Call for testing: OpenSSH 8.2

Thu Feb 6 10:29:13 AEDT 2020

Hi,

OpenSSH 8.2p1 is almost ready for release, so we would appreciate testing
on as many platforms and systems as possible. This is a feature release.

Snapshot releases for portable OpenSSH are available from
http://www.mindrot.org/openssh_snap/

The OpenBSD version is available in CVS HEAD:
http://www.openbsd.org/anoncvs.html

Portable OpenSSH is also available via git using the
instructions at http://www.openssh.com/portable.html#cvs
At https://anongit.mindrot.org/openssh.git/ or via a mirror at Github:
https://github.com/openssh/openssh-portable

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 mindrot.org. Security bugs should be reported
directly to openssh at openssh.com.

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.

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 -oHostBasedKeyTypes=-ssh-rsa user at host

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

[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(1), sshd(8), ssh-keygen(1): this release removes the "ssh-rsa"
   (RSA/SHA1) algorithm from those accepted for certificate signatures
   (i.e. the client and server CASignatureAlgorithms option) and will
   use the rsa-sha2-512 signature algorithm by default when the
   ssh-keygen(1) CA signs new certificates.

   Certificates are at special risk to the aforementioned SHA1
   collision vulnerability as an attacker has effectively unlimited
   time in which to craft a collision that yields them a valid
   certificate, far more than the relatively brief LoginGraceTime
   window that they have to forge a host key signature.

   OpenSSH releases prior to 7.2 do not support the newer RSA/SHA2
   algorithms and will refuse to accept certificates signed by an
   OpenSSH 8.2+ CA using RSA keys. Older clients/servers may use
   another CA key type such as ssh-ed25519 (supported since OpenSSH
   6.5) or one of the ecdsa-sha2-nistp256/384/521 types (supported
   since OpenSSH 5.7) instead if they cannot be upgraded.

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

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

 * ssh(1), sshd(8): the above removal of "ssh-rsa" from the accepted
   CASignatureAlgorithms list.

 * ssh(1), sshd(8): this release removes diffie-hellman-group14-sha1
   from the default key exchange proposal for both the client and
   server.

 * ssh-keygen(1): the command-line options related to the generation
   and screening of safe prime numbers used by the
   diffie-hellman-group-exchange-* key exchange algorithms have
   changed. Most options have been folded under the -O flag.

 * sshd(8): the sshd listener process title visible to ps(1) has
   changed to include information about the number of connections that
   are currently attempting authentication and the limits configured
   by MaxStartups.

Changes since OpenSSH 8.1
=========================

This release contains some significant new features.

FIDO/U2F Support
----------------

This release adds support for FIDO/U2F hardware authenticators to
OpenSSH. U2F/FIDO are open standards for inexpensive two-factor
authentication hardware that are widely used for website
authentication.  In OpenSSH FIDO devices are supported by new public
key types "ecdsa-sk" and "ed25519-sk", along with corresponding
certificate types.

ssh-keygen(1) may be used to generate a FIDO token-backed key, after
which they may be used much like any other key type supported by
OpenSSH, so long as the hardware token is attached when the keys are
used. FIDO token also generally require the user explicitly authorise
operations by touching or tapping them.

Generating a FIDO key requires the token be attached, and will usually
require the user tap the token to confirm the operation:

  $ ssh-keygen -t ecdsa-sk -f ~/.ssh/id_ecdsa_sk
  Generating public/private ecdsa-sk key pair.
  You may need to touch your security key to authorize key generation.
  Enter file in which to save the key (/home/djm/.ssh/id_ecdsa_sk): 
  Enter passphrase (empty for no passphrase): 
  Enter same passphrase again: 
  Your identification has been saved in /home/djm/.ssh/id_ecdsa_sk
  Your public key has been saved in /home/djm/.ssh/id_ecdsa_sk.pub

This will yield a public and private key-pair. The private key file
should be useless to an attacker who does not have access to the
physical token. After generation, this key may be used like any other
supported key in OpenSSH and may be listed in authorized_keys, added
to ssh-agent(1), etc. The only additional stipulation is that the FIDO
token that the key belongs to must be attached when the key is used.

FIDO tokens are most commonly connected via USB but may be attached
via other means such as Bluetooth or NFC. In OpenSSH, communication
with the token is managed via a middleware library, specified by the
SecurityKeyProvider directive in ssh/sshd_config(5). OpenSSH includes
a middleware with support for USB tokens that is may be enabled in
portable OpenSSH via the --with-security-key-builtin configure flag
(it is enabled automatically in OpenBSD). This internal middleware
requires that libfido2 (https://github.com/Yubico/libfido2) and its
dependencies be installed. If the built-in middleware is enabled then
it will be used by default.

Note: FIDO/U2F tokens are required to implement the ECDSA-P256
"ecdsa-sk" key type, but hardware support for Ed25519 "ed25519-sk" is
less common. Similarly, not all hardware tokens support some of the
optional features such as resident keys.

The protocol-level changes to support FIDO/U2F keys in SSH are
documented in the PROTOCOL.u2f file in the OpenSSH source
distribution.

There are a number of supporting changes to this feature:

 * ssh-keygen(1): add a "no-touch-required" option when generating
   FIDO-hosted keys, that disables their default behaviour of
   requiring a physical touch/tap on the token during authentication.
   Note: not all tokens support disabling the touch requirement.

 * sshd(8): add a sshd_config PubkeyAuthOptions directive that
   collects miscellaneous public key authentication-related options
   for sshd(8). At present it supports only a single option
   "no-touch-required". This causes sshd to skip its default check for
   FIDO/U2F keys that the signature was authorised by a touch or press
   event on the token hardware.

 * ssh(1), sshd(8), ssh-keygen(1): add a "no-touch-required" option
   for authorized_keys and a similar extension for certificates. This
   option disables the default requirement that FIDO key signatures
   attest that the user touched their key to authorize them, mirroring
   the similar PubkeyAuthOptions sshd_config option.

 * ssh-keygen(1): add support for the writing the FIDO attestation
   information that is returned when new keys are generated via the
   "-O write-attestation=/path" option. FIDO attestation certificates
   may be used to verify that a FIDO key is hosted in trusted
   hardware. OpenSSH does not currently make use of this information,
   beyond optionally writing it to disk.

FIDO2 resident keys
-------------------

FIDO/U2F OpenSSH keys consist of two parts: a "key handle" part stored
in the private key file on disk, and a per-device private key that is
unique to each FIDO/U2F token and that cannot be exported. These are
combined by the hardware at authentication time to derive the real key
that is used to sign authentication challenges.

For tokens that are required to move between computers, it can be
cumbersome to have to move the private key file first. To avoid this
requirement, tokens implementing the newer FIDO2 standard support
"resident keys", where it is possible to effectively retrieve the key
handle part of the key from the hardware.

OpenSSH supports this feature, allowing resident keys to be generated
using the ssh-keygen(1) "-O resident" flag. This will produce a
public/private key pair as usual, but it will be possible to retrieve
the private key part from the token later. This may be done using
"ssh-keygen -K", which will download all available resident keys from
the tokens attached to the host and write public/private key files
for them. It is also possible to download and add resident keys
directly to ssh-agent(1) without writing files to the file-system
using "ssh-add -K".

Resident keys are indexed on the token by the application string and
user ID. By default, OpenSSH uses an application string of "ssh:" and
an empty user ID. If multiple resident keys on a single token are
desired then it may be necessary to override one or both of these
defaults using the ssh-keygen(1) "-O application=" or "-O user="
options. Note: OpenSSH will only download and use resident keys whose
application string begins with "ssh:"

Storing both parts of a key on a FIDO token increases the likelihood
of an attacker being able to use a stolen token device. For this
reason, tokens should enforce PIN authentication before allowing
download of keys, and users should set a PIN on their tokens before
creating any resident keys.

Other New Features
------------------

 * sshd(8): add an Include sshd_config keyword that allows including
   additional configuration files via glob(3) patterns. bz2468

 * ssh(1)/sshd(8): make the LE (low effort) DSCP code point available
   via the IPQoS directive; bz2986,

 * ssh(1): when AddKeysToAgent=yes is set and the key contains no
   comment, add the key to the agent with the key's path as the
   comment. bz2564

 * ssh-keygen(1), ssh-agent(1): expose PKCS#11 key labels and X.509
   subjects as key comments, rather than simply listing the PKCS#11
   provider library path. PR138

 * ssh-keygen(1): allow PEM export of DSA and ECDSA keys; bz3091

 * ssh(1), sshd(8): make zlib compile-time optional, available via the
   Makefile.inc ZLIB flag on OpenBSD or via the --with-zlib configure
   option for OpenSSH portable.

 * sshd(8): when clients get denied by MaxStartups, send a
   notification prior to the SSH2 protocol banner according to
   RFC4253 section 4.2.

 * ssh(1), ssh-agent(1): when invoking the $SSH_ASKPASS prompt
   program, pass a hint to the program to describe the type of
   desired prompt.  The possible values are "confirm" (indicating
   that a yes/no confirmation dialog with no text entry should be
   shown), "none" (to indicate an informational message only), or
   blank for the original ssh-askpass behaviour of requesting a
   password/phrase.

 * ssh(1): allow forwarding a different agent socket to the path
   specified by $SSH_AUTH_SOCK, by extending the existing ForwardAgent
   option to accepting an explicit path or the name of an environment
   variable in addition to yes/no.

 * ssh-keygen(1): add a new signature operations "find-principals" to
   look up the principal associated with a signature from an allowed-
   signers file.

 * sshd(8): expose the number of currently-authenticating connections
   along with the MaxStartups limit in the process title visible to
   "ps".

Bugfixes
--------

 * sshd(8): make ClientAliveCountMax=0 have sensible semantics: it
   will now disable connection killing entirely rather than the
   current behaviour of instantly killing the connection after the
   first liveness test regardless of success. bz2627

 * sshd(8): clarify order of AllowUsers / DenyUsers vs AllowGroups /
   DenyGroups in the sshd(8) manual page. bz1690

 * sshd(8): better describe HashKnownHosts in the manual page. bz2560

 * sshd(8): clarify that that permitopen=/PermitOpen do no name or
   address translation in the manual page. bz3099

 * sshd(8): allow the UpdateHostKeys feature to function when
   multiple known_hosts files are in use. When updating host keys,
   ssh will now search subsequent known_hosts files, but will add
   updated host keys to the first specified file only. bz2738

 * All: replace all calls to signal(2) with a wrapper around
   sigaction(2). This wrapper blocks all other signals during the
   handler preventing races between handlers, and sets SA_RESTART
   which should reduce the potential for short read/write operations.

 * sftp(1): fix a race condition in the SIGCHILD handler that could
   turn in to a kill(-1); bz3084

 * sshd(8): fix a case where valid (but extremely large) SSH channel
   IDs were being incorrectly rejected. bz3098

 * ssh(1): when checking host key fingerprints as answers to new
   hostkey prompts, ignore whitespace surrounding the fingerprint
   itself.

 * All: wait for file descriptors to be readable or writeable during
   non-blocking connect, not just readable. Prevents a timeout when
   the server doesn't immediately send a banner (e.g. multiplexers
   like sslh)

 * sshd_config(5): document the sntrup4591761x25519-sha512 at tinyssh.org
   key exchange algorithm. PR#151

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

 * sshd(8): multiple adjustments to the Linux seccomp sandbox:
   - Non-fatally deny IPC syscalls in sandbox
   - Allow clock_gettime64() in sandbox (MIPS / glibc >= 2.31)
   - Allow clock_nanosleep_time64 in sandbox (ARM) bz3100
   - Allow clock_nanosleep() in sandbox (recent glibc) bz3093

 * Explicit check for memmem declaration and fix up declaration if the
   system headers lack it. bz3102

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.