A code injection vulnerability exists in ProcessMaker Open Source versions 2.x when using the default 'neoclassic' skin. An authenticated user can execute arbitrary PHP code via multiple endpoints, including appFolderAjax.php, casesStartPageAjax.php, and casesSchedulerGetPlugins.php, by supplying crafted POST requests to parameters such as action and params. These endpoints fail to validate user input and directly invoke PHP functions like system() with user-supplied parameters, enabling remote code execution. The vulnerability affects both Linux and Windows installations and is present in default configurations of versions including 2.0.23 through 2.5.1. The vulnerable skin cannot be removed through the web interface, and exploitation requires only valid user credentials.