The blockbuster report on Chinese hacking released last week by U.S. cybersecurity firm Mandiant has focused attention on the security of data held by governments and big corporations—and by law firms.

The report linked hacking of 141 entities, mainly in the United States, to a Chinese military unit based in a suburban Shanghai neighborhood. Four of those entities were law firms. Mandiant general counsel Shane McGee declined to name them, but says law firms, which store all kinds of sensitive information for a wide variety of clients, make ideal targets for hackers.