Windows kernel zero-day exploit (CVE-2021-1732) is used by BITTER APT in targeted attack. This zero-day is a new vulnerability which caused by win32k callback, it could be used to escape the sandbox of Microsoft IE browser or Adobe Reader on the lasted Windows 10 version. The quality of this vulnerability high and the exploit is sophisticated. The use of this in-the-wild zero-day reflects the organization’s strong vulnerability reserve capability. The threat organization may have recruited members with certain strength, or buying it from vulnerability brokers. The in-the-wild zero-day: 1. It targets the latest version of Windows10 operating system 2. The vulnerability is high quality and the exploit is sophisticated 3. The attacker used it with caution