Announce: OpenSSH 8.6 released

Damien Miller djm at
Mon Apr 19 10:53:14 AEST 2021

OpenSSH 8.6 has just been released. It will be available from the
mirrors listed at shortly.

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

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:

Future deprecation notice

It is now possible[1] to perform chosen-prefix attacks against the
SHA-1 algorithm for less than USD$50K.

In the SSH protocol, the "ssh-rsa" signature scheme uses the SHA-1
hash algorithm in conjunction with the RSA public key algorithm.
OpenSSH will disable this signature scheme by default in the near

Note that the deactivation of "ssh-rsa" signatures does not necessarily
require cessation of use for RSA keys. In the SSH protocol, keys may be
capable of signing using multiple algorithms. In particular, "ssh-rsa"
keys are capable of signing using "rsa-sha2-256" (RSA/SHA256),
"rsa-sha2-512" (RSA/SHA512) and "ssh-rsa" (RSA/SHA1). Only the last of
these is being turned off by default.

This algorithm is unfortunately still used widely despite the
existence of better alternatives, being the only remaining public key
signature algorithm specified by the original SSH RFCs that is still
enabled by default.

The better alternatives include:

 * The RFC8332 RSA SHA-2 signature algorithms rsa-sha2-256/512. These
   algorithms have the advantage of using the same key type as
   "ssh-rsa" but use the safe SHA-2 hash algorithms. These have been
   supported since OpenSSH 7.2 and are already used by default if the
   client and server support them.

 * The RFC8709 ssh-ed25519 signature algorithm. It has been supported
   in OpenSSH since release 6.5.

 * The RFC5656 ECDSA algorithms: ecdsa-sha2-nistp256/384/521. These
   have been supported by OpenSSH since release 5.7.

To check whether a server is using the weak ssh-rsa public key
algorithm, for host authentication, try to connect to it after
removing the ssh-rsa algorithm from ssh(1)'s allowed list:

    ssh -oHostKeyAlgorithms=-ssh-rsa user at host

If the host key verification fails and no other supported host key
types are available, the server software on that host should be

OpenSSH recently enabled the UpdateHostKeys option by default to assist
the client by automatically migrating to better algorithms.

[1] "SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and
    Application to the PGP Web of Trust" Leurent, G and Peyrin, T


 * sshd(8): OpenSSH 8.5 introduced the LogVerbose keyword. When this
   option was enabled with a set of patterns that activated logging
   in code that runs in the low-privilege sandboxed sshd process, the
   log messages were constructed in such a way that printf(3) format
   strings could effectively be specified the low-privilege code.

   An attacker who had sucessfully exploited the low-privilege
   process could use this to escape OpenSSH's sandboxing and attack
   the high-privilege process. Exploitation of this weakness is
   highly unlikely in practice as the LogVerbose option is not
   enabled by default and is typically only used for debugging. No
   vulnerabilities in the low-privilege process are currently known
   to exist.

   Thanks to Ilja Van Sprundel for reporting this bug.

Changes since OpenSSH 8.5

This release contains mostly bug fixes.

New features

 * sftp-server(8): add a new limits at protocol extension
   that allows a client to discover various server limits, including
   maximum packet size and maximum read/write length.

 * sftp(1): use the new limits at extension (when available)
   to select better transfer lengths in the client.

 * sshd(8): Add ModuliFile keyword to sshd_config to specify the
   location of the "moduli" file containing the groups for DH-GEX.

 * unit tests: Add a TEST_SSH_ELAPSED_TIMES environment variable to
   enable printing of the elapsed time in seconds of each test.


 * ssh_config(5), sshd_config(5): sync CASignatureAlgorithms lists in
   manual pages with the current default. GHPR#174

 * ssh(1): ensure that pkcs11_del_provider() is called before exit.

 * ssh(1), sshd(8): fix problems in string->argv conversion. Multiple
   backslashes were not being dequoted correctly and quoted space in
   the middle of a string was being incorrectly split. GHPR#223

 * ssh(1): return non-zero exit status when killed by signal; bz#3281

 * sftp-server(8): increase maximum SSH2_FXP_READ to match the maximum
   packet size. Also handle zero-length reads that are not explicitly
   banned by the spec.


 * sshd(8): don't mistakenly exit on transient read errors on the
   network socket (e.g. EINTR, EAGAIN); bz3297

 * Create a dedicated contrib/gnome-ssk-askpass3.c source instead of
   building it from the same file as used for GNOME2. Use the GNOME3
   gdk_seat_grab() to manage keyboard/mouse/server grabs for better
   compatibility with Wayland.

 * Fix portability build errors bz3293 bz3292 bz3291 bz3278

 * sshd(8): soft-disallow the fstatat64 syscall in the Linux
   seccomp-bpf sandbox. bz3276

 * unit tests: enable autoopt and misc unit tests that were
   previously skipped


 - SHA1 (openssh-8.6.tar.gz) = a3e93347eed6296faaaceb221e8786391530fccb
 - SHA256 (openssh-8.6.tar.gz) = ihmgdEgKfCBRpC0qzdQRwYownrpBf+rsihvk4Rmim8M=

 - SHA1 (openssh-8.6p1.tar.gz) = 8f9f0c94317baeb97747d6258f3997b4542762c0
 - SHA256 (openssh-8.6p1.tar.gz) = w+bk2hYhdiyFDQO0fu0eSN/0zJYI3etUcgKiNN+O164=

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:

Please note that the OpenPGP key used to sign releases has been
rotated for this release. The new key has been signed by the previous
key to provide continuity.

Reporting Bugs:

- Please read
  Security bugs should be reported directly to openssh at

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