Intelligence and security experts say a recent breach of U.S. federal employee data by Chinese hackers could abruptly bring the country’s strategy of basing secret U.S. intelligence units abroad to a halt. Government records containing the Social Security numbers of 21.5 million federal employees and their spouses or partners were stolen from the computer systems of the Office of Personnel Management (OPM) in a massive hack that was first made public in June.

China’s government is considered the “leading suspect” in the hack, Director of National Intelligence James R. Clapper told CNN this month.

Last year, employee data was also stolen from two firms that conducted background checks for the Department of Homeland Security.

Intelligence and security experts are worried that the information collected through those hacks may permit the Chinese government to piece together the identities of spies who have worked in China.

“From an intelligence perspective, it gives you great insight potentially used for counterintelligence purposes,” Adm. Michael S. Rogers, the director of the National Security Agency, said during an interview at the Aspen Security Forum in Colorado this week. “If I’m interested in trying to identify U.S. persons who may be in my country and I am trying to figure out why they are there -- Are they just tourists? Are they there for some other alternative purpose? -- there are interesting insights from the data you take from OPM.”

Now, experts are concerned about the future of U.S. intelligence programs that rely on undercover officers based in foreign countries, the New York Times reported. If the identities of its undercover officers cannot be adequately protected, the U.S. government may have to rethink its longtime strategy of basing undercover officers at foreign embassies such as the American Embassy in Beijing.

Rob Knake, former director of cybersecurity policy issues at the National Security Council, told the Times that the security risks to individual officers identified through these methods could result in “a whole bunch of CIA case officers spending the rest of their careers riding desks.”