Cloud-native systems enable high-scalibility for application development and deployment with loosely coupled microservices that interact over the network, but they also introduce security risks, potentially leading to unauthorized inter-service access. To mitigate such risks, existing approaches rely on manual policy configuration or static code analysis. However, these methods are time-consuming for policy maintainance, require code avalibility and fail to support access control for hidden service invocations. To address these limitations, we propose \textit{MicroGuard}, an access control system that automatically generates and enforces access control policies in microservices. During microservice development and testing, MicroGuard captures and analyzes inter-service communication packets to identify hidden runtime service invocations. MicroGuard leverages a bidirectional prefix tree (Trie) and pre-trained language models to extract comprehensive access control policies. During execution of microservice systems, MicroGuard enforces these policies to detect and block unauthorized access requests. Our experimental results show that MicroGuard effectively captures and rejects all unauthorized attempts, introducing an average processing delay of only 2% to the original response time of microservice systems.