Web application penetration testing is the practice of simulating attacks on a system in an attempt to gain access to sensitive data, with the purpose of determining whether a system is secure. These attacks are performed either internally or externally on a system, and they help provide information about the target system, identify vulnerabilities within them, and uncover exploits that could actually compromise the system. It is an essential health check of a system that informs testers whether remediation and security measures are needed.