Solution: Debug Mode & Signed Pickled Cookies (Facebook) [Challenge A]

Tools Used:
The prompt says: "The flag is the environmental value COOKIE_SESSION_STORAGE_SIG, which can be accessed from a debug error page." So we need to make an error page appear.

When you first open the page, you see a calculator. Dividing by zero throws an error in most programming languages - do that, trigger the error page, then search the error page for the 'COOKIE_SESSION_STORAGE_SIG' string to retrieve the flag.