Increasingly, computer attackers are exploiting flaws in Web applications, exposing enterprises to significant threats, including Personally Identifiable Information breaches and uploads of malware onto vulnerable corporate Websites for distribution to customer browsers. Many of these Web application vulnerabilities are a direct result of improper input validation and output filtering, which leads to numerous kinds of attacks, including cross-site scripting (XSS), SQL injection, command injection, buffer overflows and many others. This article describes some of the best defenses against such attacks, which every Web application developer should master.