The Unmaking US Security - Made in China

...attackers did get something of value: data that amounted to a partial map of the Defense Department’s unclassified networks. Analysts were also concerned that the implant—which the attackers had taken pains to hide—might be a digital weapon that could shut down those systems during a conflict.

Without a fix on China’s ultimate purpose, U.S. leaders decided in 2013 to keep the discovery secret and let the attack run, according to three officials who were informed of the plan. Keith Alexander, then-director of the National Security Agency, played a central role in the decision, the officials said. The Pentagon devised undetectable countermeasures to protect its networks, two of them said.

The moves allowed America’s own spies to begin gathering intelligence on China’s plans without alerting Beijing, the two officials said.

A spokesman for Alexander referred questions to the NSA. The agency declined to comment beyond a one-sentence statement: “NSA cannot confirm that this incident—or the subsequent response actions described—ever occurred.”