Vulnerability-Lookup

GHSA-XW36-3G63-5WFW

Vulnerability from github – Published: 2026-10-13 03:30 – Updated: 2026-10-13 03:30

Details

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.

Severity

5.3 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

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{
  "affected": [],
  "aliases": [
    "CVE-2026-109786"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-117"
    ],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-10-13T03:16:40Z",
    "severity": "MODERATE"
  },
  "details": "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": "GHSA-xw36-3g63-5wfw",
  "modified": "2026-10-13T03:30:36Z",
  "published": "2026-10-13T03:30:36Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-109786"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fail2ban/fail2ban/commit/ddd879d22664904ace24e1255ac6571808c58187"
    },
    {
      "type": "WEB",
      "url": "https://coredump.ws/repos/mail-hardening.git"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    }
  ]
}

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

Vulnerability from cvelistv5 – Published: 2026-10-13 02:40 – Updated: 2026-10-13 02:40

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 2316 records in the last 180 days details

References

2 references

URL	Tags
https://github.com/fail2ban/fail2ban/commit/ddd87…	patch
https://coredump.ws/repos/mail-hardening.git/	related

Impacted products

1 product

Vendor	Product	Version
fail2ban	fail2ban	Affected: 0.10.5 , ≤ 1.1.1 (custom) cpe:2.3:a:fail2ban:fail2ban::::::::

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JSON

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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unknown",
          "modules": [
            "sshd filter"
          ],
          "platforms": [
            "Linux",
            "Unix"
          ],
          "product": "fail2ban",
          "programFiles": [
            "config/filter.d/sshd.conf"
          ],
          "repo": "https://github.com/fail2ban/fail2ban",
          "vendor": "fail2ban",
          "versions": [
            {
              "lessThanOrEqual": "1.1.1",
              "status": "affected",
              "version": "0.10.5",
              "versionType": "custom"
            }
          ]
        }
      ],
      "configurations": [
        {
          "lang": "en",
          "supportingMedia": [
            {
              "base64": false,
              "type": "text/html",
              "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."
            }
          ],
          "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."
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:a:fail2ban:fail2ban:*:*:*:*:*:*:*:*",
                  "versionEndIncluding": "1.1.1",
                  "versionStartIncluding": "0.10.5",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "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."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "LOW",
            "baseScore": 5.3,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "NONE",
            "integrityImpact": "NONE",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
            "version": "3.1"
          },
          "format": "CVSS",
          "scenarios": [
            {
              "lang": "en",
              "value": "GENERAL"
            }
          ]
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-117",
              "description": "CWE-117 Improper Output Neutralization for Logs",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-13T02:40:13.899Z",
        "orgId": "8254265b-2729-46b6-b9e3-3dfca2d5bfca",
        "shortName": "mitre"
      },
      "references": [
        {
          "tags": [
            "patch"
          ],
          "url": "https://github.com/fail2ban/fail2ban/commit/ddd879d22664904ace24e1255ac6571808c58187"
        },
        {
          "tags": [
            "related"
          ],
          "url": "https://coredump.ws/repos/mail-hardening.git/"
        }
      ],
      "solutions": [
        {
          "lang": "en",
          "supportingMedia": [
            {
              "base64": false,
              "type": "text/html",
              "value": "Apply upstream commit ddd879d22664904ace24e1255ac6571808c58187, or upgrade to the first fail2ban release after 1.1.1 that contains it."
            }
          ],
          "value": "Apply upstream commit ddd879d22664904ace24e1255ac6571808c58187, or upgrade to the first fail2ban release after 1.1.1 that contains it."
        }
      ],
      "workarounds": [
        {
          "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."
        }
      ],
      "x_generator": {
        "engine": "CVE-Request-form 0.0.1"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "8254265b-2729-46b6-b9e3-3dfca2d5bfca",
    "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"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

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