Vulnerability · Published Oct 13, 2026 · Assigned by mitre
CVE-2026-109786
Medium 5.3
In plain English
A medium-severity security flaw in fail2ban from fail2ban. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 24% of all known vulnerabilities).
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | fail2ban | no fix listed | DEBIAN-CVE-2026-109786 |
| Debian:13 | fail2ban | no fix listed | DEBIAN-CVE-2026-109786 |
| Debian:14 | fail2ban | no fix listed | DEBIAN-CVE-2026-109786 |
| Ubuntu:22.04:LTS | fail2ban | no fix listed | UBUNTU-CVE-2026-109786 |
| Ubuntu:24.04:LTS | fail2ban | no fix listed | UBUNTU-CVE-2026-109786 |
| Ubuntu:26.04:LTS | fail2ban | no fix listed | UBUNTU-CVE-2026-109786 |
| Ubuntu:26.10 | fail2ban | no fix listed | UBUNTU-CVE-2026-109786 |
Affected products (as reported)
Weakness: CWE-117 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L