The NSA allegedly "co-opted" more than 140,000 computers since August 2007 for the purpose of injecting them with spyware. Photo: Reuters

While US law enforcement agencies have long tried to stamp out networks of compromised computers used by cybercriminals, the National Security Agency has been hijacking the so-called botnets as a resource for spying.

The NSA sought the means to automate the deployment of its tools for capturing email, browsing history and other information in order to reach as many as millions of machines.

The NSA has "co-opted" more than 140,000 computers since August 2007 for the purpose of injecting them with spying software, according to a slide leaked byformer NSA contractor Edward Snowdenand published byThe Intercepton Wednesday.

Botnets are typically used by criminals to steal financial information from infected machines, to relay spam messages, and to conduct "denial-of-service" attacks against websites by having all the computers try to connect simultaneously, thereby overwhelming them.

In November, FBI director James Comey told the US Senate that botnets had "emerged as a global cyber security threat" and that the agency had developed a "comprehensive public-private approach to eliminate the most significant botnet activity and increase the practical consequences for those who use botnets for intellectual property theft or other criminal activities."

According to the NSA slide, one technique the intelligence agency used was called QUANTUMBOT, which "finds computers belonging to botnets, and hijacks the command and control channel." The program was described as "highly successful".

It was reported in May that US agencies had tapped botnets to harvest data from the machines' owners or to maintain the ability to issue the infected computers new commands.

The slide leaked by Snowden is the first confirmation of the practice, and underscores the complications for the NSA of balancing its major mission of providing eavesdropping capability with the less well-funded missions of protecting critical national assets and assisting law enforcement.

The top secret slide was marked for distribution to the"Five Eyes" intelligence alliance, comprising Australia, New Zealand, Canada, the United States and Britain.

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NSA 'hijacked' criminal botnets to install spyware