Executive brief
Apache Tomcat is a widely used web server and application container. A vulnerability in its LockOutRealm component, which is responsible for preventing brute-force login attempts, fails to handle username case sensitivity correctly. This could allow an attacker to bypass account lockout protections or potentially gain unauthorized access by manipulating the capitalization of usernames.
Technical details
A vulnerability classified as CWE-178 (Improper Handling of Case Sensitivity) exists in the LockOutRealm component of Apache Tomcat. The component treats usernames as case-sensitive when tracking failed login attempts, which may conflict with the underlying authentication realm's behavior if that realm is case-insensitive. An unauthenticated remote attacker can exploit this by varying the case of a username to bypass lockout thresholds or potentially access accounts if the primary authentication mechanism ignores case but the lockout mechanism does not. The issue is resolved by adding a case-sensitive attribute to LockOutRealm configuration in patched versions.
Affected products
- Apache Tomcat 11.0.0-M1 through 11.0.21, 10.1.0-M1 through 10.1.54, 9.0.0.M1 through 9.0.117, 8.5.0 through 8.5.100, 7.0.0 through 7.0.109
Timeline
- 2026-05-12: disclosed
- 2026-05-12: advisory
- 2026-05-18: patched: GitHub advisory updated with patch details
References
- https://lists.apache.org/thread/ytjcgldshj73lcnd1sh95od5hrghwogp
- http://www.openwall.com/lists/oss-security/2026/05/12/9
- https://github.com/apache/tomcat/commit/4a90d3fa93988c447cd5bb7482f76ff70d7f15c2
- https://github.com/apache/tomcat/commit/6dd75beb55bd42fc5f78e929596b25018cd17717
- https://github.com/apache/tomcat/commit/83f3e51df7b87f5f6e626951c575ded1a512e8ef