CVE-2026-55653: OpenSSH double free in DH-GEX client during FIPS validation

CVE-2026-55653 · Severity: medium · CVSS 4.3 · Published 2026-06-23

A vulnerability in OpenSSH's client software could allow a malicious server to crash the client application. This occurs when the client is running in FIPS mode and connects to a server that provides specifically crafted security parameters. While this primarily results in a service disruption (Denial of Service) for the user or automated process initiating the connection, it does not currently appear to allow for data theft or unauthorized access.

Technologies: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4. Vendors: Red Hat.

Quick answers

Is CVE-2026-55653 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-55653 being exploited in the wild.
How severe is CVE-2026-55653?
Medium severity, with a CVSS score of 4.3, published 23 June 2026.
Which products and versions are affected?
OpenSSH OpenSSH 9.9p1-22.el10_2 (Red Hat build); Red Hat Red Hat Enterprise Linux 6; Red Hat Red Hat Enterprise Linux 7; Red Hat Red Hat Enterprise Linux 8; and 3 more.
Is there a fix for CVE-2026-55653?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-55653 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-55653?
Check whether you run Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7 or one of the others listed. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A double free vulnerability exists in the `input_kex_dh_gex_group()` function within `kexgexc.c` of OpenSSH. When the client is operating in FIPS mode, it performs a known-group validation on server-supplied parameters. If this validation fails, the code jumps to a cleanup routine that frees BIGNUM objects (`p` and `g`) that have already been assigned to the `kex->dh` structure. A second free occurs during the subsequent teardown of the SSH state in `kex_free()`. This is most reliably triggered in non-fatal client flows like `ssh-keyscan`. Exploitation requires the client to negotiate `diffie-hellman-group-exchange-sha256` and receive attacker-controlled DH-GEX parameters. The primary impact is a crash (SIGABRT) of the client process.

Affected products

  • OpenSSH OpenSSH 9.9p1-22.el10_2 (Red Hat build)
  • Red Hat Red Hat Enterprise Linux 6
  • Red Hat Red Hat Enterprise Linux 7
  • Red Hat Red Hat Enterprise Linux 8
  • Red Hat Red Hat Enterprise Linux 9
  • Red Hat Red Hat Enterprise Linux 10
  • Red Hat Red Hat OpenShift Container Platform 4

Timeline

  • 2026-04-26: disclosed: Initial report to Red Hat Bugzilla
  • 2026-06-23: advisory: CVE published by NVD/Red Hat

References

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