Nothing stops, but it will be a different CSRF token, which will not match to the one generated for the original page.

Server keeps track of which CSRF token is given to which client using cookies (usually some form of SessionID), and stores it on the server somewhere.

It is a very common pattern and all frameworks support it with the concept of "sessions" on the back end.

> stores it on the server somewhere

you don't need to store anything on the server. cookies for that domain are sent with the request and it is enough for the server to check its cookie with the csrf request data.

browsers would send the bank.com cookies with the bank.com request. It is security built into the browser which is why its so important to use secure browsers and secure cookies.

If the malicious user convinces the user to use an insecure browser you can circumvent CSRF, but at that point there are probably other exploits you can do.

How does server know the cookie is valid if it doesn't store it and how does it know csrf token is valid if it doesn't store it and finally how does it know that this csrf token relates to this cookie session token if it doesn't store it?

The CSRF token can have nothing to do with the cookie session information. you can store CSRF as a separate cookie.

You can validate the CSRF is valid by keeping a key on your server and matching that the token you get can be derived from that key.

See Django's implementation of CSRF for more details. CSRF tokens are separate from session and no CSRF information needs to be stored in database to validate CSRF.

> How does server know the cookie is valid if it doesn't store it

depending on why you'are asking the question, * because it decrypts correctly * because it contains some user identifier

People don't usually store sessions in cookies because cookies can't be very big, and session do become big. So what people do instead they store cookies in databases, and put session identifiers into cookies.

You don't need to store CSRF in sessions. Django doesn't by default.

CSRF token can be entirely separate from sessions.

not even you don't need to, you shouldn't. Sessions shouldn't be accessible to js at all

> What stops evil.com from having an API endpoint called evil.com/returnBankCSRFToken that goes to bank.com, scrapes token and returns it?

so evil.com will now require some sort of authentication mechanism with bank.com to scrape a valid CSRF token. If this authentication works (either if the user willingly gave their login information to evil.com, or they have a deal with bank.com directly), then there's no issues, and it works as expected.

The key is that CSRF is protecting against the browser's automatic behavior of sending cookies along with cross-origin requests.

A form on evil.com posted to bank.com does send your cookies by default.

But not for fetch() in the browser. And certainly not for a request made from evil.com's server.

Also, bank.com would want to create CSRF tokens tied to current user. Or ensure its CORS protections never allow cross-origin requests (the default) if it were to use generic CSRF tokens.

Or just use same-origin cookies.

Or don't even use cookies at all.