Over the weekend, a researcher demonstrated two unpatched weaknesses that Web masters can exploit to track millions of people who visit their sites. Taken together, the attacks allow websites to compile a list of previously visited domains, even when users have flushed their browsing history, and to tag visitors with a tracking cookie that will persist even after users have deleted all normal cookies. Ironically, the techniques abuse relatively new security features that are already built into Google Chrome and Mozilla Firefox and that may make their way into other mainstream browsers in the future.