A vulnerability in the way Firefox handles CSS allows an attacker to take advantage of an integer overflow and execute arbitrary code. In order for the attack to be successful, a user must browse to a malicious site. The security advisory is available here. This critical vulnerability was reported to Mozilla before details were available publicly. By keeping the details of the issue private until a fix was made available, TippingPoint and Mozilla were able to minimize the risk to users. For more information, please see the Mozilla Security Blog.