Chinese online-shopping giant Alibaba Group Holding Ltd. recently came under attack from hackers who tried to gain access to about 20 million user accounts of the popular Taobao online marketplace in China, the company said Thursday. As WSJ's Juro Osawa reports:

Alibaba said it protected the vast majority of those Taobao accounts by blocking hackers’ access attempts, but some attacks on a small number of accounts weren’t intercepted. An Alibaba spokesman declined to disclose the number of user accounts that may have been accessed by the hackers. He also declined to comment on whether any data has been compromised from those accounts. Suspects have already been arrested, he said.

The incident is the latest reminder of cybersecurity threats facing China’s popular online services that accumulate massive troves of data from hundreds of millions of users.