ModSecurity, often just called ModSec, is an open-source web application firewall (WAF) that checks all incoming HTTP requests, and their corresponding responses, to a website against various criteria. Among other things, this checks for the following:
Suspicious spam keywords such as drug names, known malicious URLs or similar.
Known patterns for exploiting vulnerabilities (cross-site scripting (XSS), SQL injection, command injection, etc.).
Check for incorrect logins to websites.
Here you can see which websites are actively protected by ModSecurity.
