CVE-2026-55653: 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

Is it exploited?

Not on CISA's list. CISA has not added it to the Known Exploited Vulnerabilities catalog as of 29 Oct 2026. If CISA adds it, this page shows it as known exploited.

How likely is exploitation in the next 30 days?

0.5%

Up from 0.4% on 22 Oct 2026.

EPSS score, percentile 44.7 of all scored records. A probability estimated by FIRST, not a report of exploitation.

How severe?

4.3 Medium

CVSS v3.1 base score, given by the CNA

What should I do?

CISA has set no required action, because the vulnerability is not in KEV.

Advisories and patches (12)

Which versions are affected?

Unaffected 0:9.9p1-25.el10_2; 0:9.9p1-7.el10_0.5; 0:8.0p1-30.el8_10; 0:8.0p1-30.el8_10; 0:9.9p1-9.el9_8; 0:9.9p1-9.el9_8; and 14 more

Versions by product

Source: CISA KEV 2026.10.29, as of 29 Oct 2026; FIRST EPSS, as of 29 Oct 2026; CVE List, as of 29 Oct 2026. About the data.

What to do

Workaround (from the CVE record)

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.

Advisories and patches

• RHSA-2026:36759 (Vendor advisory)
• RHSA-2026:47755 (Vendor advisory)
• RHSA-2026:47756 (Vendor advisory)
• RHSA-2026:47757 (Vendor advisory)
• RHSA-2026:54387 (Vendor advisory)
• RHSA-2026:58981 (Vendor advisory)
• RHSA-2026:76748 (Vendor advisory)
• RHSA-2026:76993 (Vendor advisory)
• RHSA-2026:80513 (Vendor advisory)
• RHSA-2026:82749 (Vendor advisory)
• RHSA-2026:82750 (Vendor advisory)
• RHSA-2026:82751 (Vendor advisory)

This summary is based on the CVE record as of 29 Oct 2026. Check the vendor's advisory before you act: affected versions and fixes can change.

Affected products and versions

As listed in the CVE record.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions not listed are: affected.

Versions the record lists as unaffected: 0:9.9p1-25.el10_2, 0:9.9p1-7.el10_0.5, 0:8.0p1-30.el8_10, 0:9.9p1-9.el9_8, 0:8.7p1-30.el9_2.13, 0:8.7p1-38.el9_4.10, 0:8.7p1-45.el9_6.5, 10.3p1-6.hum1.

Source: CVE List, as of 29 Oct 2026. About the data.

About this vulnerability

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 D…

Summary: CVE record, as of 29 Oct 2026.

• CISA has not added CVE-2026-55653 to its Known Exploited Vulnerabilities (KEV) catalog, as of 29 Oct 2026.
• The CVE record was published on 23 Jun 2026.
• The CVE record carries a CVSS v3.1 base score of 4.3 (Medium), given by its CNA.
• FIRST's EPSS model puts the probability of exploitation in the next 30 days at 0.5% (percentile 44.7), as of 29 Oct 2026. EPSS is a statistical estimate, not a report of exploitation.
• CISA's SSVC assessment in the CVE record: exploitation none, automatable no, technical impact partial.
• The credits in the CVE record name AISLE as finder.

Source: CISA KEV 2026.10.29, as of 29 Oct 2026; CVE List, as of 29 Oct 2026; FIRST EPSS, as of 29 Oct 2026. About the data.

Description

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).

Source: CVE List, as of 29 Oct 2026. About the data.

Timeline

Known exploited Red Hat vulnerabilities

• CVE-2015-3246: Red Hat Libuser Race Condition Vulnerability (added 26 Aug 2026)
• CVE-2015-5287: Red Hat Automatic Bug Reporting Tool Privilege Escalation Vulnerability (added 26 Aug 2026)
• CVE-2018-14667: Red Hat JBoss RichFaces Framework Expression Language Injection Vulnerability (added 28 Sep 2023)
• CVE-2021-3560: Red Hat Polkit Incorrect Authorization Vulnerability (added 12 May 2023)
• CVE-2021-4034: Red Hat Polkit Out-of-Bounds Read and Write Vulnerability (added 27 Jun 2022)