Manual:$wgDisableCookieCheck

Details
If set to true, MediaWiki will assume that the user has a valid session cookie after logging in or creating an account.

That is, if the cookie is not present (for example, because the user has disabled cookies), the user will appear to be logged in immediately when returning from login, but will be logged out as soon as they navigate to another page.

If set to false, MediaWiki attempts to detect this situation and give the user an informative message.

This setting does not affect security; it is a usability setting.