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 '<applet>' is used to define the applet inserted in webpage. Attackers can use '<applet' to insert malicious applet to harm users. This rule detects '<applet' in HTTP request. This rule supports to defend the A3: Injection of OWASP Top 10 - 2021. Other reference:None