session variables

I need to define a session variable with no value until a user login and then once they log off it removes the value.
How do I create a session variable for this?
Ex
New use access our default webpage
session.security=
User logs in and the security setting is set based on a cfquery
session.security=1
or
session.security=2
When the user logs off the session variable is set to
session.security=

Currently the only time the session.security is defined is after a user logs in. I need to be define prior to login but no value set and after logoff.

Will this cause any security issues also?

Thanks!