CVE-2025-55668 is a session fixation vulnerability identified in the Apache Tomcat server, specifically affecting versions from 11.0.0-M1 through 11.0.7, 10.1.0-M1 through 10.1.41, and 9.0.0.M1 through 9.0.105. The vulnerability arises due to improper handling of session identifiers in the rewrite valve component of Apache Tomcat. Session fixation occurs when an attacker is able to set or fixate a user's session ID before the user logs in, allowing the attacker to hijack the authenticated session once the user logs in. This type of vulnerability undermines the integrity of session management, potentially allowing unauthorized access to user accounts or sensitive data. The rewrite valve is a feature in Tomcat that allows URL rewriting and manipulation of request parameters, and improper session ID management here can lead to the fixation issue. Although no known exploits are currently reported in the wild, the vulnerability is publicly disclosed and fixed in Apache Tomcat versions 11.0.8, 10.1.42, and 9.0.106. Older or end-of-life versions may remain vulnerable. The lack of a CVSS score indicates that the vulnerability is newly published and has not yet been fully assessed for severity, but the nature of session fixation vulnerabilities generally poses a significant risk to web applications relying on Tomcat for session management.

For European organizations, the impact of this vulnerability can be substantial, especially for those relying on Apache Tomcat as a core component of their web infrastructure. Exploitation of session fixation can lead to unauthorized access to user sessions, potentially exposing sensitive personal data, financial information, or internal business data. This can result in breaches of GDPR compliance, leading to regulatory fines and reputational damage. Additionally, compromised sessions can facilitate further attacks such as privilege escalation or lateral movement within the network. Organizations in sectors such as finance, healthcare, government, and e-commerce, which often use Tomcat for hosting critical web applications, are particularly at risk. The absence of known exploits in the wild currently reduces immediate risk, but the public disclosure increases the likelihood of future exploitation attempts, especially if patches are not applied promptly.

European organizations should prioritize upgrading affected Apache Tomcat instances to the fixed versions: 11.0.8, 10.1.42, or 9.0.106. For environments where immediate upgrade is not feasible, organizations should review and harden session management configurations, including enforcing regeneration of session IDs upon authentication and ensuring that session IDs are not accepted from untrusted sources or URLs manipulated by the rewrite valve. Implementing web application firewalls (WAFs) with rules to detect and block suspicious session fixation attempts can provide additional protection. Regular security audits and penetration testing focused on session management can help identify residual risks. Monitoring logs for unusual session activity and educating developers and administrators about secure session handling best practices are also recommended. Finally, organizations should maintain an inventory of Tomcat versions in use and establish patch management processes to ensure timely updates.