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Using Cookies For Selective DoS

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One of the things Josh Sokol and I talked about in our presentation at Blackhat was a way to use over-sized cookies to cause a DoS on the site. The web server sees the overlong cookie and stops the request from completing. This is not new and has certainly been discussed before. However, one thing that wasn’t discussed is that using the path an attacker can selectively cause the website to stop displaying portions of the site. For instance, if the attacker wants to shut down /javascript/ or /logout.aspx or /reportabuse.aspx or whatever, they can by setting an overly-long cookie for that particular path.

Setting cookies on the target sub domain would require something like header injection/Response splitting, XSS, or a MitM attack. It should be noted though that it doesn’t have to be on the target sub domain - it can be an exploit in another sub domain because cookies don’t follow the same origin policy if the cookie is scoped to the parent domain. In this way an attacker could turn off Clickjacking prevention code (deframing scripts), or turn off other client side protections or parts of the site that are bad from an attacker’s perspective. The only real solution to this is for all browsers to start making the absolute maximum size of cookies smaller than the smallest that web servers will allow (Apache was smaller than IIS by default for instance).

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