Exposing GraphQL subgraphs publicly can lead to serious security breaches due to misconfigured federation, which enables splitting up business logic and improving performance. However, this also opens the door to unauthorized access to sensitive data and capabilities through various means such as direct querying, entity resolvers, or overridden schema fields. To mitigate these threats, it's essential to protect subgraphs at the application level by adding extra layers of authorization, using unique secrets per subgraph, and configuring routers to send headers containing shared secrets to each subgraph. This approach is considered the most consistent and effective way to prevent undesired access to subgraph capabilities, and implementing it immediately is crucial for securing GraphQL microservices.