Abstract In recent years, protocol state fuzzing has emerged as an effective technique to analyze and test network protocol implementations, uncovering numerous security vulnerabilities, bugs, and non-conformance issues in them. This paper presents ProtocolState-Fuzzer ( PSF ), an open source, generic, modular, and extensible framework for state machine learning and testing of network protocol implementations. We describe the distinctive features and support that PSF offers, its architecture and implementation, and briefly overview the protocol-specific state fuzzers that currently use PSF as their basis.