CVE-2026-44430: MCP Registry has an unauthenticated SSRF: HTTP namespace verification dials 6to4 / NAT64 / site-local IPv6 addresses, bypassing private-address allowlist

The Registry’s HTTP-based namespace verification (POST /v0/auth/http, POST /v0.1/auth/http) uses safeDialContext (internal/api/handlers/v0/auth/http.go:67-110) to refuse dialling private/internal addresses when fetching the well-known public-key file from a publisher-supplied domain. The blocklist (isBlockedIP, lines 125-133) relies entirely on Go stdlib’s IsLoopback / IsPrivate / IsLinkLocalUnicast / IsMulticast / IsUnspecified plus a manual CGNAT range. None of these cover IPv6 6to4 (2002::/16), NAT64 (64:ff9b::/96 and 64:ff9b:1::/48 per RFC 8215), or deprecated site-local (fec0::/10) — all of which encode arbitrary IPv4 in the address bits and tunnel to RFC1918 / cloud-metadata services on dual-stack / NAT64-enabled hosts.

This is the same CWE-918 SSRF class fixed in GHSA-56c3-vfp2-5qqj on czlonkowski/n8n-mcp (CVSS 8.5 HIGH). The remediation pattern is identical: extend the blocklist with the IPv6 prefix families that embed IPv4.

The endpoint is unauthenticated — it is the login flow itself — so attack complexity is low aside from the host-level routing dependency.

Affected: latest main HEAD 23f4fda and current production v1.7.6 deployment at https://registry.modelcontextprotocol.io/v0/auth/http.