Wireless communications coprocessors are a vital component of numerous Internet of Things and mobile devices. These subsystems enable devices to communicate directly with peers and supporting network infrastructures. Previous research has shown that wireless communications coprocessors lack fundamental security mechanisms to combat attacks originating from the air-interface and application processor (main CPU). To mitigate the risk of exploitation, methods are needed to retroactively add security mechanisms to communications coprocessors.This chapter focuses on securing a cellular baseband processor from attacks by hostile applications in the application processor. Such attacks often leverage attention (AT) commands to exploit vulnerabilities in baseband firmware. The attacks are mitigated by installing an AT command intrusion prevention system between the application processor and baseband processor interface.