The Qualys Security team discovered two vulnerabilities in the roaming
code of the OpenSSH client (an implementation of the SSH protocol
suite).
SSH roaming enables a client, in case an SSH connection breaks
unexpectedly, to resume it at a later time, provided the server also
supports it.
The OpenSSH server doesn’t support roaming, but the OpenSSH client
supports it (even though it’s not documented) and it’s enabled by
default.
This security update completely disables the roaming code in the OpenSSH
client.
It is also possible to disable roaming by adding the (undocumented)
option UseRoaming no to the global /etc/ssh/ssh_config file, or to the
user configuration in ~/.ssh/config, or by passing -oUseRoaming=no on
the command line.
Users with passphrase-less privates keys, especially in non interactive
setups (automated jobs using ssh, scp, rsync+ssh etc.) are advised to
update their keys if they have connected to an SSH server they don’t
trust.
More details about identifying an attack and mitigations can be found in
the Qualys Security Advisory.