Client ID Metadata Documents (CIMD) is the identity mechanism that became the default for MCP clients in the November 2025 specification update, replacing Dynamic Client Registration as the preferred way to identify clients to authorization servers. Defined in the IETF draft draft-ietf-oauth-client-id-metadata-document, CIMD lets clients use a URL as their client_id, where the URL itself resolves to a JSON document describing the client’s metadata. The authorization server fetches the document on first use, validates it against signed assertions, and treats the URL as the client identifier from that point forward. CIMD removes the registration step entirely and shifts client identity to infrastructure-asserted proof.
Why CIMD Replaced DCR as the MCP Default
Dynamic Client Registration’s openness caused real harm. Attackers registered phishing clients with target authorization servers, leading to one-click account takeover incidents Obsidian Security documented in 2025. CIMD closes most of those vectors because client identity is bound to a URL the client must actually control. Domain takeover becomes a prerequisite for impersonation, not just a registration call. The MCP November 2025 spec made CIMD the preferred default for new deployments while keeping DCR as a backward-compatible fallback. Authorization servers that support both should give CIMD higher trust by default.
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How to Implement CIMD Correctly for MCP
Host the metadata document over HTTPS with a valid certificate and a stable URL. Sign the document where possible using a key tied to your organization’s identity. Include only redirect URIs and grant types you actually use. Cache the document on the authorization server side with reasonable TTLs but refresh on signature failure. Treat CIMD client_ids as first-class identities and apply per-client policies based on the URL’s domain.
Conclusion
Client ID Metadata Documents (CIMD) became the MCP default in November 2025, replacing Dynamic Client Registration with a URL-anchored, infrastructure-asserted client identity model that closes the registration-abuse attack vectors that hit early MCP deployments. The Certified MCP Security Expert (CMCPSE) certification trains engineers to implement CIMD correctly and migrate existing DCR-based MCP deployments safely.