US surveillance agency may be using Google's advertising cookies to track and "pinpoint" targets for government hacking and location-tracking. According to Snowden's leaked presentation slides, both the NSA and the British equivalent, the GCHQ, are using a Google-specific ad cookie (know as "PREF") as a way of homing in on specific surveillance targets. While Google's cookie doesn't contain personal information like a name or email address, it does contain numeric codes that uniquely identify a user's browser.
