<p>A closed-source ICS communication is a fundamental component of supervisory software and PLCs operating critical infrastructure or configuring devices. As this is a vital communication, a compromised protocol can allow attackers to take over the entire critical infrastructure network and maliciously manipulate field device values. Thus, it is crucial to conduct security assessments of these closed-source protocol communications before deploy?ing them in a production environment to ensure the safety of critical infrastructure. However, Fuzzing closed-source communication without understanding the protocol structure or state is ineffective, making testing such closed-source communications a challenging task. </p> <p><br></p> <p>This research study introduces the APEX-ICS framework, which consists of two significant components: Automatic closed-source ICS protocol reverse-engineering and stateful black-box fuzzing. The former aims to reverse-engineer the protocol communication, which is critical to effectively performing the fuzzing technique. The latter component leverages the generated grammar to detect vulnerabilities in communication between supervisory software and PLCs. The framework prototype was implemented using the Codesys v3.0 closed-source protocol communication to conduct reverse engineering and fuzzing and successfully identified 4 previously unknown vulnerabilities, which were found to impact more than 400 manufacturer’s devices. </p>