Cross-site scripting (XSS) vulnerabilities allow attackers to inject malicious scripts into web pages viewed by other users. This type of security breach can lead to unauthorized access to user sessions or sensitive information and manipulate client-side scripts. Testing for XSS vulnerabilities is critical to ensure that any input received from the UI is appropriately sanitized, preventing malicious code from executing within the system. This is a fundamental aspect of security testing to protect the integrity and confidentiality of user data and system functionality.