Observations and insights from the Security PS Team.

CSRF Tokens

One of the many interesting discussions at Defcon recently was a discussion of CSRF by Mike Bailey and Russ McRee. They talked about a variety of real-world CSRF attacks and pointed out that a lot of companies simply accept CSRF vulnerabilities because they don't understand the risks they pose. To help people prevent this attack, frameworks like ESAPI have implemented CSRF token generators that produce a long random value to be included as a hidden field in each sensitive form. This value is then checked when the form is submitted, preventing attacks like CSRF from working.

While I was reading about this, I came across an interesting attack against CSRF tokens that was published last month. It's a cool idea. He uses the CSS properties of the browser's history to brute force token values. This works a little better than previous techniques because it generates no traffic to the server and likely isn't detectable by server-side defenses.

As other authors have already pointed out, this doesn't doom CSRF tokens to uselessness. They're still the most effective defense against CSRF attacks as long as they have a sufficient key space to prevent brute force attacks. Just make sure you use a long random value for your CSRF tokens and you'll be fine.
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