DNN (formerly DotNetNuke) is an open-source web content management platform (CMS) in the Microsoft ecosystem. Prior to 9.13.2, when uploading files (e.g. when uploading assets), the file extension is checked to see if it's an allowed file type but the actual contents of the file aren't checked. This means that it's possible to e.g. upload an executable file renamed to be a .jpg. This file could then be executed by another security vulnerability. This vulnerability is fixed in 9.13.2.