title: Cookie deleted in conditional GET request
body: <p>Recently our website hosted on Azure encountered a strange problem. After logging in, the user's immediately unauthenticated, and when they click on any link to go to other page, they will be redirected to login page due to unauthenticated. It doesn't happen all the time, but the consequence is serious, because we don't even know when it happens, there's no exception to log, just suddenly lost of authentication
  Yesterday I encountered this issue on production, and captured the requests sequence during the time I got kicked out using Fiddler, and find something unusual:</p>
  <p><a href="https://i.stack.imgur.com/fYqBg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fYqBg.png" alt="Fiddler captured" /></a></p>
  
  <p>The page I landed after login in /my-quizzes (colored in green). When I click on a link to go to different page (&quot;/group&quot; link, with green color), I'm redirected to sign in page. Between my requests to /my-quizzes and /group, there are some other requests for javascript, images, etc. The strange thing is in the highlighted request for &quot;uncheckCheckBox.png&quot;, in the response, the authentication cookie (.SFAUTH-XXX) and some other cookies are deleted (set to empty string, with expiry date = 12/10/1999). This is the reason that subsequent request to /group page is unauthenticated</p>
  <p>Our website is built on top of Sitefinity, using ASP.NET MVC</p>
  <p>My questions are:</p>
  <ul>
  <li>Is it normal to have cookies deleted in an image request like this? And this is just a conditional GET to check whether the image is modified, which is quite unusual to me. But I cannot explain why it happens</li>
  <li>Any suggestion on how we can debug this issue of cookies deleted?</li>
  </ul>