Cross-device sync is the process of keeping the same data aligned across multiple connected devices, such as a phone, tablet, and PC. In practice, it can copy messages, notifications, files, settings, or account state so the user sees a consistent experience wherever they sign in.
In cyber security, cross-device sync matters because it expands the trust boundary. A conversation that was once visible only on a locked handset may also appear on a Windows desktop, browser session, or shared workstation. That creates new exposure points for shoulder surfing, unattended sessions, weak sign-in controls, and overly permissive notification previews. Attackers may target the synced device with malware, account takeover, or session theft to reach data that is protected on the original device. Defenders treat synced endpoints as part of the same security zone: they enforce strong authentication, device encryption, screen locking, least-privilege access, and careful review of what content is mirrored between devices.