Initially, our team member, Đỗ Minh Tuấn, wanted to write about the RCA (Root Cause Analysis) of CVE-2021-1870 which APT used. But Maddie Stone pointed it to us that it was actually CVE-2021-1789. None-the-less, we would still want to share with everyone the analysis done by Đỗ Minh Tuấn. The bug is assigned CVE-2021-1789 in security content of Safari 14.0.3. We successfully exploited it on WebKitGTK <= 2.30.5 or equivalent on WebKit.