San Francisco - While US law enforcement agencies have long tried to stamp out networks of compromised computers used by cyber criminals, the National Security Agency has been hijacking the so-called botnets as a resource for spying.
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 by former NSA contractor Edward Snowden and published by The Intercept news website on 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, Federal Bureau of Investigation Director James Comey told the 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 published by The Intercept, 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".
Reuters 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, which includes the US and Britain.
The NSA declined to confirm or deny the existence of the programme. It is not known if the botnets hijacked by the agency were in other counties or in the United States, or if the botnets could have been recaptured by criminals.
Many botnet operations disable the machines' security software, leaving them vulnerable to new attacks by others.
In a written statement, an NSA spokesperson said: "As the President affirmed on 17 January, signals intelligence shall be collected exclusively where there is a foreign intelligence or counterintelligence purpose to support national and departmental missions, and not for any other purposes.
"Moreover, Presidential Policy Directive 28 affirms that all persons - regardless of nationality - have legitimate privacy interests in the handling of their personal information, and that privacy and civil liberties shall be integral considerations in the planning of US signals intelligence activities."
The Intercept article and supporting slides showed that the NSA had sought the means to automate the deployment of its tools for capturing e-mail, browsing history and other information in order to reach as many as millions of machines.
It did not say whether such widespread efforts, which included impersonating web pages belonging to Facebook and other companies, were limited to computers overseas.
If it did pursue US computers, the NSA also could have minimised information about those users.