Security advisory

{
  "id": "abab692d24128e8bb3dbaf19b53ddbb79591152c15b73aeb69f240f7045119fb",
  "url": "https://osv.dev/vulnerability/CVE-2026-109786",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "CVE-2026-109786",
    "summary": "In fail2ban 0.10.5 through 1.1.1 before ddd879d, the sshd filter (config/filter.d/sshd.conf) accepts an \"on \" suffix after the host address. Because OpenSSH logs the offered username verbatim, a remote unauthenticated attacker can use a username such as \"x VICTIM-SOURCE-IP-ADDRESS on\" to make fail2ban associate failed login attempts with an arbitrary source IP address and ban it (e.g., to block logins from a legitimate user). The default filter mode is affected.",
    "evidence": "osv:CVE-2026-109786",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-109786",
      "aliases": [
        "CVE-2026-109786"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/fail2ban/fail2ban",
              "type": "GIT",
              "events": [
                {
                  "introduced": "b25d8565fc23da58f97d1da649b46d27fb80b64a"
                },
                {
                  "fixed": "ddd879d22664904ace24e1255ac6571808c58187"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0.10.5"
                      },
                      {
                        "last_affected": "1.1.1"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
                  },
                  {
                    "range": [
                      {
                        "fixed": "ddd879d22664904ace24e1255ac6571808c58187"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  }
                ]
              }
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-109786.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:36.900172142Z",
      "provider": "osv",
      "references": [
        "https://coredump.ws/repos/mail-hardening.git/",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/109xxx/CVE-2026-109786.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-109786",
        "https://github.com/fail2ban/fail2ban/commit/ddd879d22664904ace24e1255ac6571808c58187",
        "https://github.com/fail2ban/fail2ban"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-117"
        ],
        "cna_assigner": "mitre",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/109xxx/CVE-2026-109786.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-109786",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 504369,
  "published_at": "2026-10-13T02:40:13.899+00:00"
}