This exercise of hacthissite is about cookie manipulation. When you open the exercise page it’ll insert into your system a cookie that says “User is not authorized”.So if you’re working on firefox you might use the add-on “Cookies Manager +” to edit the content of any cookie.
Now open the Cookies Manager from tools menu. Search for the “hackthissite” domain. You’ll see a cookie that includes “10” inside its domain name. (I plainly too lazy to open it and see the exact name 🙂 ). Find it and open edit on it. You’ll see that it says “no” for authorized. Make it “yes” then refresh the page and push to submit.