MCPP-PRM09: Unsafe signed_metadata¶


Family	Protected Resource Metadata (RFC 9728)
Default severity	`medium`
Confidence	`high`
Mode	passive (default scan)
Spec revisions	2025-06-18 and later

Why it matters¶

Signed metadata takes precedence over plain values (RFC 9728 §2.2). It MUST be a JWS with an iss claim; alg=none, or claims that contradict the JSON, defeat its purpose.

Remediation¶

Sign metadata with an asymmetric algorithm, include iss, and keep the signed claims identical to the plain document (or drop signed_metadata).

References¶

RFC 9728 Protected Resource Metadata

Suppressing¶

# .mcp-posture-ignore
[[ignore]]
check = "MCPP-PRM09"
target = "https://mcp.example.com/*"
justification = "Why this is acceptable here"
expires = 2026-12-31