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

{
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "cveMetadata": {
    "cveId": "CVE-2026-55653",
    "assignerOrgId": "53f830b8-0a3f-465b-8143-3b8a9948e749",
    "assignerShortName": "redhat",
    "dateUpdated": "2026-06-23T03:36:22.741Z",
    "dateReserved": "2026-06-16T23:55:05.737Z",
    "datePublished": "2026-06-23T03:36:22.741Z",
    "state": "PUBLISHED"
  },
  "containers": {
    "cna": {
      "providerMetadata": {
        "orgId": "53f830b8-0a3f-465b-8143-3b8a9948e749",
        "shortName": "redhat",
        "dateUpdated": "2026-06-23T03:36:22.741Z"
      },
      "datePublic": "2026-06-22T23:17:43.088Z",
      "title": "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",
      "descriptions": [
        {
          "lang": "en",
          "value": "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)."
        }
      ],
      "affected": [
        {
          "vendor": "Red Hat",
          "product": "Red Hat Enterprise Linux 10",
          "collectionURL": "https://access.redhat.com/downloads/content/package-browser/",
          "packageName": "openssh",
          "cpes": [
            "cpe:/o:redhat:enterprise_linux:10"
          ],
          "defaultStatus": "affected"
        },
        {
          "vendor": "Red Hat",
          "product": "Red Hat Enterprise Linux 6",
          "collectionURL": "https://access.redhat.com/downloads/content/package-browser/",
          "packageName": "openssh",
          "cpes": [
            "cpe:/o:redhat:enterprise_linux:6"
          ],
          "defaultStatus": "affected"
        },
        {
          "vendor": "Red Hat",
          "product": "Red Hat Enterprise Linux 7",
          "collectionURL": "https://access.redhat.com/downloads/content/package-browser/",
          "packageName": "openssh",
          "cpes": [
            "cpe:/o:redhat:enterprise_linux:7"
          ],
          "defaultStatus": "affected"
        },
        {
          "vendor": "Red Hat",
          "product": "Red Hat Enterprise Linux 8",
          "collectionURL": "https://access.redhat.com/downloads/content/package-browser/",
          "packageName": "openssh",
          "cpes": [
            "cpe:/o:redhat:enterprise_linux:8"
          ],
          "defaultStatus": "affected"
        },
        {
          "vendor": "Red Hat",
          "product": "Red Hat Enterprise Linux 9",
          "collectionURL": "https://access.redhat.com/downloads/content/package-browser/",
          "packageName": "openssh",
          "cpes": [
            "cpe:/o:redhat:enterprise_linux:9"
          ],
          "defaultStatus": "affected"
        },
        {
          "vendor": "Red Hat",
          "product": "Red Hat Hardened Images",
          "collectionURL": "https://access.redhat.com/downloads/content/package-browser/",
          "packageName": "openssh",
          "cpes": [
            "cpe:/a:redhat:hummingbird:1"
          ],
          "defaultStatus": "affected"
        },
        {
          "vendor": "Red Hat",
          "product": "Red Hat OpenShift Container Platform 4",
          "collectionURL": "https://access.redhat.com/downloads/content/package-browser/",
          "packageName": "rhcos",
          "cpes": [
            "cpe:/a:redhat:openshift:4"
          ],
          "defaultStatus": "affected"
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "lang": "en",
              "description": "Double Free",
              "cweId": "CWE-415",
              "type": "CWE"
            }
          ]
        }
      ],
      "references": [
        {
          "url": "https://access.redhat.com/security/cve/CVE-2026-55653",
          "tags": [
            "vdb-entry",
            "x_refsource_REDHAT"
          ]
        },
        {
          "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2462351",
          "name": "RHBZ#2462351",
          "tags": [
            "issue-tracking",
            "x_refsource_REDHAT"
          ]
        }
      ],
      "metrics": [
        {},
        {
          "format": "CVSS",
          "cvssV3_1": {
            "version": "3.1",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L",
            "attackVector": "NETWORK",
            "attackComplexity": "LOW",
            "privilegesRequired": "NONE",
            "userInteraction": "REQUIRED",
            "scope": "UNCHANGED",
            "confidentialityImpact": "NONE",
            "integrityImpact": "NONE",
            "availabilityImpact": "LOW",
            "baseScore": 4.3,
            "baseSeverity": "MEDIUM"
          }
        }
      ],
      "workarounds": [
        {
          "lang": "en",
          "value": "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:\n\n```\nKexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group14-sha1\n```\n\n(Note: The above example `KexAlgorithms` list is illustrative and should be adjusted based on your environment's security requirements.)\n\nAdditionally, 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."
        }
      ],
      "timeline": [
        {
          "time": "2026-04-26T18:54:39.000Z",
          "lang": "en",
          "value": "Reported to Red Hat."
        },
        {
          "time": "2026-06-22T23:17:43.088Z",
          "lang": "en",
          "value": "Made public."
        }
      ]
    }
  }
}