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 its an allowed file type but the actual contents of the file arent checked. This means that its 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.
The product does not properly distinguish between different types of elements in a way that leads to insecure behavior.