CVE-2025-31651 is a critical vulnerability in the Apache Software Foundation's Apache Tomcat server, identified as CWE-116: Improper Encoding or Escaping of Output. This vulnerability arises from improper neutralization of escape, meta, or control sequences in certain rewrite rule configurations. Specifically, for a subset of unlikely rewrite rule setups, 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 security policies implemented via rewrite rules, potentially leading to unauthorized access or manipulation of web application behavior. The affected Apache Tomcat versions include 11.0.0-M1 through 11.0.5, 10.1.0-M1 through 10.1.39, and 9.0.0.M1 through 9.0.102, with older end-of-life versions possibly also vulnerable. The vulnerability has a CVSS 3.1 base score of 9.8, indicating critical severity, with attack vector network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), and high impact on confidentiality, integrity, and availability (C:H/I:H/A:H). No known exploits have been reported in the wild yet, but the potential impact is severe. Users are strongly recommended to upgrade to the fixed version once available to mitigate this issue.

For European organizations, this vulnerability poses a significant risk due to the widespread use of Apache Tomcat as a Java servlet container in enterprise web applications and services. Successful exploitation could allow attackers to bypass security constraints, potentially leading to unauthorized data access, data modification, or service disruption. This could result in breaches of sensitive personal data protected under GDPR, leading to regulatory penalties and reputational damage. Critical infrastructure, financial institutions, healthcare providers, and government agencies using affected Tomcat versions are particularly at risk. The ability to exploit this vulnerability remotely without authentication and user interaction increases the threat level, making it feasible for attackers to launch automated attacks at scale. The bypass of rewrite rules may also facilitate further attacks such as privilege escalation, injection attacks, or lateral movement within networks, amplifying the overall impact.

1. Immediate upgrade to the fixed Apache Tomcat version once officially released by the Apache Software Foundation. Monitor Apache security advisories closely for the patch release. 2. In the interim, review and tighten rewrite rule configurations to minimize complexity and avoid configurations that could be bypassed; consider disabling rewrite rules if not essential. 3. Employ web application firewalls (WAFs) with custom rules to detect and block suspicious requests that attempt to exploit rewrite rule bypasses. 4. Conduct thorough security testing and code review of web applications relying on Tomcat rewrite rules to identify potential security constraints that could be circumvented. 5. Implement network segmentation and strict access controls to limit exposure of Tomcat servers to untrusted networks. 6. Enable detailed logging and monitoring of Tomcat server access and rewrite rule processing to detect anomalous activities indicative of exploitation attempts. 7. Educate security teams and developers about the nature of CWE-116 vulnerabilities and the importance of proper encoding and escaping in web applications.