that require root for PTY allocation, the post-authentication
   sshd-session process retains root privilege, whereas on other
   platforms this process runs with the privilege of the logged-in
   user. When sshd-session was run with elevated privlege, it could
   perform certain actions as root and circumvent controls that
   would normally have applied to the user, such as making unix
   domain socket connections or binding (via -R forwarding) to low-
   numbered TCP ports.

   For this reason, this release disables the GatewayPorts and
   StreamLocalForwarding options and support for these (few)
   platforms will be removed in future if no alteratives to
   requiring privilege in the post-authentication process are found.
   Affected platforms include QNX 6, SCO OpenServer 5 and builds
   that were made with the --disable-fd-passing configure option.

   This problem was reported separately by sn0x-sharma and by Dark
   River.

New features
------------

 * All: enable hybrid post-quantum ssh-mldsa44-ed25519 signature
   algorithm. Note that this no longer uses the "@openssh.com"
   vendor extension suffix that the previous experimental
   implementation used. Keys generated with the previous experimental
   support must be regenerated and/or removed.

 * sshd(8): Add the WarnWeakCrypto option to sshd_config(5). This
   option was previously available for the client only. This option
   is enabled by default and will log when the client uses a key
   agreement scheme that is not post-quantum safe.

 * ssh-keygen(1), ssh-add(1): preserve user-verification (PIN or
   biometric) requirement for resident keys loaded from a FIDO
   token, by checking the credential's credProtect policy.
   GHPR701 from Savely Krasovsky

 * ssh(1): include local and remote version strings in the ~I
   connection information display.

 * ssh-add(1): add a -P flag to skip PIN entry for FIDO and PKCS#11
   tokens that do not require it.

 * sftp(1): add '-p' flag for mkdir/lmkdir to create directories as
   required. This flag has similar ergonomics to mkdir(1), and
   previously-existing directories do not cause an error.

 * ssh-keygen(1): add a "hexdump" key export mode that dumps the SSH
   wire-formatted key blob in hex format. Useful when writing
   documentation, tests, etc. E.g. `ssh-keygen -em hexdump -f /key`

 * ssh(1), sshd(8): ChannelTimeout now accepts timeouts with
   fractional seconds.

 * sshd(8): allow specification of the location of $SSH_AUTH_SOCK
   used for agent forwarding using a new AgentSocketPath option.
   This supports both using a user-specific path, such as the
   default of a subdirectory of $HOME ("user:.ssh/agent"), and
   the previous approach of allowing agent forwarding sockets to be
   located in a shared directory (e.g. "shared:/tmp"). Sockets
   created in shared directories will be created inside a
   subdirectory with a randomised name. bz3860

 * ssh-agent(1): allow specification of agent socket directories
   using a -A flag. It accepts "user:" and "shared:" directory
   styles similar to the sshd AgentSocketPath option.

 * sshd(8): account for public key authentication "key ok" tests
   separately to auth attempts. Add a `PubkeyOptions max-pk-ok:nnnn`
   option to allow a number of PK_OK tests (asking whether the
   server might accept a given public key) that do not count against
   MaxAuthTries, defaulting to 6 attempts. After these attempts are
   exhausted, further attempts count as failed authentications
   against MaxAuthTries. Practically, this allows more keys on disk
   or held in ssh-agent to be checked for use before the server
   disconnects.

 * ssh(1), sshd(8): extend the existing TCPKeepAlive option to also
   support setting keepalives on sockets created for forwarding
   connections. Previously this option controlled keepalives on the
   connection socket only. TCPKeepAlive "yes" or "transport" enables
   keepalives on the connection socket. "TCPKeepAlive all" additionally
   enables them for forwarding sockets. bz3921

Bugfixes
--------

 * sshd(8), ssh(1): fix configuration matching on more Turkic
   languages which have disjoint dotted and dotless i/I characters,
   specifically Azerbaijani and Crimean Tatar. bz3991

 * ssh(1), sshd(8): don't attempt to set TCP_NODELAY on non-IP/IPv6
   sockets. Eliminates some noise in debug logs.

 * ssh(1): fix case for ssh -G option output; bz4005

 * ssh(1), sshd(8): Fix ChannelTimeout specificity; previously a
   more specific channel type (e.g. "session:shell") could clobber
   a user-specified ChannelTimeout if it was less specific (e.g.
   "session").  Also, in some cases, the debug messages were
   printing 0 instead of the effective timeout. bz3994

 * sftp(1): avoid NULL dereference crash in some circumstances when
   a server fails a stat/lstat operation. GHPR707

 * sshd(8): close a race condition where a SIGTERM/SIGQUIT would be
   ignored if it was received by the server while it was processing a
   SIGHUP restart request. bz3981

 * sshd(8), ssh(1): check key and CA signature types during key
   parsing against allowlists (PubkeyAcceptedAlgorithms, etc) as
   early as possible. This reduces the attack surface presented by
   disabled algorithms. Suggested by and with extensive feedback
   from Chris Rohlf in collaboration with Claude and Anthropic
   Research.

 * All: switch the fallback implementation of the ed25519 signature
   algorithm used when libcrypto is disabled from SUPERCOP ed25519
   to libsodium. The libsodium implementation includes a number of
   strictness checks over the original reference implementation we
   have used to this point and a more ergonomic API.

 * ssh-add(1), ssh(1), ssh-keygen(1): fix spin on password entry when
   the program attempting to read a password was started in a
   background process group, with no TTY and with certain signals
   ignored. bz3995

 * scp(1): disallow nul byte in received scp -O filename. This was
   not reachable in normal operation. Reported by Chua Wei Xun.

 * ssh-keygen(1), ssh(1), sshd(8): implement a maximum number of KDF
   rounds that will be accepted when writing an OpenSSH-format
   private key or when loading one. This limit is set quite high
   (1M), but ensures that a service that is passed a bad key with a
   ridiculously high number of rounds will eventually complete
   parsing it.

 * ssh-keygen(1): bump the default number of KDF rounds from 24 to
   32 (this is a linear increase, not like bcrypt(3) which is
   exponential).

 * ssh(1): make StreamLocalBindMask properly respect Host/Match
   blocks and make it first-match-wins as documented. bz4013

 * sshd(8): make StreamLocalBindMask properly first-match-wins.

Portability
-----------

 * All: remove the NetBSD BROKEN_READ_COMPARISON workaround. This
   appears to be no longer required and caused pre-auth CPU spinning.

 * sshd(8): don't link sshd against libselinux when SELinux support
   is enabled (note: this library is still linked for the sshd-auth
   and sshd-session helper binaries).

 * sshd(8): allow madvise(..., MADV_DONTNEED_LOCKED) in the seccomp
   sandbox; needed by GrapheneOS' hardened allocator. bz4001

 * sshd(8): restrict mremap(2) flags accepted by the seccomp
   sandbox. Only MREMAP_MAYMOVE is now accepted as other flags may
   have some utility in attack chains. Reported by: Mohammad Hossein
   Abedini.

 * sshd(8): allow PAMServiceName in Match (regressed in 10.4).
   During the refactor of server option parsing, the ability to set
   PAMServiceName in Match blocks was accidentally disabled.

 * sshd(8): the --disable-fd-passing configure option has been
   removed.

Checksums:
==========

 - SHA1 (openssh-10.6.tar.gz) = 096e9cd60cd08e0acacfe16b615144f8c0463da5
 - SHA256 (openssh-10.6.tar.gz) = 0lP9h9/QH3k667vqbxKhkkusc3CN5iKUiDNI9fsR06A=
 - SHA1 (openssh-10.6p1.tar.gz) = e6a34f5625e20172b214c50ebd4e0dcb4fd18013
 - SHA256 (openssh-10.6p1.tar.gz) = qdyVZd/+hkD2TYY80poyvEo9vewFZqf8RMXW7nZ9Xzk=

Please note that the SHA256 signatures are base64 encoded and not
hexadecimal (which is the default for most checksum tools). The PGP
key used to sign the releases is available from the mirror sites:
https://cdn.openbsd.org/pub/OpenBSD/OpenSSH/RELEASE_KEY.asc

Reporting Bugs:
===============

- Please read https://www.openssh.com/report.html
  Security bugs should be reported directly to openssh () openssh com