A Chinese app called Zao, which lets users swap their own faces with celebrities, has received backlash over the weekend over privacy concerns.

The face-swapping app, released on Friday, Aug. 30, immediately went viral, skyrocketing to the top of Apple's App Store in China within only a few days.

According to App Annie, a firm that tracks downloads all over the world, Zao has become the most downloaded free app in China's App Store as of Monday, Sept. 2.

Swapping Faces With Celebrities

To use Zao, users sign up with their phone numbers and then upload photos to the app. They can either use an existing photo or create a series of photos where they blink their eyes and open their mouths. From there, they can choose from a number of videos on which they can superimpose their own faces.

In addition to popular Chinese celebrities, the app also allows users to swap faces with Hollywood icons like Marilyn Monroe and Leonardo DiCaprio.

Over the weekend, internet users shared videos generated using the app. A tweet below showed the user replacing Academy Award winner DiCaprio in scenes from Titanic, Romeo + Juliet, Django Unchained, and other movies.

Users Bring Up Potential Privacy issues

However, the viral new app immediately received online backlash. Users noticed that the early version of the app's privacy policy stated that Zao has "free, irrevocable, permanent, transferable, and relicense-able" rights to photos uploaded by users.

As a result, Zao now has a 1.9/5 stars on the App Store and has received over 4,000 user reviews.

In a statement published on the Chinese social media site Weibo, the developers behind the face-swapping app addressed the issues.

"We understand the concerns about privacy. We've received the feedback, and will fix the issues that we didn't take into consideration, which will take some time," the statement read (via The Guardian).

The app's privacy policy has since been edited to assure users that the photos they upload will not be used for purposes other than to improve the service. It also said that videos deleted from the app will also be erased from the servers.

However, according to Bloomberg, the China E-Commerce Research Center urged authorities to investigate the app, which, they said, violated certain laws.

Zao was published by Momo Inc.

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