The limited success in cracking Tor relies on the massive internet cable taping program and secret nodes set up on the anonymizing network. By monitoring traffic exiting the network the NSA could potentially identify some of the sources by looking for patterns, however, the documents state that success rates were "negligible." Like other encryption tools, there does appear to be some effort on the part of the NSA and GCHQ to secure backdoors through influencing the development of Tor, though it's unclear how much, if any, success the agencies have had in this area. The most successful efforts to thwart Tor relied on exploiting weaknesses in other software, such as injecting malware when users visit sites with older versions of Firefox. This is similar to an exploit used by the FBI to identify Tor users earlier in the year. Both vulnerabilities have since been fixed.

While these efforts obviously raise concern for Tor's many users, there could be fallout from from other parts of the government. Tor was created and is still largely funded by other agencies within the US. The NSA has already found itself at the center of a firestorm thanks to the documents leaked by Mr. Snowden, but the most recent revelations show it actively trying to undermine tools that other defense and diplomatic agencies rely on.