Menu
If I understand correctly, in a CSRF attack a malicious website A tells my browser to send a request to site B. My browser will automatically include my B cookies in that request. Although A cannot see those cookies, if I'm already authenticated in B the request will look legit, and whatever action was asked will be successfully performed. To avoid this, every time that I visit a page of B containing a form, I receive a CSRF token. This token is associated to my session, so if I make a POST to B I MUST include such token; otherwise B rejects my request. The benefit of this scheme is that A will not have access to that token.
I have two questions:
Thanks!
391
The attack will only be successful if the user is in an active session with the vulnerable application. An attacker must find a valid URL to maliciously craft. The URL needs to have a state-changing effect on the target application. An attacker also needs to find the right values for the URL parameters.
"CSRF attacks are also very difficult to detect, because they look very much like a legitimate request from a trusted user." OWASP currently ranks CSRF attacks as the number eight most common and critical Web application vulnerability, down from the five spot since the last list was compiled.
CSRF tokens prevent CSRF because without token, attacker cannot create a valid requests to the backend server. CSRF tokens should not be transmitted using cookies. The CSRF token can be added through hidden fields, headers, and can be used with forms, and AJAX calls.
Your description is correct.
If site A tells your browser to go to B and get the token, that's fine, but as it is a cross-domain request, A will not have access to the token in Javascript (this is a browser feature). So when A tells your browser to go back to B and actually do something, it still cannot include the token in the request.
That is, unless B set the token as a cookie. Evidently, that would be flawed, because the token cookie would also be sent, thus negating any protection. So the token in this case must be sent as either a form value or a request header (or something else that is not sent automatically like a cookie).
This also means that if B is vulnerable to cross-site scripting, it is also vulnerable to CSRF, because the token can then be stolen, but CSRF is the smaller problem then. :)
110
Correct.
Site A can't get site B's csrf token because of the browser's CORS strategy.
And we need to validate the request's referer(It can be forged).
https://en.wikipedia.org/wiki/HTTP_referer
It is also a good practice to validate the crsf token in url(AKA query string).
FYI,Laravel, a popular web framework, uses a hidden CSRF token field in the form to prevent csrf attack.
32
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
