Announce: OpenSSH 8.7 released

Fri Aug 20 15:28:29 AEST 2021

OpenSSH 8.7 was released on 2021-08-20. It is available from the
mirrors listed at https://www.openssh.com/.

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

Imminent deprecation notice
===========================

OpenSSH will disable the ssh-rsa signature scheme by default in the
next release.

In the SSH protocol, the "ssh-rsa" signature scheme uses the SHA-1
hash algorithm in conjunction with the RSA public key algorithm.
It is now possible[1] to perform chosen-prefix attacks against the
SHA-1 algorithm for less than USD$50K.

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
upgraded.

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

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

This release includes a number of changes that may affect existing
configurations:

 * scp(1): this release changes the behaviour of remote to remote
   copies (e.g. "scp host-a:/path host-b:") to transfer through the
   local host by default. This was previously available via the -3
   flag. This mode avoids the need to expose credentials on the
   origin hop, avoids triplicate interpretation of filenames by the
   shell (by the local system, the copy origin and the destination)
   and, in conjunction with the SFTP support for scp(1) mentioned
   below, allows use of all authentication methods to the remote
   hosts (previously, only non-interactive methods could be used).
   A -R flag has been added to select the old behaviour.

 * ssh(1)/sshd(8): both the client and server are now using a
   stricter configuration file parser. The new parser uses more
   shell-like rules for quotes, space and escape characters. It is
   also more strict in rejecting configurations that include options
   lacking arguments. Previously some options (e.g. DenyUsers) could
   appear on a line with no subsequent arguments. This release will
   reject such configurations. The new parser will also reject
   configurations with unterminated quotes and multiple '='
   characters after the option name.

 * ssh(1): when using SSHFP DNS records for host key verification,
   ssh(1) will verify all matching records instead of just those
   with the specific signature type requested. This may cause host
   key verification problems if stale SSHFP records of a different
   or legacy signature type exist alongside other records for a
   particular host. bz#3322

 * ssh-keygen(1): when generating a FIDO key and specifying an
   explicit attestation challenge (using -Ochallenge), the challenge
   will now be hashed by the builtin security key middleware. This
   removes the (undocumented) requirement that challenges be exactly
   32 bytes in length and matches the expectations of libfido2.

 * sshd(8): environment="..." directives in authorized_keys files are
   now first-match-wins and limited to 1024 discrete environment
   variable names.

Changes since OpenSSH 8.6
=========================

This release contains a mix of new features and bug-fixes.

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

 - scp(1): experimental support for transfers using the SFTP protocol
   as a replacement for the venerable SCP/RCP protocol that it has
   traditionally used. SFTP offers more predictable filename handling
   and does not require expansion of glob(3) patterns via the shell
   on the remote side.

   SFTP support may be enabled via a temporary scp -s flag. It is
   intended for SFTP to become the default transfer mode in the
   near future, at which time the -s flag will be removed. The -O
   flag exists to force use of the original SCP/RCP protocol for
   cases where SFTP may be unavailable or incompatible.

 - sftp-server(8): add a protocol extension to support expansion of
   ~/ and ~user/ prefixed paths. This was added to support these
   paths when used by scp(1) while in SFTP mode.

 - ssh(1): add a ForkAfterAuthentication ssh_config(5) counterpart to
   the ssh(1) -f flag. GHPR#231

 - ssh(1): add a StdinNull directive to ssh_config(5) that allows the
   config file to do the same thing as -n does on the ssh(1) command-
   line. GHPR#231

 - ssh(1): add a SessionType directive to ssh_config, allowing the
    configuration file to offer equivalent control to the -N (no
    session) and -s (subsystem) command-line flags. GHPR#231

 - ssh-keygen(1): allowed signers files used by ssh-keygen(1)
   signatures now support listing key validity intervals alongside
   they key, and ssh-keygen(1) can optionally check during signature
   verification whether a specified time falls inside this interval.
   This feature is intended for use by git to support signing and
   verifying objects using ssh keys.

 - ssh-keygen(8): support printing of the full public key in a sshsig
   signature via a -Oprint-pubkey flag.

Bugfixes
--------

 * ssh(1)/sshd(8): start time-based re-keying exactly on schedule in
   the client and server mainloops. Previously the re-key timeout
   could expire but re-keying would not start until a packet was sent
   or received, causing a spin in select() if the connection was
   quiescent.

 * ssh-keygen(1): avoid Y2038 problem in printing certificate
   validity lifetimes. Dates past 2^31-1 seconds since epoch were
   displayed incorrectly on some platforms. bz#3329

 * scp(1): allow spaces to appear in usernames for local to remote
   and scp -3 remote to remote copies. bz#1164

 * ssh(1)/sshd(8): remove references to ChallengeResponseAuthentication
   in favour of KbdInteractiveAuthentication. The former is what was in
   SSHv1, the latter is what is in SSHv2 (RFC4256) and they were
   treated as somewhat but not entirely equivalent. We retain the old
   name as a deprecated alias so configuration files continue to work
   as well as a reference in the man page for people looking for it.
   bz#3303

 * ssh(1)/ssh-add(1)/ssh-keygen(1): fix decoding of X.509 subject name
   when extracting a key from a PKCS#11 certificate. bz#3327

 * ssh(1): restore blocking status on stdio fds before close. ssh(1)
   needs file descriptors in non-blocking mode to operate but it was
   not restoring the original state on exit. This could cause
   problems with fds shared with other programs via the shell,
   bz#3280 and GHPR#246

 * ssh(1)/sshd(8): switch both client and server mainloops from
   select(3) to pselect(3). Avoids race conditions where a signal
   may arrive immediately before select(3) and not be processed until
   an event fires. bz#2158

 * ssh(1): sessions started with ControlPersist were incorrectly
   executing a shell when the -N (no shell) option was specified.
   bz#3290

 * ssh(1): check if IPQoS or TunnelDevice are already set before
   overriding. Prevents values in config files from overriding values
   supplied on the command line. bz#3319

 * ssh(1): fix debug message when finding a private key to match a
   certificate being attempted for user authentication. Previously it
   would print the certificate's path, whereas it was supposed to be
   showing the private key's path. GHPR#247

 * sshd(8): match host certificates against host public keys, not
   private keys. Allows use of certificates with private keys held in
   a ssh-agent.  bz#3524

 * ssh(1): add a workaround for a bug in OpenSSH 7.4 sshd(8), which
   allows RSA/SHA2 signatures for public key authentication but fails
   to advertise this correctly via SSH2_MSG_EXT_INFO. This causes
   clients of these server to incorrectly match
   PubkeyAcceptedAlgorithmse and potentially refuse to offer valid
   keys. bz#3213

 * sftp(1)/scp(1): degrade gracefully if a sftp-server offers the
   limits at openssh.com extension but fails when the client tries to
   invoke it. bz#3318

 * ssh(1): allow ssh_config SetEnv to override $TERM, which is
   otherwise handled specially by the protocol. Useful in ~/.ssh/config
   to set TERM to something generic (e.g. "xterm" instead of
   "xterm-256color") for destinations that lack terminfo entries.

 * sftp-server(8): the limits at openssh.com extension was incorrectly
   marked as an operation that writes to the filesystem, which made it
   unavailable in sftp-server read-only mode. bz#3318

 * ssh(1): fix SEGV in UpdateHostkeys debug() message, triggered when
   the update removed more host keys than remain present.

 * many manual page fixes.

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

 * ssh(1): move closefrom() to before first malloc. When built against
   tcmalloc, the closefrom() would stomp on file descriptors created
   for tcmalloc's internal use. bz#3321

 * sshd(8): handle GIDs > 2^31 in getgrouplist. When compiled in 32bit
   mode, the getgrouplist implementation may fail for GIDs greater than
   LONG_MAX.

 * ssh(1): xstrdup environment variable used by ForwardAgent. bz#3328

 * sshd(8): don't sigdie() in signal handler in privsep child process;
   this can end up causing sandbox violations per bz3286

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

 - SHA1 (openssh-8.7.tar.gz) = 61bfa5a55e2b7e851b1d463aa432e4ff508f61cc
 - SHA256 (openssh-8.7.tar.gz) = Q5jPfCaYhTKhkDPYH32jtjFN7qhNhkNHYC9awg2THLs=

 - SHA1 (openssh-8.7p1.tar.gz) = 8719032c1e47732c8fdb14adfb24b5e9e71de802
 - SHA256 (openssh-8.7p1.tar.gz) = fKNLi7JK6eUPM3krcJGzhB1+G0QP9XvJ+r3fAeLtHiQ=

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

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