Compromised SIM Cards Exploit Proactive SIM to Steal Files, Execute Code and Force 2G on Smartphones and IoT Devices

Researchers from the University of Birmingham and Fuzzware have shown that a compromised SIM card can take control of certain smartphones and IoT devices by abusing a built-in cellular standard feature. The attack allows file theft, disabling of connectivity, forced downgrade to 2G networks and even arbitrary code execution.

The technique relies on the Proactive SIM capability, which lets the SIM card itself send instructions to the modem. Specifically, the RUN AT command forces the modem to execute AT commands, a control language dating back to the 1980s. This creates a direct, standards-compliant channel from the SIM card to the device hardware.

To validate the findings, the researchers developed the CATana toolkit and evaluated 18 smartphones plus eight cellular-connected devices. The AT command interface was reachable from the SIM on two phones and seven IoT units. In total, four vulnerabilities were discovered and weaponized to achieve code execution, file exfiltration, denial of service and forced 2G downgrade.

On an AUTEL charging station equipped with a Quectel EC25-AFX module, commands from the SIM card resulted in command injection and code execution. The OPPO Reno14 F 5G accepted 198 AT commands and variants, allowing the device to be powered off, the modem stopped or the connection forced to 2G. The downgrade persisted across airplane mode, SIM removal and network setting changes, opening the door to rogue base-station attacks because 2G lacks network authentication.

In the Quectel EG25-G modem, researchers combined SIM-issued commands with a malicious symbolic link to exfiltrate chosen files to an attacker-controlled server. All attacks require prior control of the SIM card, which could be obtained through existing vulnerabilities, physical replacement, remote management infrastructure or supply-chain tampering.

Some manufacturers have already issued patches. GSMA has registered the issue as CVD-2026-0122. The researchers argue that repeatedly patching RUN AT is insufficient and recommend removing the function from device firmware altogether.