Excessive permissions: applying least privilege to MCP servers

Introduction

A server can work correctly and still have access to more data or actions than it needs. Least privilege does not remove every risk, but it bounds the impact of an unexpected call.

From general permission to specific capabilities

Start with a tool matrix: for each one, define purpose, invoking identity, resources, operations, effects and returned data. Separate reading from writing and prevent a query tool from inheriting administrative permissions. Limit data by user, project or resource and validate the requested object in the server, not only in the interface.

Credentials should belong to a bounded service identity, with rotation and expiry appropriate to the case. For HTTP transport with MCP authorization, the protocol describes scopes and flow requirements; the security guide recommends requesting scopes that match the operation. An authenticated connection must not be interpreted as unrestricted authorization for every function.

Operational controls

Disable unnecessary tools, restrict network destinations and file system access, and isolate processes when appropriate. Set limits on request volume, size and frequency. For high-impact actions, add informed confirmation and a rollback path when possible.

Review permissions periodically and after changes of server, owner or integration. Test access-denied cases as well as the happy path. A test should confirm that the server cannot reach resources outside the task, even when it receives unexpected parameters.

Conclusion

Least privilege applies to every leg: process, tool, token, file, API and user. Reducing the available authority is more reliable than hoping the model always chooses not to use it.