An issue was discovered in GetSimple CMS through 3.3.15. insufficient input sanitation in the theme-edit.php file allows upload of files with arbitrary content (PHP code, for example). This vulnerability is triggered by an authenticated user; however, authentication can be bypassed. According to the official documentation for installation step 10, an admin is required to upload all the files, including the .htaccess files, and run a health check. However, what is overlooked is that the Apache HTTP Server by default no longer enables the AllowOverride directive, leading to data/users/admin.xml password exposure. The passwords are hashed but this can be bypassed by starting with the data/other/authorization.xml API key. This allows one to target the session state, since they decided to roll their own implementation. The cookie_name is crafted information that can be leaked from the frontend (site name and version). If a someone leaks the API key and the admin username, then they can bypass authentication. To do so, they need to supply a cookie based on an SHA-1 computation of this known information. The vulnerability exists in the admin/theme-edit.php file. This file checks for forms submissions via POST requests, and for the csrf nonce. If the nonce sent is correct, then the file provided by the user is uploaded. There is a path traversal allowing write access outside the jailed themes directory root. Exploiting the traversal is not necessary because the .htaccess file is ignored. A contributing factor is that there isn't another check on the extension before saving the file, with the assumption that the parameter content is safe. This allows the creation of web accessible and executable files with arbitrary content.