MCP-SEC-003

Error messages do not echo request headers or credentials

This rule's verbatim spec quote hasn't been captured yet — see validator/README.md. It is not yet counted toward coverage or graded.

How to fix this

House security rule, not a literal MCP spec requirement.

Example: failing and passing

The error body repeats a request header value verbatim, so a secret in that header ends up in logs or error pages.

Violates MCP-SEC-003 · HTTP

POST /mcp HTTP/1.1
Host: mcp.example.com
Mcp-Method: tools/list
X-Tenant-Key: tk_live_93f1c2
...

HTTP/1.1 400 Bad Request
Content-Type: text/plain

Unknown tenant key tk_live_93f1c2

The error describes the problem without copying the header value back.

Complies with MCP-SEC-003 · HTTP

POST /mcp HTTP/1.1
Host: mcp.example.com
Mcp-Method: tools/list
X-Tenant-Key: tk_live_93f1c2
...

HTTP/1.1 400 Bad Request
Content-Type: text/plain

Unknown tenant key

How the validator checks this

Probe P3.1, against a streamable-http server on the modern protocol. What it reports:

pass: A distinctive request header was not reflected in the response body
warn: The response body reflected a request header value back verbatim. Echoing request headers is how a proxy-injected or credential-bearing header ends up in a log or an error page.
skip: The server returned no body, so there was nothing that could echo a request header

Quoted from the probe that runs this check, so it cannot drift from what the validator actually reports.

This rule is checked heuristically: a fail is inferred rather than certain, so it bumps the overall grade up one level instead of being treated as certain non-compliance.

Checked in the same request as MCP-HDR-001.

Check your own server against MCP-SEC-003

Probe P3.1 checks MCP-SEC-003 in the same request as 1 other rule, so one run shows your server's result for this rule next to the rest. Validate a server or read how the validator works.