Do I need a CSRF token for jQuery .ajax()?

Question

So I've got a basic .ajax() POST method to a PHP file.

What security measures do I need?

A few posts around were mentioning using a hidden MD5 input field that you send via AJAX and verify in the PHP file. Is this a good enough method?

Answer 1

The risk from CSRF is that an external site could send data to yours and the users browser will automatically send the authentication cookie along with it.

What you need is some way for the receiving action (that your $.ajax() method is sending POST data to) to be able to check that the request has come from another page on your site, rather than an external site.

There are a couple of ways to do this, but the recommended way is to add a token to the request that you can check for and that the hackers can't get to.

At its simplest:

• On log on create a long random string token and save it against the user.
• Add a parameter to the $.ajax() request that includes the token.
• On request check that the token matches the one that you have saved for the user.
• If the token doesn't match you have a CSRF hack.

The hacker can't get to your DB and can't actually read the page you've sent to the user (unless they get an XSS attack in, but that's another problem) so can't spoof the token.

All that matters with the token is that you can predict (and validate) it and that the hacker can't.

For this reason it's easiest to generate something long and random and store it in the DB, but you could build up something encrypted instead. I wouldn't just MD5 the username though - if the CSRF attackers figure out how to generate your tokens you'll be hacked.

Another way is to store the token is in a cookie (rather than your database), as the attackers can't read or change your cookies, just cause them to be re-sent. Then you're the token in the HTTP POST data matches token in the cookie.

You can make these a lot more sophisticated, for instance a token that changes every time it's successfully used (preventing resubmission) or a token specific to the user and action, but that's the basic pattern.