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TI-2026-71528 Aliases CVE-2026-109786

Log injection in fail2ban sshd filter enables IP ban abuse

Priority LOW 30CWE-117Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Impact type
—
Confidentiality · integrity · availability
None · None · Low
Fix status
No fix listed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA.

Summary

fail2ban from 0.10.5 through 1.1.1 before ddd879d accepts an "on <token>" suffix after the host address in its sshd filter, and the default filter mode is affected. Because OpenSSH logs offered usernames verbatim, the filter fails to neutralize log output (CWE-117), so crafted input is misread. A remote unauthenticated attacker can supply a crafted username so that failed logins are attributed to an arbitrary source IP address and banned, blocking that address's legitimate logins.

Written with Claude Haiku 5.5 from the sources below · 2026-10-27

Description from the CNA

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00314 24th percentileScored 2026-11-07
Fix No fix listed No fixed version in the sources yet

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
Yes
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
fail2banfail2ban0.10.5 – 1.1.1—No fix listedCNA
  • fail2ban Vendorfail2ban Affected 0.10.5 – 1.1.1 Fixed in — No fix listed CNA

Remediation

The sources list no fixed version for fail2ban.

Written with Claude Haiku 5.5 from the sources below · 2026-10-27

Vendor assessments

Red Hat

Red Hat's page for this CVE →

Not affected: 1 product
  • Red Hat

Timeline

  1. 2026-10-13 Threatist record created from CVE-2026-109786
  2. 2026-10-13 Affected versions changed · Red Hat
  3. 2026-10-13 CVSS score added · Red Hat
  4. 2026-10-16 CISA SSVC decision points changed

References

Patch 1

Other 1

  • TI-2021-12417 Possible RCE vulnerability in mailing action using mailutils (mail-whois) CVSS 3.1HIGH 8.1
  • TI-2007-04181 fail2ban 0.8 and earlier does not properly parse sshd log files, which allows remote… CVSS 2.0MEDIUM 6.8
  • TI-2012-04743 server/action.py in Fail2ban before 0.8.8 does not properly handle the content of the… CVSS 2.0HIGH 7.5
  • TI-2013-05868 config/filter.d/cyrus-imap.conf in the cyrus-imap filter in Fail2ban before 0.8.11 allows… CVSS 2.0MEDIUM 5.0
  • TI-2013-05867 config/filter.d/postfix.conf in the postfix filter in Fail2ban before 0.8.11 allows… CVSS 2.0MEDIUM 5.0
  • TI-2013-01945 The apache-auth.conf, apache-nohome.conf, apache-noscript.conf, and apache-overflows.conf… CVSS 2.0MEDIUM 5.0
See all 9 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences