The vulnerability is an SSRF in the DnnImageHandler. The commit 4721dd9eef846936d3b1a3676499e46968d15feb addresses this by hardening the URL validation.

1. The DnnImageHandler.GenerateImage method is the entry point that receives and processes the URL from user input (passed in parameters["Url"]). It was vulnerable because it relied on an inadequate validation mechanism.
2. The DnnImageHandler.UriBelongsToSite method (a private static method within DnnImageHandler before the patch) was responsible for the actual URL validation. The patch removes this method entirely and replaces its functionality with a new, separate UriValidator class and its UriBelongsToSite method. This indicates that the original DnnImageHandler.UriBelongsToSite method contained the flawed logic that led to the SSRF vulnerability. Both functions would be involved during the exploitation of the vulnerability: GenerateImage as the function processing the malicious input, and the original UriBelongsToSite as the function performing the failed validation.