ModSecurity is a plugin for Apache web servers that functions as a web application layer firewall. It's employed to prevent attacks against script-driven websites through the use of security rules which contain specific expressions. This way, the firewall can prevent hacking and spamming attempts and protect even websites which are not updated often. For instance, several failed login attempts to a script administrative area or attempts to execute a specific file with the objective to get access to the script shall trigger specific rules, so ModSecurity shall stop these activities the minute it detects them. The firewall is quite efficient as it monitors the whole HTTP traffic to an Internet site in real time without slowing it down, so it could prevent an attack before any harm is done. It furthermore maintains a very comprehensive log of all attack attempts which contains more info than standard Apache logs, so you could later examine the data and take additional measures to improve the security of your Internet sites if needed.