USIM Application Toolkit (USAT) is the standardized toolkit interface for modern USIM cards. It lets the card send proactive commands to the phone or modem, rather than remaining a passive identity token. Those commands can request actions such as displaying prompts, opening menus, or, on some implementations, interacting with modem control paths like AT commands.
USAT matters because it creates a control boundary inside cellular devices. If a modem or embedded module accepts toolkit commands too broadly, a compromised or malicious SIM can influence radio-stack behavior and potentially affect the host system that depends on it. Defenders should treat USAT as a privileged input channel: inventory where it is enabled, disable it when unnecessary, and test modem firmware for strict validation and isolation. In practice, security comes from limiting which proactive commands are supported and ensuring the baseband cannot be steered into dangerous actions by SIM-originated input.