CVE-2022-39957 is a high-severity vulnerability affecting the OWASP ModSecurity Core Rule Set (CRS), specifically versions 3.0.x, 3.1.x, 3.2.1, and 3.3.2. The vulnerability stems from a protection mechanism failure (CWE-693) related to how the CRS processes HTTP response bodies when the client includes an HTTP Accept header with an optional "charset" parameter. This parameter can cause the web application firewall (WAF) to receive the response in an encoded form that the CRS cannot decode properly. As a result, the WAF may fail to detect access to restricted resources that it would normally block or log. This bypass undermines the CRS's ability to enforce security policies effectively, potentially allowing attackers to access sensitive resources without triggering alerts or blocks. The issue affects legacy and currently supported CRS versions, with recommended upgrades to versions 3.2.2 and 3.3.3 to mitigate the vulnerability. The CVSS v3.1 base score is 7.3, indicating a high severity, with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), and impacts on confidentiality, integrity, and availability (C:L/I:L/A:L). No known exploits are reported in the wild as of the publication date, but the vulnerability presents a significant risk due to its potential to bypass WAF protections silently.

For European organizations, this vulnerability poses a substantial risk, especially for those relying on the OWASP ModSecurity CRS to protect web applications from attacks such as unauthorized access, data leakage, and injection attacks. The bypass can allow attackers to access restricted resources without detection, potentially leading to data breaches, unauthorized data modification, or service disruption. Organizations in sectors with strict data protection regulations, such as finance, healthcare, and government, could face compliance violations and reputational damage if exploited. The vulnerability's ability to affect confidentiality, integrity, and availability simultaneously increases the risk profile. Since the CRS is widely used as a community-driven WAF rule set integrated into various ModSecurity deployments, many European enterprises and service providers could be impacted if they have not applied the recommended updates. The lack of required privileges or user interaction for exploitation further increases the threat, as attackers can remotely exploit this vulnerability without authentication.

European organizations should prioritize upgrading their OWASP ModSecurity CRS installations to versions 3.2.2 or 3.3.3, which contain fixes addressing this response body bypass vulnerability. It is critical to verify that all ModSecurity deployments using affected CRS versions are identified through asset inventories and patch management systems. Additionally, organizations should audit their WAF configurations to ensure that response body inspection is enabled and functioning correctly, and consider implementing additional security layers such as runtime application self-protection (RASP) or enhanced logging to detect anomalous access patterns. Network-level monitoring and anomaly detection systems should be tuned to identify unusual Accept header usage or encoded response patterns that could indicate exploitation attempts. Regular security assessments and penetration tests should include checks for this bypass scenario. Finally, organizations should maintain awareness of updates from OWASP and ModSecurity communities and subscribe to relevant vulnerability advisories to respond promptly to emerging threats.