security - Understanding CSRF

Question

I don't understand how using a 'challenge token' would add any sort of prevention: what value should compared with what?

From OWASP:

In general, developers need only generate this token once for the current session. After initial generation of this token, the value is stored in the session and is utilized for each subsequent request until the session expires.

If I understand the process correctly, this is what happens.

I log in at http://example.com and a session/cookie is created containing this random token. Then, every form includes a hidden input also containing this random value from the session which is compared with the session/cookie upon form submission.

But what does that accomplish? Aren't you just taking session data, putting it in the page, and then comparing it with the exact same session data? Seems like circular reasoning. These articles keep talking about following the "same-origin policy" but that makes no sense, because all CSRF attacks ARE of the same origin as the user, just tricking the user into doing actions he/she didn't intend.

Is there any alternative other than appending the token to every single URL as a query string? Seems very ugly and impractical, and makes bookmarking harder for the user.

Answer 1

The attacker has no way to get the token. Therefore the requests won't take any effect.

I recommend this post from Gnucitizen. It has a pretty decent CSRF explanation: http://www.gnucitizen.org/blog/csrf-demystified/