The online anonymity network Tor is a high-priority target for the National Security Agency. The work of attacking Tor is done by the NSA's application vulnerabilities branch, which is part of the systems intelligence directorate, or SID. The majority of NSA employees work in SID, which is tasked with collecting data from communications systems around the world.
According to a top-secret NSA presentation provided by the whistleblower Edward Snowden, one successful technique the NSA has developed involves exploiting the Tor browser bundle, a collection of programs designed to make it easy for people to install and use the software. The trick identified Tor users on the internet and then executes an attack against their Firefox web browser.
The NSA refers to these capabilities as CNE, or computer network exploitation.
Source: The Guardian. Read full article. (link)