Today I came across a problem where all of a sudden Uploadify stopped working with both Chrome and Firefox but continued to work with IE. I could have sworn that it was working on all 3 browsers previously and had now idea what had changed.
So I started doing some digging and found that Uploadify was trying to upload the file to the correct controller and action but Forms Authentication was rejecting it, even though I was logged in correctly!
After doing a few Google searches I came across this blog post which explains the problem and even has code for a new AuthorizationAttribute which solves the problem.
Basically the issue is with flash where it will ignore the browser’s session state and grab the cookies from IE, which is a known and active bug. This means that both Chrome and Firefox won’t work with Uploadify and authorisation because flash will send no cookies! It also means it is entirely possible for it to have previously work for me while testing because I probably also had a IE window open and logged in while testing, which would have given me a valid cookie.