The vulnerability stems from the dol_check_secure_access_document function's handling of the 'userphoto' modulepart. Before patching, it granted unconditional access ($accessallowed = 1) to user photos without validating the file path structure. Attackers could bypass authorization by manipulating the file parameter to access arbitrary files in user directories. The patch added a regex check (preg_match('/^\d+/photos//') to enforce proper path structure, confirming the original vulnerability existed in this authorization check logic. Other file modifications were secondary path construction fixes that depended on this core authorization vulnerability.