Critical Undertow Vulnerability Exposes User Sessions in Multiple Ubuntu Releases

A security vulnerability has been identified in the Undertow Java web server affecting multiple Ubuntu releases, including 24.04 LTS, 22.04 LTS, 20.04 LTS, 18.04 LTS, and 16.04 LTS. The flaw arises from improper validation of the Host header in incoming HTTP requests, potentially allowing remote attackers to gain unintended access to user sessions. This vulnerability is tracked as CVE-2025-12543, published on January 7, 2026. Users are advised to update their systems to specific package versions to mitigate the risk. The affected package versions are available through Ubuntu Pro, which offers extended security maintenance. A standard system update will apply the necessary changes to address this issue. The vulnerability poses a significant risk due to the potential for unauthorized access to sensitive user data.

Key Points: • Undertow vulnerability allows remote access to user sessions on multiple Ubuntu LTS versions. • Affected versions include Ubuntu 24.04, 22.04, 20.04, 18.04, and 16.04 LTS. • Users should update to specified package versions to mitigate the risk of exploitation.