ModSecurity is a plugin for Apache web servers which functions as a web application layer firewall. It's used to stop attacks towards script-driven websites by using security rules that contain certain expressions. That way, the firewall can block hacking and spamming attempts and shield even Internet sites which are not updated on a regular basis. As an example, a number of failed login attempts to a script administrative area or attempts to execute a certain file with the intention to get access to the script shall trigger specific rules, so ModSecurity will block these activities the moment it discovers them. The firewall is extremely efficient because it monitors the entire HTTP traffic to a website in real time without slowing it down, so it will be able to stop an attack before any harm is done. It also keeps an incredibly comprehensive log of all attack attempts which contains more information than traditional Apache logs, so you can later check out the data and take further measures to increase the security of your websites if required.