Executive brief
Apache Tomcat is a widely used web server and application container for Java applications. A security flaw in its Digest authentication mechanism allows unauthorized individuals to log in as any non-existent user by providing a specific password. This could allow attackers to bypass security controls and gain unauthorized access to protected web resources and data.
Technical details
An authentication bypass vulnerability exists in Apache Tomcat's Digest authenticator. When Digest authentication is enabled, the system fails to correctly handle users not present in the configured Realm. If an attacker attempts to authenticate as a non-existent user and provides the string "null" as the password, the authenticator incorrectly validates the request. This allows a remote, unauthenticated attacker to bypass security constraints and gain access to protected resources. The issue is rooted in improper handling of null return values from the user realm during the digest calculation process. Patches are available in versions 11.0.22, 10.1.55, and 9.0.118.
Affected products
- Apache Tomcat 11.0.0-M1 to 11.0.21, 10.1.0-M1 to 10.1.54, 9.0.0.M1 to 9.0.117, 8.5.0 to 8.5.100, 7.0.0 to 7.0.109
Timeline
- 2026-05-12: disclosed
- 2026-05-12: advisory
- 2026-05-18: patched: GitHub advisory reviewed and updated with patch information
References
- https://lists.apache.org/thread/7x09x7o12solvclslw3sz0288xc8wx73
- http://www.openwall.com/lists/oss-security/2026/05/12/8
- https://github.com/apache/tomcat/commit/3d4d3fae07a6cd9c2eb193c5491001740ec64448
- https://github.com/apache/tomcat/commit/6565a6cb6499e56fe2f34457cec99f9d1c4f39e9
- https://github.com/apache/tomcat/commit/a99c355e8199adbfd67c9a1fffbd85b810b196cd