CVE-2025-31651 is a critical security vulnerability classified under CWE-116 (Improper Encoding or Escaping of Output) affecting multiple versions of Apache Tomcat, specifically from 8.5.0 through 11.0.5. The flaw arises from improper neutralization of escape, meta, or control sequences in the handling of rewrite rules. For a subset of uncommon rewrite rule configurations, an attacker can craft specially designed HTTP requests that bypass these rules. Since rewrite rules are often used to enforce security constraints such as access controls or URL filtering, bypassing them can lead to unauthorized access or exposure of sensitive resources. The vulnerability does not require any authentication or user interaction, and the attack vector is network-based, making it remotely exploitable. The CVSS v3.1 score of 9.8 reflects the critical nature of this vulnerability, impacting confidentiality, integrity, and availability. Affected versions include all releases from 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. Although no public exploits have been reported yet, the vulnerability's characteristics suggest that exploitation could lead to significant security breaches. The Apache Software Foundation has released fixed versions to address this issue, and users are strongly advised to upgrade. This vulnerability is particularly relevant for organizations relying on Apache Tomcat for web application hosting, especially those using rewrite rules to enforce security policies.

The impact of CVE-2025-31651 on European organizations can be severe due to the widespread use of Apache Tomcat in enterprise, government, and public sector web applications. Successful exploitation allows attackers to bypass rewrite rules that enforce security constraints, potentially leading to unauthorized access to sensitive data, privilege escalation, or disruption of services. This can compromise confidentiality, integrity, and availability of critical systems. In sectors such as finance, healthcare, and government, where data protection regulations like GDPR impose strict requirements, such breaches could result in legal penalties, reputational damage, and financial losses. Additionally, the vulnerability’s remote exploitability without authentication increases the risk of automated attacks and large-scale scanning campaigns targeting vulnerable Tomcat instances. European organizations with complex rewrite rule configurations are at higher risk, as these configurations may inadvertently enable the bypass. The lack of known exploits in the wild currently provides a window for proactive mitigation, but the critical severity demands immediate attention to prevent potential exploitation.

To mitigate CVE-2025-31651, European organizations should take the following specific actions: 1) Immediately identify all Apache Tomcat instances in use, including legacy and EOL versions, and prioritize upgrading to the latest patched versions provided by the Apache Software Foundation. 2) Review and audit all rewrite rule configurations to identify any that could be susceptible to bypass, especially those enforcing critical security constraints. 3) Implement strict input validation and output encoding practices in web applications to reduce reliance solely on rewrite rules for security enforcement. 4) Deploy web application firewalls (WAFs) with updated signatures capable of detecting and blocking suspicious requests that attempt to exploit rewrite rule bypasses. 5) Monitor web server logs for anomalous or malformed requests that could indicate exploitation attempts. 6) Establish network segmentation and least privilege principles to limit the impact of any potential compromise. 7) Educate development and operations teams about the risks associated with rewrite rule misconfigurations and the importance of timely patching. 8) For organizations unable to upgrade immediately, consider temporary mitigations such as disabling or simplifying rewrite rules that enforce security constraints until patches can be applied.