The vulnerability stems from improper input neutralization in file upload processing. The patch adds strtolower() calls to filename extensions in two locations, indicating the original code lacked case normalization. This allowed XSS vectors through file extensions that bypassed MIME type checks. While no specific named function is called out, the procedural code in plupload.php responsible for handling uploaded files' naming and validation is clearly the vulnerable component, as shown by the patch modifying extension handling logic.