China Mobile Users Hit by Bug
The devices had been infected by a Trojan-based attack first discovered in 2011, news agency Xinhua reported.
The botnet can allow the smartphones to be hijacked remotely and potentially be used for fraudulent purposes, the BBC reported.
The warning comes as mobile Internet use in China has soared, growing by more than 18 percent in the past year.
There are now more than 420 million mobile users, according to the China Internet Network Information Center (Cinic).
The surge has attracted the attention of Apple chief executive Tim Cook, who met with the chairman of China Mobile last week, the report said.
Details of the meeting were scant, but a China Mobile spokesman said it was regarding 'matters of co-operation' in the region.
According to the report, in China specifically, local authorities even went as far as to warn operators to clean up security weaknesses in their mobile app stores.
Despite warnings at the time, it is believed that the Trojan is still very much active and enabling the growth of the botnet, the report said.
Users have been advised to monitor their call and data logs for unusual activity, it added.