Yikes.

Hackers working for the Chinese state breached the computer system of the Office of Personnel Management in December, U.S. officials said Thursday, and the agency will notify some 4 million current and former federal employees that their personal data may have been compromised.

The hack was the second major intrusion of the same agency by China in less than a year and the second significant foreign breach into U.S. government networks in recent months. Russia last year compromised White House and State Department e-mail systems in a campaign of cyber-espionage.