I am accessing Linux box from my Windows box using putty through SSH (port 22).
Is there any way I can hack this service session from backtrack 5? If yes then how? What kind of countermeasures can we take to safeguard this kind of issue.
Knowing C will help little with this unless you take the long way around. You are looking at either A) finding a vulnerability in openssh or, B) the more practical option or brute-forcing usernames/passwords. Will take a while but hey you're not going anywhere. I'd suggest checking out Hydra or xHydra from THC.