CVE-2026-109786

Arbitrary IP Banning via Username Injection in fail2ban SSHD Filter

Published Oct 13, 2026 CVSS 3.1 · 5.3 Medium EPSS < 1% KEV · Not listed
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.

Remediation

Vendor Solution

Apply upstream commit ddd879d22664904ace24e1255ac6571808c58187, or upgrade to the first fail2ban release after 1.1.1 that contains it.


Vendor Workaround

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.

Tracking

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Advisories

No advisories available
History
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
Title Arbitrary IP Banning via Username Injection in fail2ban SSHD Filter
Type Values Removed Values Added
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.
First Time appeared Fail2ban
Fail2ban fail2ban
Weaknesses CWE-117
CPEs cpe:2.3:a:fail2ban:fail2ban:*:*:*:*:*:*:*:*
Vendors & Products Fail2ban
Fail2ban fail2ban
References
Metrics cvssV3_1

{'score': 5.3, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
mitre
Published
Updated
Oct 15, 2026, 3:04 PM
Reserved
Oct 13, 2026, 2:40 AM
Status
Awaiting Analysis
Published
Oct 13, 2026, 3:16 AM
Modified
Oct 16, 2026, 7:55 PM