OCPPuzz: Specification-Driven Fuzzing of Charging Station Management Systems with Large Language Model

Electric vehicles (EVs) are being rapidly adopted, with over 61,000 publicly accessible charging stations deployed across the United States as of 2024. A core component of this infrastructure is the Charging Station Management System (CSMS), which is responsible for security-critical tasks such as user authentication and billing. Given its importance, the CSMS has become a target of real-world attacks that have resulted in financial losses, data breaches, and denial-of-service (DoS) incidents. Nevertheless, research on CSMS security remains limited, and automated testing tools are lacking. Testing CSMS is challenging because they communicate with charging stations (CS) using the Open Charge Point Protocol (OCPP). Effective testing must contend with OCPP's complexity: 1) messages containing up to 48 fields, 2) inter- and intra-message field dependencies, and 3) its stateful nature, which requires tracking the states of both CS and CSMS during testing. To address these challenges, we present OCPPuzz, a specification-based fuzzing framework for CSMS. OCPPuzz automatically extracts message structures, field constraints, and dependency rules from the OCPP specification, as well as valid CS-CSMS state transitions described in its use case diagrams. To handle specifications expressed in natural language and semi-formal diagrams, OCPPuzz combines heuristic rule-based extraction with a large language model (LLM). We evaluated OCPPuzz on four open-source CSMS implementations and uncovered numerous deviations from the OCPP specification that led to critical security issues, including DoS and free charging. We reported 930 implementation bugs to the corresponding vendors, of which 492 have been acknowledged so far. In addition, we reported 155 specification bugs in OCPP to the Open Charge Alliance (OCA); 78 have been committed for fixes and 82 acknowledged for further investigation. We expect additional acknowledgments and fixes in the near future.