Zao is an app that went viral overnight because it allows people to insert their face into various famous movies, all directly from their phone. In the video clip above you have a demonstration of its functionality, a user swapping his face with that of Leonardo DiCaprio in Titanic, and other movies.
Zao was created by a company in China, which collects phone numbers and pictures of users who want to use it after installation. People who want to be included in famous films must take a selfie and upload it to the Chinese servers, then choose the film sequences from the existing list.
Zao has been downloaded since then by millions upon millions of people who were eager to brag on social networks about what the application can do. Functionality is reminiscent of that of FaceApp, which allowed the modification of faces only in pictures, but Zao is much more complex.
Zao sends pictures and phone numbers to China, to be used in who knows what ways, FaceApp sending people's pictures to Russia, so it's just as bad. Zao remains in the App Store at the moment, and continues to be popular, and those from Apple will probably not remove it in order not to risk entering the "eyes" of the Chinese government.
