CVE Catalog

CVE-2026-109786

MediumCVSS 5.3
Published: Updated: Translated: NVD NIST

Exploitation Probability (EPSS)

Low risk
0.31%

23th percentile - higher than 23% of all known CVEs

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.

Risk Assessment

An attacker can cause legitimate users or IP addresses to be banned, leading to service unavailability and potential DoS attacks.

Recommendation

Update fail2ban to a patched version. Additionally, consider modifying the sshd filter configuration to not accept the "on" suffix.

Related vulnerabilities

Original NVD description (English source)

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.

Vulnerability data from NVD (NIST) · CISA KEV · EPSS