How to prevent CSRF attacks

  • Gérald Barré

This post is part of the series 'Vulnerabilities'. Be sure to check out the rest of the blog posts of the series!

#What's XSRF?

CSRF, Cross-Site Request Forgery, vulnerabilities allow us to impersonate a user to make a query on a website. This happens when a website allows an authenticated user to do a sensitive action but the website does not verify that the user triggered this action. Instead, they only check that the query comes from the browser of an authorized user.

The attacker aims to use our authentication cookie already stored by the browser for the targeted website. To do this, it is enough for them to make the browser request the target site. There are several possibilities for this:

  • Convincing the user to click on a link,
  • Inserting HTML/JavaScript code into a website that the user visits. One can for example find an image whose link is a link executing an action on the targeted site. Example <img src="https://example.com/delete/1">. When the browser receives an img tag, it retrieves the image specified in the source using a GET request and therefore requests the desired site.

One can say that to avoid this kind of attack it is enough to replace all the actions realizable by the method GET in POST. Well, that's not enough. Indeed, the attacker can use JavaScript:

<body onload="document.getElementById('form').submit()">
    <form id="form" action="https://example.com/delete/1" method="post">
        <input name="delete" value="Delete" />
    </form>
</body>

The form will be submitted automatically using the POST method. It's better than using GET, but it doesn't solve the problem.

#How to guard against it?

  • Check the HTTP referrer. Basically, if it's not our domain there's a problem. However this method can be annoying to the user. Indeed some enterprise proxy removes the referrer from all HTTP requests. So we will find ourselves blocking legitimate requests. Moreover, it is possible to forge its request and thus define the referrer. This method is therefore fickle.
  • Add an invisible field in the form. The concept is to generate a string of random characters and add it to the form. When the client sends the form, the server verifies that the value of the hidden field is the same as what was generated. Example: <input id="a" name="a" type="hidden" value="hjkguyez721g45654gfd12cwx400huiyu" /> a bit like the captcha.
  • Add a cookie. When the page is created, the server sends a cookie. When submitting the client returns it and the server checks that the content of the cookie is identical to what it generated.

Here you have all the cards in hand to protect your websites from this type of attack.

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