CVE-2025-54571 is a vulnerability identified in the OWASP ModSecurity web application firewall (WAF) engine, affecting versions 2.9.11 and earlier. ModSecurity is deployed across major web servers including Apache, IIS, and Nginx, serving as a critical security layer for web applications. The vulnerability stems from an unchecked return value in the code handling HTTP response headers, specifically allowing an attacker to override the Content-Type header of HTTP responses. This flaw is categorized under CWE-252, which involves failure to check the return value of a function, leading to improper handling of security-critical operations. By manipulating the Content-Type header, an attacker can induce browsers to interpret responses in unintended ways, enabling cross-site scripting (XSS) attacks or disclosure of arbitrary script source code. These attacks can compromise the confidentiality and integrity of client-side data and potentially lead to further exploitation of the victim’s environment. The vulnerability is remotely exploitable without requiring authentication or user interaction, increasing its risk profile. Although no active exploits have been reported, the issue is addressed in ModSecurity version 2.9.12. The CVSS v4.0 score of 6.9 reflects a medium severity, driven by network attack vector, low complexity, and no privileges or user interaction needed. The scope is local to the client side, affecting the confidentiality of data processed by the browser. This vulnerability highlights the importance of rigorous input validation and error handling in security middleware components.

For European organizations, this vulnerability poses a significant risk to web application security, especially those relying on ModSecurity as a frontline defense. Successful exploitation could lead to client-side XSS attacks, enabling attackers to steal session tokens, perform unauthorized actions on behalf of users, or disclose sensitive script code. This undermines user trust and may lead to data breaches involving personal or financial information, contravening GDPR requirements. The impact extends to web service availability indirectly, as exploited XSS can facilitate further attacks or cause reputational damage. Organizations in sectors such as finance, healthcare, e-commerce, and government, which heavily depend on secure web applications, are particularly vulnerable. The lack of authentication or user interaction for exploitation increases the threat surface, making automated attacks feasible. Additionally, compromised client browsers can serve as pivot points for broader network intrusions. Therefore, the vulnerability could have cascading effects on confidentiality, integrity, and indirectly on availability of services.

European organizations should immediately upgrade ModSecurity to version 2.9.12 or later to remediate this vulnerability. In addition to patching, organizations should implement strict validation and sanitization of HTTP headers within their WAF configurations to prevent unauthorized header manipulation. Deploy Content Security Policy (CSP) headers to mitigate the impact of potential XSS attacks by restricting script execution sources. Regularly audit and monitor web traffic for anomalies, specifically looking for unusual Content-Type header changes or suspicious payloads. Employ runtime application self-protection (RASP) tools to detect and block exploitation attempts in real time. Conduct security awareness training for developers and administrators to recognize and respond to WAF misconfigurations. Finally, integrate vulnerability scanning and penetration testing focused on header manipulation and XSS vectors into the security lifecycle to proactively identify similar issues.