restrict page access using session variables

I'm updating an ASP net website app that provides group discussions.  It uses membership login for "subscribers" and a group discussion URL to a non membership login for "participants".   When users passed through these login screens a set of user session variables is created for them.  Any page that they might try to access inside the application will test to see if these user session variables are there.  If not they get redirected to a application exit page.

What is the risk/is there anyway/How might an evildoer get around this safety check on session variables to access the application pages without going through the subscribers or participant login pages?
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David Johnson, CD, MVPOwnerCommented:
they really can't.

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