CVE-2026-109786 Business logic in fail2ban
Remote attackers can ban arbitrary IP addresses by manipulating SSH login usernames, causing denial of service for legitimate users.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-109786?
CVE-2026-109786 is a medium-severity vulnerability in fail2ban, published on 13 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-117
Which versions are affected by CVE-2026-109786?
How do I fix CVE-2026-109786?
No fix has been published for CVE-2026-109786 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-109786 being exploited?
There is no known exploitation of CVE-2026-109786 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-109786, GHSA-xw36-3g63-5wfw, UBUNTU-CVE-2026-109786.