CakePHP Session data being lost on redirect

Having successfully enabled access using LTI to a local version of iCases – see https://learntech.medsci.ox.ac.uk/wordpress-blog/?p=229 – I got it set up on a live server, assuming that it would work without any trouble. However, I was unable to login successfully through WebLearn, which is our Tool Consumer.

It turned out that this was due to the session data being lost when redirecting from the login page within the CakePHP app to the scenario page. Authentication using LTI relies on session data, as the LTI context information is saved to the session. Therefore, when the session data was lost, the app could no longer tell that the user had accessed it through a valid LTI request, and so the user was denied access.

I fixed this by changing Security.level in core.php to ‘low’ (it had previously been medium). From the CakePHP docs, this increases the multiplier for the ‘Session.timeout’ value (from 100 to 300) and disables (or, to be pedantic, does not enable) PHP’s session.referer_check. It seems to be the latter that was the problem. However, in the php.ini file we have ‘session.referer_check = ‘, which should mean that session.referer_check is not enabled anyway. So I am not sure why changing the security level had an effect, unless setting the Security.level to medium enables session.referer_check, even if it was not already enabled.

As far as I can tell from reading around, disabling session.referer_check should not cause any problems, as it is only possible to access the LTI-ed iCases through WebLearn. Checking that the LTI launch is valid includes checking that the launch request has come from a valid location.

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