A Step-by-Step Walkthrough Solution to CTF LEVEL 9 INFOSECINSTITUTE (Practical Web HACKING)

The level says we need to login as Mary Jane and we check cookies we get a cryptic looking string replace %3D with =, if you are using firebug cookie explorer it will do it automatically for you , decode the string as base 64 you will get the output.

So now all you need to do is encode the string MARY JANE as base 64 and replace the cookie and refresh the page and you get

Also we learn that authentication using cookies is not at all a secure practice.