OpenSSH client: double free on attacker-controlled DH-GEX parameters crashes the client in FIPS mode

CVSS 6.5CVE-2026-55653Kernel, userspace & hypervisorcurated

Impact

A malicious SSH server can trigger a double free in the OpenSSH client's Diffie-Hellman Group Exchange path during FIPS-mode known-group validation, using group parameters it controls. The confirmed outcome is client-side process termination - a denial of service, not server compromise. What makes it worth tracking on a GPU fleet is where ssh clients run unattended: job launchers, Slurm prologue and epilogue scripts, rsync and scp data movers, Ansible and configuration runs, and CI agents that reach nodes over ssh. A hostile or compromised endpoint they connect out to can reliably kill those connections, which shows up as failed jobs and stalled orchestration rather than as a security event. It only bites clients connecting outward, in FIPS mode, and there is no reported memory-disclosure or code-execution outcome in the record.

Who can reach it

Network, outbound: the operator's OpenSSH client must connect to an SSH server the attacker controls (the vector records required user interaction). No authentication to the victim host is needed; a listening SSH service does not expose you.

What to do

Apply the distribution openssh update - Red Hat has shipped RHSA-2026:36759, 47755, 47756, 47757 and 54387 across RHEL 6-10, Hardened Images and Update Infrastructure 5. This is a client-side package upgrade; new ssh invocations pick up the fix immediately, so no reboot or node drain is required, though long-running agents holding an old binary should be restarted.

References

https://nvd.nist.gov/vuln/detail/CVE-2026-55653 https://access.redhat.com/errata/RHSA-2026:36759 https://access.redhat.com/errata/RHSA-2026:54387

This entry is curated: imported from vendor advisories with machine assistance, not yet individually verified. Confirm against your vendor's advisory before acting, and report anything wrong.