CVE-2022-39956 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 arises from incorrect authorization handling (CWE-863) that allows a partial bypass of the CRS when processing HTTP multipart requests. Attackers can craft multipart payloads using specific character encoding schemes via the Content-Type or the deprecated Content-Transfer-Encoding MIME headers. These encodings are not properly decoded or inspected by the ModSecurity engine and its rule set, allowing malicious payloads to evade detection. This bypass can be exploited if the backend application supports these encoding schemes, potentially leading to unauthorized access or manipulation of data. The vulnerability affects both legacy and currently supported CRS versions, with mitigations available by upgrading to CRS versions 3.2.2 and 3.3.3 and ensuring the underlying ModSecurity engine is updated to versions 2.9.6 or 3.0.8. The CVSS v3.1 score of 7.3 reflects the network attack vector, low attack complexity, no privileges or user interaction required, and impacts on confidentiality, integrity, and availability. No known exploits are reported in the wild yet, but the vulnerability poses a significant risk due to its ability to bypass web application firewall protections, which are critical for defending web applications against various attacks.

For European organizations, this vulnerability can have serious implications. Many enterprises and public sector entities rely on ModSecurity with the OWASP CRS as a frontline defense to protect web applications from injection attacks, data exfiltration, and unauthorized access. A successful bypass could allow attackers to deliver malicious payloads undetected, potentially leading to data breaches, service disruptions, or unauthorized data manipulation. This is particularly critical for sectors handling sensitive personal data under GDPR, such as finance, healthcare, and government services. The ability to bypass WAF protections undermines trust in security controls and increases the risk of compliance violations and reputational damage. Additionally, the vulnerability could be leveraged in targeted attacks against European organizations with web applications that accept multipart HTTP requests, such as file upload functionalities, increasing the attack surface.

To mitigate this vulnerability effectively, European organizations should: 1) Immediately upgrade the OWASP ModSecurity Core Rule Set to versions 3.2.2 or 3.3.3, which contain fixes addressing this bypass. 2) Ensure the underlying ModSecurity engine is updated to at least version 2.9.6 or 3.0.8 to support the patched CRS versions properly. 3) Review and harden web application configurations to restrict or validate multipart request encodings, especially those using Content-Type or Content-Transfer-Encoding headers that are deprecated or uncommon. 4) Implement additional monitoring and logging for multipart HTTP requests to detect anomalous encoding patterns that could indicate exploitation attempts. 5) Conduct thorough security testing, including fuzzing multipart request handling, to identify any residual weaknesses. 6) Coordinate with application developers to validate backend support for multipart encodings and apply input validation or sanitization as needed. 7) Maintain an incident response plan that includes scenarios involving WAF bypasses to respond swiftly if exploitation is detected.