A malicious or compromised SIM card can use standard cellular functions to push some phones and cellular-connected devices into unsafe behavior, according to The Register’s report on research presented this week at the USENIX WOOT conference in Baltimore. The work examines “proactive SIM” functionality, a standards-defined mechanism that lets a SIM issue commands to the device that hosts it. One command path, RUN AT, allows the SIM to request execution of AT commands — the long-running instruction set used to control modems. The researchers’ finding is not that SIMs are exploiting a stray debug feature, but that specification-compliant behavior can create an attack surface when a hostile SIM is present. The Register identifies the researchers as Tomasz Piotr Lisowski and Marius Muench of the University of Birmingham, working with Fuzzware’s Kristian Covic. They built a toolkit called CATANA to test what a malicious SIM could do across 26 devices: 18 smartphones and eight Internet of Things modems. The results were uneven but material. According to The Register, nine of the 26 devices exposed an AT command interface to the SIM. The IoT devices were more permissive in the test set: seven of the eight modems exposed that interface. The researchers reported four vulnerabilities and demonstrated attacks including code execution, arbitrary file reads, denial of service, and forcing a connection down to 2G. The demonstrations show why the issue is more than theoretical for connected infrastructure. On an Autel electric-vehicle charger fitted with a Quectel EC25-AFX cellular module, the researchers used commands from the SIM to exploit a command injection flaw in the modem’s Linux-based application processor and obtain code execution, The Register reports. On a Quectel EG25-G modem, they combined a malicious symbolic link with SIM-originating commands to email a targeted file to an attacker-controlled server. The phone-side findings were narrower but still notable. On an Oppo Reno14 F 5G, the researchers found 198 AT commands and variants available through the SIM interface, according to the report. Some could power down the handset, kill the modem, or push the device back onto 2G. The Register says the 2G downgrade was persistent enough that airplane mode, disabling the SIM, and changing network settings did not reverse it. The attack model has an important constraint: the attacker needs control of the SIM. The Register lists possible paths as compromised SIM software, physical tampering, abuse of remote administration by a malicious or breached operator, or supply-chain compromise. That makes this different from a drive-by mobile exploit, but relevant to fleets and embedded devices where SIM provisioning, remote management, and hardware custody are part of the security boundary. The research also found that vulnerable Android versions allowed a hostile SIM to use the standardized LAUNCH BROWSER command to open an attacker-controlled website without user interaction, even while the phone was locked, according to The Register. Google tracked that issue as CVE-2025-48618 and patched Android 13 through 16 in December 2025. The researchers disclosed their findings to Google, Oppo, Quectel, Semtech, and Qualcomm in March, The Register reports. The provided material does not include each vendor’s response or whether all affected modem behaviors have been remediated, so the current takeaway is bounded: some tested devices exposed a standards-defined SIM command path in ways that enabled serious attacks under a hostile-SIM threat model. Who benefits: Security teams with cellular fleets benefit from a clearer test target: whether their devices expose AT command interfaces to the SIM and how modem commands are filtered. Researchers and tooling vendors may also gain from CATANA-style testing of SIM-to-device behavior. Who's exposed: The exposure is not universal; The Register reports nine exposed devices in a 26-device test set. The devices most exposed in that set were IoT modems, with seven of eight allowing the SIM to reach an AT command interface.