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 security vulnerabilities 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 the frontend requests and the backend responses from dependent services. We also present the results of our fuzz testing, which reported and got fixed thousands of security vulnerabilities in real-world microservice applications.