OpenEMR before 8.4.0 contains a local file inclusion in the ESign router (library/ESign/Router.php), which passes the unsanitized module parameter into a require_once of '<module>/Controller.php', and the fax module's faxProcessUploads writes attacker-named files to a predictable temp path without extension checks. Authenticated attackers can upload a PHP file named Controller.php, then request interface/esign/index.php with a traversing module parameter to include it and execute arbitrary code.