The vulnerability stems from improper neutralization of user input in the login form. The provided POC shows a POST request with an XSS payload in the 'module' parameter being reflected in the response. While the exact code isn't available, XSS in login forms typically occurs when user-controlled input is echoed back without escaping (e.g., in error messages or form repopulation). The confidence is medium because the description explicitly implicates the login form, and the attack pattern matches unescaped output of request parameters, but without patch details or code access, we infer based on standard PHP CMS patterns.