Bug 2550173 (CVE-2026-109786)

Summary: CVE-2026-109786 fail2ban: fail2ban: Denial of Service via improper host parsing in sshd filter
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
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Priority: medium    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in fail2ban. The OpenSSH service filter improperly parses log entries when identifying offending network addresses. A remote, unauthenticated attacker can supply a crafted username during login attempts, causing the service to associate failed authentication events with an arbitrary source IP address. Consequently, this flaw can lead to the automated banning of legitimate users or systems, resulting in a Denial of Service (DoS).
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Depends On: 2550189, 2550190    
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Description OSIDB Bzimport 2026-10-13 02:51:23 UTC 
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.