Last Wednesday, The Intercept published an expose on the NSA's XKeyscore program. Along with information on the breadth and scale of the NSA's metadata collection, The Intercept revealed how the NSA relies on unencrypted cookie data to identify users. As The Intercept says:
"The NSA’s ability to piggyback off of private companies’ tracking of their own users is a vital instrument that allows the agency to trace the data it collects to individual users. It makes no difference if visitors switch to public Wi-Fi networks or connect to VPNs to change their IP addresses: the tracking cookie will follow them around as long as they are using the same web browser and fail to clear their cookies."
The NSA slides released by The Intercept give detailed guides to understanding the data transmitted by these cookies, as well as how to find unique machine identifiers that analysts can use to differentiate between multiple machines using the same IP address. We've written before about how spy agencies piggyback on social media account data to find Internet users' names or other identifying info, and these slides drive home the point that HTTP cookies leave users vulnerable to government surveillance, since any intermediary (or spy agency) can read the sensitive data they contain.
Worse yet, most of the time these identifying cookies come from third-party sources on webpages, and users have no meaningful way to opt out of receiving them (short of blocking all third party cookies) since advertisers (the main server of these types of cookies) refuse to honor the Do Not Track header.
Browser makers could help address this sort of non-consensual tracking by both advertisers and the NSA with some simple technical changes—changes that have been shown to reduce the number of third party cookies received by 67%. So far, though, they've been unwilling to build privacy protecting features in by default. Until they do, the best way for users to protect themselves is by installing a privacy protecting app like Privacy Badger, which is designed to block these types of uniquely identifying tracking cookies, or HTTPS Everywhere to block the transmission of HTTP cookies.