Length of CSRF Token

Question

Is there any standard length of the token which should be used while generating the random tokens? Should we use the same standard which we use for generating Session IDs?

Answer 1

I would consider a 128 bits of entropy in a token to be the de-facto standard. OWASP and CWE both recommend this as a minimum. 20 characters of Base64 (capable of 120 bits) is also handy for something in the URL. I would also note that in many cases poor seeding for those tokens creates problems. For one bit of reference, see the (kind of tastelessly decorated, but very informative) slides from http://samy.pl/bh10/.

Make sure you choose your entropy source well.

Answer 2

In CSRF an attacker can make many guesses. What if an employee visits an attackers site and then goes on Christmas vacation? An attacker could make many millions of cross-site requests. We have a similar concern for session id's. If either value is obtained then session is compromised. The same standards of strength should be applied to both CSRF tokens and Session IDs.

In both cases make sure the value expires. Can the attacker can't make 2^128 requests in a week, but eventually he will be able to. If your random number generator is weak, then you may think you have a 2^128th cryptographic nonce, but it might be much less.