We study the ability of a passive eavesdropper to leverage \third-party" HTTP tracking cookies for mass surveillance. If two web pages embed the same tracker which tags the browser with a unique cookie, then the adversary can link visits to those pages from the same user (i.e., browser instance) even if the user's IP address varies. Further, many popular websites leak a logged-in user's identity to an eavesdropper in unencrypted trafic. To evaluate the effectiveness of our attack, we introduce a methodology that combines web measurement and network measurement. Using OpenWPM, our web privacy measurement platform, we simulate users browsing the web and find that the adversary can reconstruct 62|73% of a typical user's browsing history. We then analyze the effect of the physical location of the wiretap as well as legal restrictions such as the NSA's "one-end foreign" rule. Using measurement units in various locations|Asia, Europe, and the United States|we show that foreign users are highly vulnerable to the NSA's dragnet surveillance due to the concentration of third-party trackers in the U.S. Finally, we find that some browser-based privacy tools mitigate the attack while others are largely ineffective.