The White House on Monday played down a report that hackers tied to China's government penetrated a U.S. government system, saying the attack hit an unclassified network and that there is no sign that any secrets were stolen.
The conservative Washington Free Beacon reported on Sunday that the hackers broke into a system used by the White House Military Office for nuclear commands.
But a White House official said Monday that "the attack was identified, the system was isolated and there is no indication whatsoever that any exfiltration of data took place."
"Moreover, there was never any impact or attempted breach of any classified system," the official said on condition of anonymity. Asked whether the White House had concluded that the attack came from China or was linked to that country's government, the official said the administration was "not pointing to any actors."
"This was a spear phishing attack against an unclassified network. These types of attacks are not infrequent and we have mitigation measures in place," the official added.
Spear phishing typically involves a hacker spoofing an official email account in order to get access to a secure system. When the recipients open the message or click on any link in what they think is a valid email, it can download malicious software.
The Free Beacon, citing unnamed defense and intelligence officials familiar with the incident, described the attack as having occurred in September. It involved hackers thought to have used computer servers in China. The hackers broke into a computer network used by the White House Military Office, which handles, among other things, strategic nuclear commands.