Cross-site scripting (XSS) vulnerabilities (even in other applications running on the same domain) allow attackers to bypass essentially all CSRF preventions.

The Authenticity Token is a countermeasure to Cross-Site Request Forgery (CSRF). What is CSRF, you ask? It's a way that an attacker can potentially hijack sessions without even knowing session tokens.

Rails does not issue the same stored token with every form. Neither does it generate and store a different token every time. It generates and stores a cryptographic hash in a session and issues new cryptographic tokens, which can be matched against the stored one, every time a page is rendered.

Since the authenticity token is stored in the session, the client cannot know its value. This prevents people from submitting forms to a Rails app without viewing the form within that app itself. Imagine that you are using service A, you logged into the service and everything is ok. Now imagine that you went to use service B, and you saw a picture you like, and pressed on the picture to view a larger size of it. Now, if some evil code was there at service B, it might send a request to service A (which you are logged into), and ask to delete your account, by sending a request to http://serviceA.com/close_account. This is what is known as CSRF (Cross Site Request Forgery). If service A is using authenticity tokens, this attack vector is no longer applicable, since the request from service B would not contain the correct authenticity token, and will not be allowed to continue.