The vulnerability stems from improper neutralization of user-controlled parameters (Locale/DropdownField and FailedLoginCount/NumericField) in form validation messages. SilverStripe's security advisory explicitly cites these FormField subclasses as vulnerable points where validation responses containing user input were rendered without proper encoding. The XSS triggers when invalid input is reflected in error messages, making the validate() methods of these classes the logical vulnerable functions responsible for handling and displaying unsanitized user input.