"He made a terrible mistake in having these files on his computer and going to China, " Tunick said.

WSJ: 5-plus years for Ill. man in military secrets case

The little-known software, which Maxthon says has some 120 million monthly users, more than half of them in China, aims to capitalize on consumers' desires to access their files from a range of devices.

WSJ: Web Browsers Are Reinvented