CVE-2025-31651 is a critical security vulnerability identified in the Apache Software Foundation's Apache Tomcat server, affecting multiple versions including 8.5.0 through 8.5.100, 9.0.0.M1 through 9.0.102, 10.1.0-M1 through 10.1.39, and 11.0.0-M1 through 11.0.5. The vulnerability is classified under CWE-116, which pertains to improper encoding or escaping of output. Specifically, this flaw arises from improper neutralization of escape, meta, or control sequences within certain rewrite rule configurations. In these configurations, a specially crafted HTTP request can bypass some rewrite rules that are intended to enforce security constraints. This bypass could allow an attacker to circumvent access controls or other security policies implemented via rewrite rules, potentially leading to unauthorized access or manipulation of web applications hosted on Tomcat. The vulnerability has a CVSS v3.1 base score of 9.8, indicating a critical severity level, with an attack vector that is network-based, requiring no privileges or user interaction, and impacting confidentiality, integrity, and availability. Although no known exploits are currently reported in the wild, the ease of exploitation and the critical impact make this a significant threat. Users are strongly advised to upgrade to the fixed version once available to mitigate this vulnerability. The issue affects both current and some end-of-life versions, highlighting the importance of patching even older deployments where feasible.

For European organizations, the impact of CVE-2025-31651 can be substantial. Apache Tomcat is widely used across Europe in enterprise environments, government agencies, financial institutions, and critical infrastructure sectors to serve Java-based web applications. The ability to bypass rewrite rules that enforce security constraints could lead to unauthorized data access, data manipulation, or service disruption. Confidentiality breaches could expose sensitive personal data protected under GDPR, leading to regulatory penalties and reputational damage. Integrity violations could allow attackers to alter application behavior or data, potentially undermining business operations or enabling further attacks. Availability impacts could disrupt essential services, especially in sectors like healthcare, finance, and public administration. Given the vulnerability requires no authentication or user interaction and can be exploited remotely, the risk of automated scanning and exploitation attempts is high. This elevates the threat level for organizations that have not yet applied patches or mitigations, particularly those with internet-facing Tomcat servers or complex rewrite rule configurations.

Beyond the essential step of upgrading Apache Tomcat to the fixed version once released, European organizations should implement several targeted mitigations: 1) Conduct a thorough audit of all rewrite rules in use to identify and remediate any configurations that could be bypassed, applying stricter validation and sanitization of inputs. 2) Employ Web Application Firewalls (WAFs) configured to detect and block anomalous or suspicious requests that attempt to exploit rewrite rule bypasses. 3) Restrict access to Tomcat management and administrative interfaces through network segmentation, IP whitelisting, or VPN access to reduce exposure. 4) Enable detailed logging and monitoring of HTTP requests and rewrite rule processing to detect potential exploitation attempts early. 5) Implement defense-in-depth by combining Tomcat-level security with application-level authentication and authorization controls, ensuring that even if rewrite rules are bypassed, other security layers prevent unauthorized access. 6) Regularly update and patch all components of the web application stack to minimize the attack surface. 7) Educate development and operations teams about secure rewrite rule practices and the risks of improper encoding or escaping.