SIM cards: The root of trust in modern cellular networks. Phones, cars, trains, payment terminals, 5G routers, smart devices - chances are that they use a SIM to connect to the internet via mobile networks. In this talk, we will show that SIMs are more than passive storage devices containing secrets. We will discuss their inner workings and provide a reverse-engineering guided run-down of the interactions between modern phones and SIMs. We then highlight our approach for finding vulnerabilities in baseband implementations, triggerable from hostile SIM cards. Furthermore, we take a look at the high-impact vulnerabilities we found during our research, affecting millions of devices. Lastly, we show how we repurposed commercially available SIM interposers including patching their firmware at binary level. Our modified interposer acts as a malicious machine-in-the-middle device, attacking the baseband.