HONG KONG — For Apple in China, trouble seems to be the new normal.

Cybersecurity monitoring groups and security experts said on Monday that people trying to use Apple’s online data storage service, known as iCloud, were the target of a new attack that sought to steal users’ passwords and then spy on their activities.

Starting over the weekend, when many users across China tried to sign into their iCloud accounts, they may have been giving away login information to a third party, in what is called a man-in-the-middle attack.

“You think you are getting information directly from Apple, but in fact the authorities are passing information between you and Apple, and snooping on it the whole way,” said a spokesman for an independent censorship-monitoring website, GreatFire, who declined to be named because of fear of reprisal.

The back-end I.P. address targeted by the attack was changed Tuesday by Apple, according to a tweet from GreatFire.