CVE-2025-48866: 
ModSecurity vulnerability analysis and mitigation

ModSecurity, an open source cross-platform web application firewall (WAF) engine for Apache, IIS, and Nginx, contains a high-severity denial-of-service vulnerability (CVE-2025-48866) in versions prior to 2.9.10. The vulnerability exists in the sanitiseArg (and its alias sanitizeArg) action, which is vulnerable to adding an excessive number of arguments, leading to denial of service (NVD, GitHub Advisory).

The vulnerability stems from ModSecurity's sanitiseArg action, which is designed to mask sensitive data in audit logs by replacing matched arguments with asterisks. When processing excessive arguments (e.g., 500 parameters), the engine enters a quadratic complexity loop where each argument triggers a sanitization check. For example, 500 arguments result in 500 × 500 = 250,000 memory operations, potentially allocating gigabytes of RAM per request. The vulnerability has been assigned a CVSS v3.1 base score of 7.5 (HIGH) with vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (GBHackers, GitHub Advisory).

Successful exploitation of this vulnerability can lead to denial of service through memory exhaustion (up to 2GB per request) and CPU spikes from the APR table resizing, resulting in service unavailability. The attack does not require authentication and can be executed remotely (GBHackers).

The primary mitigation is to upgrade to ModSecurity version 2.9.10 which contains the fix. If immediate upgrading is not possible, organizations can avoid using rules that contain the sanitiseArg (or sanitizeArg) action. Additional recommendations include implementing rate limiting for POST requests and monitoring memory usage patterns during peak traffic (GitHub Advisory, GBHackers).