CVE-2022-39958 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 an incorrect authorization mechanism (CWE-863) that allows an attacker to bypass the web application firewall (WAF) protections by exploiting the handling of the HTTP Range header. By submitting repeated HTTP requests with carefully crafted small byte ranges in the Range header, an attacker can sequentially exfiltrate small portions of a restricted resource from the backend server. This data exfiltration occurs despite the presence of the CRS-based WAF, as the small subsections of data evade pattern matching and detection mechanisms. The vulnerability essentially allows unauthorized access to sensitive backend data that should be protected by the WAF. The issue affects legacy CRS versions 3.0.x and 3.1.x as well as currently supported versions 3.2.1 and 3.3.2. The recommended remediation is to upgrade to CRS versions 3.2.2 or 3.3.3, which include fixes for this vulnerability, and to configure the CRS paranoia level to 3 or higher to enhance detection capabilities. The CVSS 3.1 base score is 7.5, reflecting a network attack vector with low attack complexity, no privileges or user interaction required, and a high impact on confidentiality. There are no known exploits in the wild at the time of publication, but the vulnerability poses a significant risk due to its ability to bypass WAF protections and leak sensitive data incrementally and stealthily.

For European organizations, this vulnerability poses a significant risk to the confidentiality of sensitive data protected by web application firewalls using the OWASP ModSecurity CRS. Many European enterprises, government agencies, and critical infrastructure providers deploy ModSecurity CRS as part of their web security stack. The ability to bypass WAF protections and exfiltrate data in small undetectable chunks could lead to leakage of intellectual property, personal data protected under GDPR, or other sensitive information. This could result in regulatory penalties, reputational damage, and financial losses. The incremental nature of the data exfiltration makes detection difficult, increasing the window of exposure. Organizations relying on legacy CRS versions or those who have not updated to the patched versions remain vulnerable. Additionally, the vulnerability could be leveraged in targeted attacks against high-value European targets, including financial institutions, healthcare providers, and government portals, where data confidentiality is paramount.

European organizations should immediately assess their use of OWASP ModSecurity CRS and identify if they are running affected versions (3.0.x, 3.1.x, 3.2.1, 3.3.2). The primary mitigation is to upgrade to the patched versions 3.2.2 or 3.3.3. Beyond upgrading, organizations should configure the CRS paranoia level to 3 or higher to improve detection of suspicious requests involving Range headers or other evasion techniques. It is advisable to implement strict monitoring and alerting on unusual HTTP Range header usage patterns, such as repeated small byte-range requests from the same source IP. Web server and application logs should be analyzed for anomalies consistent with incremental data exfiltration attempts. Network intrusion detection systems (NIDS) can be tuned to detect suspicious Range header usage. Additionally, organizations should review backend server configurations to limit or disable support for HTTP Range requests where feasible, or implement rate limiting on such requests to reduce the risk of sequential data leakage. Finally, regular security audits and penetration testing should include attempts to exploit this vulnerability to verify the effectiveness of mitigations.