You could, hypothetically, move from CookieStore to a different session store, but I am not positive that this will mitigate the vulnerability (by making it more difficult for the attacker to get write access to the session, not by making it more difficult to turn a session into code execution), and do not have a few hours to reacquaint myself with the code to give you a firmer answer there.

I am weakly confident, on the basis of some spelunking in the Rails internals in February, that if you get attacker chosen data into the session you're uniformly hosed on (at least) Rails 2 and Rails 3.