Call for testing: OpenSSH 8.4

Damien Miller djm at
Sun Sep 20 16:02:17 AEST 2020


OpenSSH 8.4p1 is almost ready for release, so we would appreciate testing
on as many platforms and systems as possible. This is a bugfix release.

Snapshot releases for portable OpenSSH are available from

The OpenBSD version is available in CVS HEAD:

Portable OpenSSH is also available via git using the
instructions at
At or via a mirror at Github:

Running the regression tests supplied with Portable OpenSSH does not
require installation and is a simply:

$ ./configure && make tests

Live testing on suitable non-production systems is also appreciated.
Please send reports of success or failure to
openssh-unix-dev at Security bugs should be reported
directly to openssh at

Below is a summary of changes. More detail may be found in the ChangeLog
in the portable OpenSSH tarballs.

Thanks to the many people who contributed to this release.


 * ssh-agent(1): restrict ssh-agent from signing web challenges for
   FIDO/U2F keys.

   When signing messages in ssh-agent using a FIDO key that has an
   application string that does not start with "ssh:", ensure that the
   message being signed is one of the forms expected for the SSH protocol
   (currently public key authentication and sshsig signatures).

   This prevents ssh-agent forwarding on a host that has FIDO keys
   attached granting the ability for the remote side to sign challenges
   for web authentication using those keys too.

   Note that the converse case of web browsers signing SSH challenges is
   already precluded because no web RP can have the "ssh:" prefix in the
   application string that we require.

 * ssh-keygen(1): Enable FIDO 2.1 credProtect extension when generating
   a FIDO resident key.

   The recent FIDO 2.1 Client to Authenticator Protocol introduced a
   "credProtect" feature to better protect resident keys. We use this
   option to require a PIN prior to all operations that may retrieve
   a resident key from a FIDO token.

Potentially-incompatible changes

This release includes a number of changes that may affect existing

 * For FIDO/U2F support, OpenSSH recommends the use of libfido2 1.5.0
   or greater. Older libraries have limited support at the expense of
   disabling particular features. These include resident keys, PIN-
   required keys and multiple attached tokens.

 * ssh-keygen(1): the format of the attestation information optionally
   recorded when a FIDO key is generated has changed. It now includes
   the authenticator data needed to validate attestation signatures. 

 * The API between OpenSSH and the FIDO token middleware has changed
   and the SSH_SK_VERSION_MAJOR version has been incremented as a
   result. Third-party middleware libraries must support the current
   API version (7) to work with OpenSSH 8.4.

 * The portable OpenSSH distribution now requires automake to rebuild
   the configure script and supporting files. This is not required when
   simply building portable OpenSSH from a release tar file.

Changes since OpenSSH 8.3

New features

 * ssh(1), ssh-keygen(1): support for FIDO keys that require a PIN for
   each use. These keys may be generated using ssh-keygen using a new
   "verify-required" option. When a PIN-required key is used, the user
   will be prompted for a PIN to complete the signature operation.

 * sshd(8): authorized_keys now supports a new "verify-required"
   option to require FIDO signatures assert that the token verified
   that the user was present before making the signature. The FIDO
   protocol supports multiple methods for user-verification, but
   currently OpenSSH only supports PIN verification.

 * sshd(8), ssh-keygen(1): add support for verifying FIDO webauthn
   signatures. Webauthn is a standard for using FIDO keys in web
   browsers. These signatures are a slightly different format to plain
   FIDO signatures and thus require explicit support.

 * ssh(1): allow some keywords to expand shell-style ${ENV}
   environment variables. The supported keywords are CertificateFile,
   ControlPath, IdentityAgent and IdentityFile, plus LocalForward and
   RemoteForward when used for Unix domain socket paths. bz#3140

 * ssh(1), ssh-agent(1): allow some additional control over the use of
   ssh-askpass via a new $SSH_ASKPASS_REQUIRE environment variable,
   including forcibly enabling and disabling its use. bz#69

 * ssh(1): allow ssh_config(5)'s AddKeysToAgent keyword accept a time
   limit for keys in addition to its current flag options. Time-
   limited keys will automatically be removed from ssh-agent after
   their expiry time has passed.

 * scp(1), sftp(1): allow the -A flag to explicitly enable agent
   forwarding in scp and sftp. The default remains to not forward an
   agent, even when ssh_config enables it.

 * ssh(1): add a '%k' TOKEN that expands to the effective HostKey of
   the destination. This allows, e.g., keeping host keys in individual
   files using "UserKnownHostsFile ~/.ssh/known_hosts.d/%k". bz#1654

 * ssh(1): add %-TOKEN, environment variable and tilde expansion to
   the UserKnownHostsFile directive, allowing the path to be
   completed by the configuration (e.g. bz#1654)

 * ssh-keygen(1): allow "ssh-add -d -" to read keys to be deleted
   from stdin. bz#3180

 * sshd(8): improve logging for MaxStartups connection throttling.
   sshd will now log when it starts and stops throttling and periodically
   while in this state. bz#3055


 * ssh(1), ssh-keygen(1): better support for multiple attached FIDO
   tokens. In cases where OpenSSH cannot unambiguously determine which
   token to direct a request to, the user is now required to select a
   token by touching it. In cases of operations that require a PIN to
   be verified, this avoids sending the wrong PIN to the wrong token
   and incrementing the token's PIN failure counter (tokens
   effectively erase their keys after too many PIN failures).

 * sshd(8): fix Include before Match in sshd_config; bz#3122

 * ssh(1), sshd(8): limit the amount of channel input data buffered,
   avoiding peers that advertise large windows but are slow to read
   from causing high memory consumption.

 * ssh-agent(1): handle multiple requests sent in a single write() to
   the agent.

 * sshd(8): allow sshd_config longer than 256k

 * sshd(8): avoid spurious "Unable to load host key" message when sshd
   load a private key but no public counterpart

 * ssh(1): prefer the default hostkey algorithm list whenever we have
   a hostkey that matches its best-preference algorithm.

 * sshd(1): when ordering the hostkey algorithms to request from a
   server, prefer certificate types if the known_hosts files contain a key
   marked as a @cert-authority; bz#3157

 * ssh(1): perform host key fingerprint comparisons for the "Are you
   sure you want to continue connecting (yes/no/[fingerprint])?"
   prompt with case sensitivity.

 * sshd(8): ensure that address/masklen mismatches in sshd_config
   yield fatal errors at daemon start time rather than later when
   they are evaluated.

 * ssh-keygen(1): ensure that certificate extensions are lexically
   sorted. Previously if the user specified a custom extension then
   the everything would be in order except the custom ones. bz#3198

 * ssh(1): also compare username when checking for JumpHost loops.

 * ssh-keygen(1): preserve group/world read permission on known_hosts
   files across runs of "ssh-keygen -Rf /path". The old behaviour was
   to remove all rights for group/other. bz#3146

 * ssh-keygen(1): Mention the [-a rounds] flag in the ssh-keygen
   manual page and usage().

 * sshd(8): explicitly construct path to ~/.ssh/rc rather than
   relying on it being relative to the current directory, so that it
   can still be found if the shell startup changes its directory.

 * sshd(8): when redirecting sshd's log output to a file, undo this
   redirection after the session child process is forked(). Fixes
   missing log messages when using this feature under some

 * sshd(8): start ClientAliveInterval bookkeeping before first pass
   through select() loop; fixed theoretical case where busy sshd may
   ignore timeouts from client.

 * ssh(1): only reset the ServerAliveInterval check when we receive
   traffic from the server and ignore traffic from a port forwarding
   client, preventing a client from keeping a connection alive when
   it should be terminated. bz#2265

 * ssh-keygen(1): avoid spurious error message when ssh-keygen
   creates files outside ~/.ssh

 * sftp-client(1): fix off-by-one error that caused sftp downloads to
   make one more concurrent request that desired. This prevented using
   sftp(1) in unpipelined request/response mode, which is useful when
   debugging. bz#3054

 * ssh(1), sshd(8): handle EINTR in waitfd() and timeout_connect()
   helpers. bz#3071

 * ssh(1), ssh-keygen(1): defer creation of ~/.ssh until we attempt to
   write to it so we don't leave an empty .ssh directory when it's not
   needed. bz#3156

 * ssh(1), sshd(8): fix multiplier when parsing time specifications
   when handling seconds after other units. bz#3171


 * sshd(8): always send any PAM account messages. If the PAM account
   stack returns any messages, always send them to the user and not
   just if the check succeeds. bz#2049

 * Implement some backwards compatibility for libfido2 libraries
   older than 1.5.0. Note that use of an older library will result
   in the loss of certain features including resident key support,
   PIN support and support for multiple attached tokens.

 * configure fixes for XCode 12

 * gnome-ssh-askpass3: ensure the "close" button is not focused by
   default for SSH_ASKPASS_PROMPT=none prompts. Avoids space/enter
   accidentally dismissing FIDO touch notifications.

 * gnome-ssh-askpass3: allow some control over textarea colour via
   environment variables.

 * sshd(8): document another PAM spec problem in a frustrated comment

 * sshd(8): support NetBSD's utmpx.ut_ss address field. bz#960

 * Add the ssh-sk-helper binary and its manpage to the RPM spec file

 * Detect the Frankenstein monster of Linux/X32 and allow the sandbox
   to function there. bz#3085

OpenSSH is brought to you by Markus Friedl, Niels Provos, Theo de
Raadt, Kevin Steves, Damien Miller, Darren Tucker, Jason McIntyre,
Tim Rice and Ben Lindstrom.

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