Announce: OpenSSH 8.4 released

[Damien Miller]

OpenSSH 8.4 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.

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
=========================

It is now possible[1] to perform chosen-prefix attacks against the
SHA-1 algorithm for less than USD$50K. For this reason, we will be
disabling the "ssh-rsa" public key signature algorithm by default in a
near-future release.

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.

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 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
upgraded.

We intend to enable UpdateHostKeys by default in the next OpenSSH
release. This will assist the client by automatically migrating to
better algorithms. Users may consider enabling this option manually.

[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
    (2020) https://eprint.iacr.org/2020/014.pdf

Security
========

 * 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
configurations:

 * 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

Bugfixes
--------

 * 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): close stdin/out/error when forking after authentication
   completes ("ssh -f ...") bz#3137

 * 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.
   bz#3057

 * 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.
   bz#3185

 * 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
   circumstances.

 * 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

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

 * 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
   $GNOME_SSH_ASKPASS_FG_COLOR and $GNOME_SSH_ASKPASS_BG_COLOR
   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

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

- SHA1 (openssh-8.4.tar.gz) = 71675139df6807f396e6bd92ff8cb9b0356385d8
- SHA256 (openssh-8.4.tar.gz) = JhBgLYkyRge/zQK8ylBSRcOYvrV/tHwQcvVXfExGB70=

- SHA1 (openssh-8.4p1.tar.gz) = 69305059e10a60693ebe6f17731f962c9577535c
- SHA256 (openssh-8.4p1.tar.gz) = WgHSLkB+scBbqKj3xlTTiKE+nyJuTtM704dI2vodKyQ=

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 as RELEASE_KEY.asc from
the mirror sites.

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

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