The vulnerability stems from improper sanitization of user-controlled input (page titles) in tcemain flash messages. The DataHandler (t3lib_tcemain.php) processes backend operations like copy/move, which includes unsanitized page titles in flash messages. These messages are then rendered without escaping in the backend via FlashMessageQueue. While the exact method for message construction isn't specified, process_cmdmap is central to command execution, and addMessage handles message storage. The high confidence for process_cmdmap comes from its role in handling the vulnerable operation flow, while addMessage has medium confidence due to potential sanitization occurring earlier in the chain.