Libpng Security Vulnerability Detected by Play Store using Unity 5.6.0f3

I have recently submitted my app to the Play Store for closed beta testing but I got denied because the Play Store detected I was using an old version of libpng that has a security vulnerability. I tried to find it from the assets I've made (which include a bunch of native plugins) but I can't seem to find it. I've also asked the creators of Assets I bought from the Asset Store but they have all told me it has something to do with the engine itself. One of them saying it has to do something with the Texture2D.LoadImage and Texture2D.EncodeToPNG API's. I was wondering what I should do about this?