The microservice paradigm is a popular software development pattern that breaks down a large application into smaller, independent services. While this approach offers several advantages, such as scalability, agility, and flexibility, it also introduces new security challenges. This paper presents a novel approach to securing microservice architectures using fuzz testing. Fuzz testing is known to find programming errors in software by feeding it with unexpected or random inputs. In this paper, we propose a zero-config fuzz test generation technique for microservices that can maximize coverage of internal states by mutating both the incoming requests and the backend responses from dependent services. We successfully deployed our technique to over 95% of C++ services built on Google's internal microservice platform. It reported and got fixed thousands of errors in real-world microservice applications.