Executive brief
ModSecurity is a widely used web application firewall (WAF) that protects web servers like Apache and Nginx from malicious traffic. A flaw in how the firewall processes certain data transformations allows an attacker to crash the firewall's worker processes by sending a specially crafted web request. This results in a denial of service, preventing legitimate users from accessing the protected website until the attack stops and the processes restart.
Technical details
A vulnerability exists in libModSecurity3 (ModSecurity v3) due to an out-of-bounds read (CWE-125) in the hex_decode.cc component. When a rule is configured to use the t:hexDecode transformation, the engine fails to properly handle query string parameters containing only a single character. This leads to a segmentation fault in the modsecurity::utils::string::x2c function during processing. A remote, unauthenticated attacker can exploit this by sending repeated HTTP requests with a single-character parameter to crash WAF worker processes, resulting in a denial of service (DoS). This issue is resolved in version 3.0.15; ModSecurity v2 is not affected.
Affected products
- OWASP libModSecurity3 < 3.0.15
Timeline
- 2026-04-28: patched: Version 3.0.15 released
- 2026-05-05: advisory: NVD and GitHub advisories published