Modern connected vehicles often prioritize defending against external attacks while overlooking internal vulnerabilities within in-vehicle networks. Electronic Control Units (ECUs), the computational backbone of these networks, are critical targets; their compromise can cascade adverse effects across the vehicle, jeopardizing safety and performance. However, analyzing and exploiting ECU firmware is challenging due to system complexity and security mechanisms. This paper introduces X-ECU, a practical framework for analyzing and exploiting ECU firmware. X-ECU identifies internal vulnerabilities, extracts critical information (e.g., CAN frame IDs, receiving masks, clock frequency settings), and enables in-vehicle network attacks. Unlike traditional reverse engineering, X-ECU reduces errors and time complexity while bypassing ECU protections like firmware security strategies and compiler optimizations. Leveraging binary code similarity, X-ECU maps intermediate files from firmware development to the final firmware, identifying library functions, inline functions, and critical configurations. Its instruction-set-based design ensures compatibility across automotive chip architectures, making it applicable to mainstream ECUs. Evaluations on Infineon Tricore and TI MSP432 demonstrate X-ECU’s effectiveness in extracting key data and enabling targeted attacks. The framework advances understanding of in-vehicle network vulnerabilities and serves as a tool for both security analysis and exploitation.