CVE-2026-109786 (GCVE-0-2026-109786)

Vulnerability from cvelistv5 – Published: 2026-10-13 02:40 – Updated: 2026-10-13 02:40
VLAI
Summary
In fail2ban 0.10.5 through 1.1.1 before ddd879d, the sshd filter (config/filter.d/sshd.conf) accepts an "on <token>" 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.
Severity
5.3 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
CWE
  • CWE-117 - Improper Output Neutralization for Logs
Assigner
mitre CNA under the mitre root
CNA scorecard E 28/100 over 2311 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
fail2ban fail2ban Affected: 0.10.5 , ≤ 1.1.1 (custom)
    cpe:2.3:a:fail2ban:fail2ban:*:*:*:*:*:*:*:*
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unknown",
          "modules": [
            "sshd filter"
          ],
          "platforms": [
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            "Unix"
          ],
          "product": "fail2ban",
          "programFiles": [
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          ],
          "repo": "https://github.com/fail2ban/fail2ban",
          "vendor": "fail2ban",
          "versions": [
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              "value": "The sshd jail is enabled with the stock filter.d/sshd.conf (any mode: normal, the default, extra, or aggressive). Before 1.1.1 the filter only matches log lines from a process named sshd (OpenSSH before 10, or systems that log as sshd); 1.1.1 also matches OpenSSH 10\u0027s sshd-session."
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          "value": "The sshd jail is enabled with the stock filter.d/sshd.conf (any mode: normal, the default, extra, or aggressive). Before 1.1.1 the filter only matches log lines from a process named sshd (OpenSSH before 10, or systems that log as sshd); 1.1.1 also matches OpenSSH 10\u0027s sshd-session."
        }
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          "value": "In fail2ban 0.10.5 through 1.1.1 before ddd879d, the sshd filter (config/filter.d/sshd.conf) accepts an \"on \u003ctoken\u003e\" 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."
        }
      ],
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            "attackVector": "NETWORK",
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            "baseScore": 5.3,
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        "orgId": "8254265b-2729-46b6-b9e3-3dfca2d5bfca",
        "shortName": "mitre"
      },
      "references": [
        {
          "tags": [
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          "url": "https://github.com/fail2ban/fail2ban/commit/ddd879d22664904ace24e1255ac6571808c58187"
        },
        {
          "tags": [
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          "url": "https://coredump.ws/repos/mail-hardening.git/"
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              "type": "text/html",
              "value": "Apply upstream commit ddd879d22664904ace24e1255ac6571808c58187, or upgrade to the first fail2ban release after 1.1.1 that contains it."
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          ],
          "value": "Apply upstream commit ddd879d22664904ace24e1255ac6571808c58187, or upgrade to the first fail2ban release after 1.1.1 that contains it."
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          "lang": "en",
          "supportingMedia": [
            {
              "base64": false,
              "type": "text/html",
              "value": "Override __on_port_opt and __suff in a local filter.d/sshd.local so that only an optional \"port N\" may follow the host address, as in the upstream fix: __on_port_opt = (?: port \\d+)? and __suff = %(__on_port_opt)s(?: \\[preauth\\])?\\s*. Add the administrator\u0027s own addresses to ignoreip."
            }
          ],
          "value": "Override __on_port_opt and __suff in a local filter.d/sshd.local so that only an optional \"port N\" may follow the host address, as in the upstream fix: __on_port_opt = (?: port \\d+)? and __suff = %(__on_port_opt)s(?: \\[preauth\\])?\\s*. Add the administrator\u0027s own addresses to ignoreip."
        }
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    "assignerShortName": "mitre",
    "cveId": "CVE-2026-109786",
    "datePublished": "2026-10-13T02:40:13.899Z",
    "dateReserved": "2026-10-13T02:40:12.792Z",
    "dateUpdated": "2026-10-13T02:40:13.899Z",
    "state": "PUBLISHED"
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      "percentile": "0.2254"
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    "nvd": {
      "cve": {
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                  "Unix"
                ],
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                "vendor": "fail2ban",
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        "cveTags": [],
        "descriptions": [
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            "value": "In fail2ban 0.10.5 through 1.1.1 before ddd879d, the sshd filter (config/filter.d/sshd.conf) accepts an \"on \u003ctoken\u003e\" 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."
          }
        ],
        "id": "CVE-2026-109786",
        "lastModified": "2026-10-13T03:16:40.943",
        "metrics": {
          "cvssMetricV31": [
            {
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        "published": "2026-10-13T03:16:40.943",
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            "url": "https://coredump.ws/repos/mail-hardening.git/"
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            "url": "https://github.com/fail2ban/fail2ban/commit/ddd879d22664904ace24e1255ac6571808c58187"
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        "vulnStatus": "Received",
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    "redhat_vex": {
      "current_release_date": "2026-10-13T05:24:10+00:00",
      "cve": "CVE-2026-109786",
      "id": "CVE-2026-109786",
      "initial_release_date": "2026-10-13T02:40:13.899000+00:00",
      "product_status:known_not_affected": "1",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "fail2ban: fail2ban: Denial of Service via improper host parsing in sshd filter",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-109786.json",
      "version": "3"
    }
  }
}



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Nomenclature

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