The vulnerability is a Server-Side Request Forgery (SSRF) in the file upload functionality of Parse Server. The root cause is the downloadFileFromURI function in src/Routers/FilesRouter.js, which makes an HTTP GET request to a URI provided by the user in the _source.uri parameter of a Parse.File object. This allows an attacker to force the server to make requests to arbitrary internal or external services.
The function addFileDataIfNeeded checks for the presence of the uri format and calls the vulnerable downloadFileFromURI function. This addFileDataIfNeeded function was called within the file creation handler inside the expressRouter method of the FilesRouter class, which is the main entry point for the vulnerability when a file is uploaded.
The patch addresses the vulnerability by completely removing the downloadFileFromURI and addFileDataIfNeeded functions, thereby disabling the feature of uploading files from a URI.