Over the past decade, there’s been a privacy arms race between unscrupulous website operators and browser makers. The former wield an ever-changing lineup of so-called zombie cookies that can’t be easily deleted and attacks that sniff thousands of previously visited sites, while browser makers aim to prevent such privacy invasions by closing the design weaknesses that make them possible. Almost as soon as one hole is closed, hackers find a new one.

http://arstechnica.com/security/2015/10/unpatched-browser-weaknesses-can-be-exploited-to-track-millions-of-web-users/