Developer Builds MCP Linter and Discovers Security Issues in Anthropic Official Servers
A developer has created mcp-linter, a zero-dependency security linter for the Model Context Protocol (MCP) introduced by Anthropic. The tool helps developers avoid common pitfalls when building servers that connect Claude Desktop, Claude Code, Cursor, and autonomous agents to external resources.
The Model Context Protocol is built on JSON-RPC 2.0 and uses stdio transport for local servers. Any unintended output to stdout, such as a stray console.log statement, immediately breaks the protocol framing and causes the client to disconnect or fail parsing.
Four hidden problems identified
The article details four recurring issues. First, stdout pollution occurs when diagnostic messages are written to the wrong stream instead of stderr. Second, missing destructiveHint and readOnlyHint annotations allow agents to invoke dangerous tools like delete_user without human oversight. Third, tool poisoning lets attackers embed prompt-injection directives inside tool descriptions that are fed directly to the model. Fourth, incomplete input schemas without descriptions force the LLM to guess argument formats, raising hallucination rates.
The linter launches the target server in an isolated child process, performs an initialize handshake, requests tools/list, resources/list and prompts/list, then evaluates 34 rules before cleanly exiting. It produces a health score from 0 to 100 and can emit SARIF reports for GitHub Code Scanning.
Results on official Anthropic servers
- @modelcontextprotocol/server-everything scored 90/100; one parameter lacked a description.
- @modelcontextprotocol/server-memory scored 66/100; four parameters were missing descriptions and top-level instructions were absent.
- @modelcontextprotocol/server-filesystem scored only 38/100; 18 parameters had no descriptions, making correct tool calls significantly harder for models.
The project is available on npm and GitHub under the MIT license. Planned enhancements include support for SSE and WebSocket transports, context-budget auditing, and LLM-assisted schema repair.
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