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

Source: https://junglewise.ai/threats/cve-2026-55653-openssh-double-free-in-dh-gex-client-during-fips-validation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-23. Updated: 2026-06-23  
License: CC BY 4.0 with a link back

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](https://junglewise.ai/threats/technologies/enterprise-linux-10), [Red Hat Enterprise Linux 6](https://junglewise.ai/threats/technologies/enterprise-linux-6), [Red Hat Enterprise Linux 7](https://junglewise.ai/threats/technologies/enterprise-linux-7), [Red Hat Enterprise Linux 8](https://junglewise.ai/threats/technologies/enterprise-linux-8), [Red Hat Enterprise Linux 9](https://junglewise.ai/threats/technologies/enterprise-linux-9), [Red Hat OpenShift Container Platform 4](https://junglewise.ai/threats/technologies/openshift-container-platform-4). Vendors: [Red Hat](https://junglewise.ai/threats/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

- [https://access.redhat.com/downloads/content/package-browser/](https://access.redhat.com/downloads/content/package-browser/)
- [https://access.redhat.com/security/cve/CVE-2026-55653](https://access.redhat.com/security/cve/CVE-2026-55653)
- [https://bugzilla.redhat.com/show_bug.cgi?id=2462351](https://bugzilla.redhat.com/show_bug.cgi?id=2462351)

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