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First of all, I'm quite new to web security so this might be a dumb question. Also English is not my native language so there might be some errors.

I understand that by having a csrf token as a hidden parameter in a form or HTTP header, an attacker cannot use something like an image tag to perform an action on the user's behalf. I also understand that if you have an XSS on the website, you can steal the csrf token and perform any action.

What I don't understand however is what prevent an attacker to his own domain, like evilsite.com, use JavaScript to make a GET request, analyses the response, extract the csrf token and submit the form to make some action. As you can make a post request with JQuery for example, you should be able to send the request and trick the user.

There should be a mechanism that I don't understand here because it makes no sense for me right now. What don't I understand?

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    The best answer: try to do it yourself! It's not a hard experiment and you'll learn a lot. – Conor Mancone Aug 13 at 23:58
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Same-origin policy prevents a website from reading another website, which also prevents it from extracting the token to perform a CSRF attack.

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    So if I understand this correctly, we have to trust the browser as this is client-side. Am I right? – o2640110 Aug 14 at 0:19
  • @o2640110: That's right. – Benoit Esnard Aug 14 at 0:21
  • Is there a way to do a server-side check to add further protection? (Other than the csrf token) – o2640110 Aug 14 at 9:35
  • @o2640110 Some people check the referer, but an attacker could force the browser to not send that header. Note that the CSRF provides enough protection already. – Benoit Esnard Aug 14 at 9:38
  • Thanks a lot for all your answers. – o2640110 Aug 14 at 14:26

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