CVE-2025-54571 is a vulnerability identified in the open source web application firewall (WAF) engine ModSecurity, specifically affecting versions prior to 2.9.12. ModSecurity is widely used across Apache, IIS, and Nginx web servers to provide security by filtering and monitoring HTTP traffic. The vulnerability stems from an unchecked return value issue (CWE-252) that allows an attacker to override the HTTP response's Content-Type header. This manipulation can lead to multiple security issues depending on the HTTP context, including Cross-Site Scripting (XSS) attacks and arbitrary disclosure of script source code. Essentially, by controlling the Content-Type header, an attacker can trick the browser into interpreting the response in a way that executes malicious scripts or exposes sensitive code. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. The CVSS 4.0 base score is 6.9 (medium severity), reflecting the network attack vector, low complexity, and no privileges or user interaction required. However, the impact on confidentiality is limited to script source disclosure, and integrity and availability impacts are not evident. No known exploits are currently reported in the wild, and the issue is resolved in ModSecurity version 2.9.12.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on ModSecurity as part of their web application security stack. Exploitation could lead to client-side attacks such as XSS, which may compromise user sessions, steal credentials, or facilitate phishing and malware distribution. Disclosure of arbitrary script source code could aid attackers in crafting more effective attacks against the web applications protected by the WAF. This risk is heightened for organizations handling sensitive personal data under GDPR, as successful exploitation could lead to data breaches and regulatory penalties. Additionally, public-facing web services in sectors like finance, healthcare, and government are particularly at risk due to the potential reputational damage and operational disruption caused by such attacks. Since ModSecurity is deployed on major web servers, the scope of affected systems is broad, increasing the potential attack surface across European enterprises.

European organizations should prioritize upgrading ModSecurity to version 2.9.12 or later to remediate this vulnerability. In addition to patching, organizations should implement strict Content Security Policies (CSP) to mitigate the impact of potential XSS attacks by restricting the execution of unauthorized scripts. Web application developers should review and sanitize all user inputs and outputs rigorously. Network-level protections such as Web Application Firewalls (WAFs) should be configured to detect and block anomalous HTTP header manipulations. Regular security audits and penetration testing focusing on HTTP response headers and client-side security controls are recommended. Monitoring web server logs for unusual Content-Type header changes or suspicious traffic patterns can help in early detection of exploitation attempts. Finally, organizations should ensure their incident response plans include scenarios involving client-side attacks facilitated by WAF misconfigurations or vulnerabilities.