DSA-5586-1 openssh – security update

Debian Security Advisory

Several vulnerabilities have been discovered in OpenSSH, an
implementation of the SSH protocol suite.


It was discovered that sshd failed to correctly initialise
supplemental groups when executing an AuthorizedKeysCommand or
AuthorizedPrincipalsCommand, where a AuthorizedKeysCommandUser or
AuthorizedPrincipalsCommandUser directive has been set to run the
command as a different user. Instead these commands would inherit
the groups that sshd was started with.


Luci Stanescu reported that a error prevented constraints being
communicated to the ssh-agent when adding smartcard keys to the
agent with per-hop destination constraints, resulting in keys being
added without constraints.


Fabian Baeumer, Marcus Brinkmann and Joerg Schwenk discovered that
the SSH protocol is prone to a prefix truncation attack, known as
the “Terrapin attack”. This attack allows a MITM attacker to effect
a limited break of the integrity of the early encrypted SSH
transport protocol by sending extra messages prior to the
commencement of encryption, and deleting an equal number of
consecutive messages immediately after encryption starts.

Details can be found at https://terrapin-attack.com/


It was discovered that when PKCS#11-hosted private keys were
added while specifying destination constraints, if the PKCS#11
token returned multiple keys then only the first key had the
constraints applied.


It was discovered that if an invalid user or hostname that contained
shell metacharacters was passed to ssh, and a ProxyCommand,
LocalCommand directive or “match exec” predicate referenced the user
or hostname via expansion tokens, then an attacker who could supply
arbitrary user/hostnames to ssh could potentially perform command
injection. The situation could arise in case of git repositories
with submodules, where the repository could contain a submodule with
shell characters in its user or hostname.