A massive data breach at Japan's largest travel agency has underscored the risks companies face when they keep sensitive data on networks connected to the internet, experts say. Some warn government systems are especially vulnerable to state-sponsored attack, including by China and North Korea.

JTB Corp. said Tuesday hackers may have obtained the passport details and other records of 7.93 million customers after a subsidiary's server was hacked in April.

On Wednesday, the government-backed Japan Tourism Agency faulted JTB for keeping personal information on a machine reachable from the global internet.