ModSecurity is a plugin for Apache web servers that functions as a web application layer firewall. It's used to prevent attacks towards script-driven websites by employing security rules which contain specific expressions. In this way, the firewall can block hacking and spamming attempts and shield even websites which aren't updated regularly. For example, several failed login attempts to a script administrative area or attempts to execute a certain file with the purpose to get access to the script shall trigger specific rules, so ModSecurity shall block these activities the instant it discovers them. The firewall is quite efficient because it screens the entire HTTP traffic to an Internet site in real time without slowing it down, so it can easily stop an attack before any harm is done. It also keeps a very detailed log of all attack attempts which contains more information than typical Apache logs, so you can later check out the data and take additional measures to increase the security of your sites if required.