The U.S. National Security Agency has repeatedly tried to compromise Tor, the government-funded online anonymity tool, but has had little success, according to a new report in the U.K.'s Guardian.

The NSA has tried multiple strategies for defeating Tor, with its most successful method focused on attacking vulnerable software on users' computers, including the Firefox browser, according to the report, published Friday. In the Firefox attack, NSA agents have been able to gain "full control" of targets' computers, said the report, citing documents given to the Guardian by former NSA contractor Edward Snowden.

NSA documents provided by Snowden, which the Guardian began publishing in June, say the agency is collecting bulk phone records in the U.S. as well as Internet communications overseas. But in many cases, the NSA has been frustrated in its efforts to target Tor users.

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