CVE-2026-55653

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).

This Moderate flaw in OpenSSH affects clients operating in FIPS mode when negotiating Diffie-Hellman Group Exchange (DH-GEX) with a malicious SSH server. While it can lead to client process termination, resulting in a denial of service, the impact is limited to availability and does not result in broader system compromise. In order to exploit this vulnerability the attacker needs to trick the user to connect to an untrusted malicious server or compromise the server first. The availability impact is considered Low as the only impacted process is the single run of the SSH client trying to connect to the malicious server.

This vulnerability affects only the OpenSSH versions shipped with Red Hat products.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

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 [email protected],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.