Medium [CVE-2026-52761] Web Application Firewall rules bypass due to incorrect UTF-8 to Unicode transformation

ModSecurity is an open source, cross platform web application firewall (WAF) engine for Apache, IIS and Nginx.

From 3.0.0 through 3.0.15, the t:utf8toUnicode transformation in src/actions/transformations/utf8_to_unicode.cc produces wrong output on i386 architecture because snprintf uses sizeof on a char pointer rather than the length of the unicode buffer, allowing rules that use this transformation to be bypassed on i386 architecture.

This issue is fixed in version 3.0.16. The `t:utf8toUnicode` transformation function, responsible for converting UTF-8 encoded characters to Unicode, generates incorrect output on i386 architectures.

This vulnerability allows an attacker to bypass WAF rules, potentially enabling malicious requests to reach the protected application. The consequence is a moderate integrity loss due to the circumvention of security controls.

Conditions for Exploitation: Exploitation is strictly limited to 32-bit (i386) architectures, which reduces the likelihood of exploitation in typical deployments. Furthermore, it requires the ModSecurity configuration to actively use rules that rely on the affected transformation function.

Impact Limitations: The vulnerability functions solely as a WAF rule bypass and does not directly cause privilege escalation, data corruption, or arbitrary code execution on the host system.