A face-swapping app that went viral on Chinese social media over the weekend sparked a privacy controversy within hours of its debut.

Just take a selfie and choose footage from a classic film or soap opera on the app, and a user can superimpose his or her face on any actor playing any role they like.

The ZAO app puts people's faces on to celebrities in videos using deepfake-style technology. (Picture: Sina)

But after generating huge interest in its first two days of circulation, users soon learned that the pictures of their faces were no longer under their control.

The ZAO app – introduced by Momo, a social media app developer – uses “deepfake”, a technique for human image synthesis based on artificial intelligence.

Since its debut on Friday, hashtags with the name ZAO have had more than 8 million views on Weibo, China’s equivalent of Twitter. It had meanwhile zoomed to No 1 on the free entertainment app list in the Apple App Store as of Sunday.

However, almost immediately after it was released, users, lawyers and industry insiders began to question a clause in the app’s user agreement.

That clause gave the developers the global right to permanently use any image created on the app for free. Moreover, the developers had the right to transfer this authorisation to any third party without further permission from the user.

Mainland legal experts said the clause was not legal.

“If a user’s private information is to be used by a third party, [the company] must get clear authorisation from the user,” lawyer Fang Chaoqiang told The Beijing News.

Moreover, if a user uploaded another person’s photo without authorisation, and a platform used that photo for commercial purposes, the platform would have infringed that person’s portrait rights, Fang added.

Under pressure, ZAO’s developer revised its user agreement on Saturday, deleting the permanent and free-use clause. It also said that if users deleted photos on the app, the platform would not keep them in its server.

On Sunday, China’s top social networking app WeChat banned sharing pictures from the ZAO app.

“Videos from the app flooded my WeChat, and they were really interesting,” said one Weibo user. “But we should care about the safety of our personal information.”

People were worried because biometrics data, including facial recognition, has been widely used in China, but the regulations have not yet caught up.

“Facial recognition is very sensitive personal information,” Li Wei, director of the technology department at the People’s Bank of China, said at a forum in July. “Once leaked, it can have a huge impact on individuals.”