According to the agency's internal presentation slides, provided by former NSA contractor Edward Snowden, when companies follow consumers on the internet to better tailor their advertising, the technique opens the door for similar tracking by the government.
The NSA and its British counterpart, GCHQ, are using the small tracking files called cookies that advertising networks place on computers to identify people browsing the internet. The intelligence agencies have made particular use of the "PREFID", part of Google-specific tracking software known as the "PREF" cookie.
This cookie typically doesn't contain personal information but it does contain numeric codes that enable websites to uniquely identify a person's browser. In addition to tracking web visits, the PREFID allows the NSA to single out an individual's communications among the sea of internet data.
Internet cookies are used to "enable remote exploitation", although the specific attacks used by the NSA against targets are not addressed in the documents. It is not clear how the NSA obtains Google PREF cookies or whether the company co-operates.