Malicious SIMs can hijack smartphones, steal files, and lock them onto 2G
Researchers have found that compromised or malicious SIM cards can issue commands to some smartphones and cellular-connected devices, allowing attackers to steal information, disrupt communications, downgrade connections to 2G, and in some cases execute code.
Tomasz Piotr Lisowski and Dr Marius Muench of the University of Birmingham, working with Kristian Covic from Fuzzware, traced this to a legitimate function already built into the cellular spec.
That function is Proactive SIM. It lets a card send instructions to a device’s modem instead of only answering the modem’s requests. One instruction, RUN AT, tells the modem to execute an AT command, the control language modems have used since the 1980s. A SIM that shouldn’t be trusted gets a direct line into the hardware.
To determine how much access this provides, the researchers developed a toolkit called CATana and tested it against 18 smartphones and eight cellular IoT devices. Two of the smartphones exposed an AT command interface to the SIM. Seven of the eight IoT devices did the same.
The testing uncovered four vulnerabilities and produced demonstrations involving code execution, file theft, denial of service, and forced network downgrades.
“The fascinating part here is that the proactive capabilities of a SIM and the resulting attack surface is explicitly defined in the technical specifications for cellular communication, resulting into ‘specification-compliant’ attacks,” said Dr Marius Muench, Assistant Professor, University of Birmingham.
“Other researchers, cybersecurity experts, and leaked intelligence documents have shown some of the dangers of hostile SIMs before us. Yet, the resulting risks have not been fully mitigated. Potentially, this is because hostile SIMs are not included in most threat models; although we slowly see a promising shift here,” added Muench.
Hostile SIM attacks in practice
On an AUTEL EV charger built around a Quectel EC25-AFX module, SIM-issued commands triggered a command injection flaw in the modem’s Linux-based application processor, giving the researchers code execution on the cellular module.
An OPPO Reno14 F 5G accepted 198 AT commands and variants through the SIM interface, including commands that could power off the phone, stop its modem, or force the connection onto 2G. The downgrade persisted after the researchers enabled airplane mode, disabled the SIM, and changed the phone’s network settings. Because 2G lacks network-to-device authentication, a device forced onto the older network can also be more susceptible to rogue base stations impersonating legitimate cellular networks.
Against a Quectel EG25-G modem, the team combined a malicious symbolic link with commands sent from the SIM to email a targeted file from the device to a server they controlled.
None of these attacks works without control of the SIM itself. The researchers describe several ways this could happen, including vulnerabilities in SIM software, physical replacement or tampering, abuse of remote SIM-management infrastructure, and modification during manufacturing or distribution.
Separately, the researchers found that affected Android versions allowed a hostile SIM to trigger the standardized LAUNCH BROWSER command and open an attacker-controlled website without user interaction, including while the phone was locked. Google tracked the issue as CVE-2025-48618 and addressed it for Android 13 through 16 in December 2025.
The team reported its findings to the GSM Association and to affected chip and device makers before publishing. Several manufacturers have already shipped patches or hardened configurations, and the wider SIM AT issue is logged by GSMA as CVD-2026-0122.
“It was great working together with the affected companies and GSMA,” Muench said. “Our reports were treated seriously, and key manufacturers made software updates and hardened configurations available to their customers. This will benefit billions of future SIM-enabled devices operating worldwide, including smartphones, connected vehicles, payment terminals, routers, critical infrastructure and EV charging systems.”
Researchers call for RUN AT to be retired
Kristian Covic called hostile SIMs “an overlooked attack vector” and said the team was glad to help demonstrate the risk through the project.
Patching individual bugs isn’t enough on its own, the team argues. Their recommendation is to deprecate the RUN AT feature altogether and strip the related code out of modem and device firmware, rather than leaving it dormant behind a filter.
“The attacks we found only scratch the surface of what is possible with hostile SIM cards. We will keep working on bringing more of the attack surface to the public light and hope to cooperate with vendors and standardization bodies to remedy the risks in today’s and future devices,” Tomasz Piotr Lisowski, concluded.