Cross-site scripting(XSS) is a type of computer security vulnerability tipically found in Web application. XSS enables attackers to inject client-side scripts into Web pages viewed by other users. HTML tag '<form>' is used to collect user's input. Attackers can use '<form>' to trick users to input username and password, then submit them to a malicious server. This rule detects '<form>' in HTTP request. This rule supports to defend the A3: Injection of OWASP Top 10 - 2021. Other reference:None