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Just today, an issue arised with a Joomla 3.6.5 installation I have.

A user was seeing other connected users' details. Something went wrong with the session table (I suppose, because I checked and there were more records than I expected) and all of a sudden, this user was able to see and edit other user data, but only in specific pages.

It looked as if it went back and forth between this user's and another user's session.

Then I updated to Joomla 3.7.0 and another issue came up: Some menu items are "external URLs" which I use as text separators, due to some issues I had with iPads. Their external URL is just a hash (#), to work as a text separator, thus pointing to my home page.
When someone clicks on that link, they are taken back to the home page and the page looks as if they were logged-out, and a prompt to login is back there. If you click on it, nothing happens and you have to use a menu item to navigate back to the site, where you find out that you are still logged-in.

The user information kept in this site is very sensitive, and suddenly my users see each other's data!
Any help is desperately appreciated.

  • What hosting is it on? I had a client several months back with a site on Network Solutions with some crazy cache/sessions problems. Never did figure out cause and there were zero issues when I cloned to my server. Might try cloning to another server to rule that out. – YellowWebMonkey Apr 28 '17 at 17:40
  • It's on Justhost. Just downloaded a backup with Akeeba Backup and absolutely no issue on my localhost. I made another backup from localhost and will re-upload, and post as soon as I see if the issue continues. – mavrosxristoforos Apr 28 '17 at 18:05
  • The issue only happens on my host. The website completely works on localhost. I asked the host and they claim it is not their issue. – mavrosxristoforos Apr 29 '17 at 14:47
  • I found out that, when I open Chrome Developer Tools and make sure that Disable Cache is enabled, the page works normally. Any help is appreciated. – mavrosxristoforos Apr 29 '17 at 15:23
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Something is caching data for authenticated users when it shouldn't be. Generally within the core Joomla extensions, they will only serve cached data to guest users.

  • Thank you for posting. This is exactly the problem. I contacted my host and finally they accepted the fact that they have some caching system called "Varnish". Do you have any clue on how to disable that? – mavrosxristoforos May 1 '17 at 16:20
  • You will need to talk with your host about it. Generally Varnish isn't a bad thing but it has to be configured correctly. – Michael May 2 '17 at 17:37
  • Thank you Michael. Your answer was very helpful. I have also posted the solution, below. – mavrosxristoforos May 5 '17 at 20:56
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After a lot of searching, my host pointed out the fact that they use Varnish, which is a cache system, incompatible with Joomla, by default. They didn't seem eager to edit its settings, but they told me to add the following to my .htaccess file.

Header add "disablevcache" "true"

After doing so, the issue was resolved within 24 to 36 hours.

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