Openssh: double free in red hat enterprise linux versions of openssh dh-gex client path during fips known-group validation leads to client-side denial of service
A flaw was found in OpenSSH. A malicious SSH server can exploit a double free vulnerability in the Diffie-Hellman Group Exchange (DH-GEX) client path. This occurs during FIPS (Federal Information Processing Standards) mode known-group validation when the client processes attacker-controlled DH-GEX group parameters. Successful exploitation leads to client-side process termination, resulting in a Denial of Service (DoS).
To mitigate this issue, OpenSSH clients operating in FIPS mode should avoid negotiating the `diffie-hellman-group-exchange-sha256` key exchange algorithm. This can be achieved by explicitly listing allowed key exchange algorithms in the client's SSH configuration file (e.g., `/etc/ssh/ssh_config` or `~/.ssh/config`), ensuring `diffie-hellman-group-exchange-sha256` is *not* included. For example, to use a subset of common algorithms, you might configure:
```
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group14-sha1
```
(Note: The above example `KexAlgorithms` list is illustrative and should be adjusted based on your environment's security requirements.)
Additionally, avoid using non-fatal client flows, such as `ssh-keyscan`, against untrusted SSH servers while FIPS mode is enabled. Changes to `ssh_config` will take effect for new SSH connections.