An issue was discovered in certain Apple products. iCloud before 6.2 on Windows is affected. iTunes before 12.6 on Windows is affected. The issue involves cleartext client-certificate transmission in the "APNs Server" component. It allows man-in-the-middle attackers to track users via correlation with this certificate.
https://support.apple.com/HT207607
https://support.apple.com/HT207599