Web Application Firewall
A WAF protects web applications from intrusions by analyzing HTTP traffic. It not only rejects suspicious requests, but can also provide real-time alerts and logging of intrusion attempts. XSS, SQL-injection, DDOS and many other forms of attacks are protected against.
A particularly useful feature of a WAF is virtual patching of old and legacy systems that cannot be easily updated. The WAF sits in front of the legacy system analyzing all traffic without needing to touch the end system itself.