{"id":"CVE-2026-109786","state":"PUBLISHED","title":"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","description":"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.","publishedAt":"2026-10-13T02:40:13.899Z","updatedAt":"2026-10-15T15:04:19.106Z","severity":{"score":5.3,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","source":"mitre"},"assigner":"mitre","products":["fail2ban fail2ban"],"affected":[{"vendor":"fail2ban","product":"fail2ban","defaultStatus":"unknown","versions":[{"version":"0.10.5","status":"affected","lessThanOrEqual":"1.1.1","versionType":"custom"}],"modules":["sshd filter"],"platforms":["Linux","Unix"],"programFiles":["config/filter.d/sshd.conf"],"repo":"https://github.com/fail2ban/fail2ban"}],"references":["https://github.com/fail2ban/fail2ban/commit/ddd879d22664904ace24e1255ac6571808c58187","https://coredump.ws/repos/mail-hardening.git/"],"weaknesses":["CWE-117"],"ingestedAt":"2026-10-29T20:10:33.477Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/109xxx/CVE-2026-109786.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-109786","state":"PUBLISHED","datePublished":"2026-10-13T02:40:13.899Z","dateUpdated":"2026-10-15T15:04:19.106Z","assignerShortName":"mitre","assignerOrgId":"8254265b-2729-46b6-b9e3-3dfca2d5bfca","dateReserved":"2026-10-13T02:40:12.792Z"},"containers":{"cna":{"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 <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."}],"affected":[{"vendor":"fail2ban","product":"fail2ban","defaultStatus":"unknown","versions":[{"version":"0.10.5","status":"affected","lessThanOrEqual":"1.1.1","versionType":"custom"}],"modules":["sshd filter"],"platforms":["Linux","Unix"],"programFiles":["config/filter.d/sshd.conf"],"repo":"https://github.com/fail2ban/fail2ban"}],"references":[{"url":"https://github.com/fail2ban/fail2ban/commit/ddd879d22664904ace24e1255ac6571808c58187","tags":["patch"]},{"url":"https://coredump.ws/repos/mail-hardening.git/","tags":["related"]}],"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":{"shortName":"mitre","orgId":"8254265b-2729-46b6-b9e3-3dfca2d5bfca","dateUpdated":"2026-10-13T02:40:13.899Z"},"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's 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's sshd-session."}],"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's 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's own addresses to ignoreip."}],"x_generator":{"engine":"CVE-Request-form 0.0.1"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:a:fail2ban:fail2ban:*:*:*:*:*:*:*:*","versionStartIncluding":"0.10.5","versionEndIncluding":"1.1.1"}]}]}]},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-10-15T15:03:56.691809Z","id":"CVE-2026-109786","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-15T15:04:19.106Z"}}]}}}