Implementation guide
OWASP AISVS v1.01
Model Context Protocol (MCP) Security · Schema, Message, and Input Validation
Reject unknown or oversized MCP parameters
Verify that MCP servers reject unrecognized or oversized parameters in function calls.
OWASP AISVS v1.01, requirement 10.4.3
In plain English
An MCP server should reject a tool call that contains parameters it does not recognise or values that are too large. Quietly ignoring extras hides probing and lets attacker-controlled data ride along. Oversized values are a cheap way to exhaust resources.
What good looks like
- A call with an unrecognised parameter is rejected.
- Every parameter has a size or length limit.
- Errors say what was wrong without revealing internals, and are logged.
How teams usually do it
- Input schemas with additionalProperties: false, plus maxLength, maxItems and numeric ranges on every field.
- Validation in shared middleware so that every tool gets it.
Suggested evidence
- Tool schemas.
- Tests with unknown and oversized parameters.
Common mistakes
- Ignoring unknown parameters silently.
- Limits on the request as a whole and none on individual fields.
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Requirement text from the OWASP AI Security Verification Standard, licensed under CC BY-SA 4.0. This guidance is shared under the same licence.