GHSA-r48c-v28r-pf6v: MCP Registry has an unauthenticated SSRF: HTTP namespace verification dials 6to4 / NAT64 / site-local IPv6 addresses, bypassing private-address allowlist
SummaryThe 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`.DetailsVulnerable code`internal/api/handlers/v0/auth/http.go:125-133`:Per Go source (`src/net/ip.go`), the relevant stdlib helpers cover:| Helper | IPv6 coverage |
|---|---|
| `IsLoopback` | `::1`, IPv4-mapped of 127/8 (via `To4()` fast-path) |
| `IsPrivate` | ULA `fc00::/7` only — `ip[0]&0xfe == 0xfc` |
| `IsLinkLocalUnicast` | `fe80::/10` only — `ip[1]&0xc0 == 0x80` (NOT `fec0::/10` which is `0xc0`) |
| `IsMulticast` | `ff00::/8` |
| `IsUnspecified` | `::` |The Registry's blocklist therefore **does not** cover:| Prefix | Defined in | Why dangerous |
|---|---|---|
| `2002::/16` | RFC 3056 (6to4) | Bits 16-47 embed an arbitrary IPv4 address. `2002:a9fe:a9fe::` is the 6to4 encoding of `169.254.169.254` (AWS / Azure metadata). `2002:0a00:0001::` encodes `10.0.0.1`. On hosts with 6to4 routing or any explicit `2002::/16` route, the dial reaches the embedded IPv4. |
| `64:ff9b::/96` | RFC 6052 (NAT64 well-known prefix) | Low 32 bits embed an IPv4 address. `64:ff9b::a9fe:a9fe` translates to `169.254.169.254` on any NAT64-enabled network — which is the **default** in IPv6-only GKE node pools, AWS IPv6-only EC2, Azure IPv6 VMs with NAT64, and DNS64/NAT64 corporate networks. |
| `64:ff9b:1::/48` | RFC 8215 (local-use NAT64) | Same tunnelling concern, intended for operator-defined NAT64. |
| `fec0::/10` | RFC 3879 (deprecated site-local) | Some BSD / older Linux stacks still honour these for routing into site-local internal networks. |`safeDialContext` resolves DNS once and dials by IP (good — pins against rebinding TOCTOU), but the IP-allowlist gate is the security boundary, and that gate is incomplete.Exposure surface`POST /v0/auth/http` (and `POST /v0.1/auth/http`) is registered in `internal/api/handlers/v0/auth/http.go:197-218` and routed unauthenticated in `internal/api/router/v0.go:24,39`:The handler builds `https://<attacker-domain>/.well-known/mcp-registry-auth` (line 143) and dials via the `safeDialContext`-equipped client. The `domain` parameter is taken verbatim from the unauthenticated POST body.Critical order-of-operations confirmation in `CoreAuthHandler.ExchangeToken` (`internal/api/handlers/v0/auth/common.go:246-265`):`ValidateDomainAndTimestamp(domain, timestamp)` — domain format check (no IP literal, must contain dot)`DecodeAndValidateSignature(signedTimestamp)` — hex decode**`keyFetcher(ctx, domain)`** ← SSRF dial happens here`VerifySignatureWithKeys(...)` ← only AFTER fetchSo the SSRF dial fires before any signature verification. Attacker needs only a valid RFC3339 timestamp (±15s window) and any hex string for `signedTimestamp`.PoCTested against `main` HEAD `23f4fda` (`make dev-compose` boots Registry on `localhost:8080`).Step 1 — Set up attacker DNSConfigure `attacker.example` with the AAAA records:(Equivalent free options: a domain on Cloudflare with manual AAAA, or a `requestbin`-style service with custom DNS.)Step 2 — Trigger the dial (no credentials required)Timestamp need only be within ±15s of server clock. `signedTimestamp` is any hex string — it is decoded but only verified AFTER `FetchKey` has already dialled.Step 3 — ObserveOn a NAT64-enabled host (default in IPv6-only GKE / AWS IPv6 nodes / Cloudflare WARP), the server-side dial reaches `169.254.169.254:443`. Tcpdump on the registry host confirms the outbound TLS handshake to the embedded IPv4. Where 169.254.169.254 listens on a TLS port (most cloud metadata services do not, but kube-apiserver, internal admin panels, and bespoke IPv4 services do), the connection completes and the response (limited to 4 KiB by `MaxKeyResponseSize`) is consumed as a key candidate.For hosts without 6to4 / NAT64 routing, the dial fails with `no route to host` rather than `refusing to connect to private or loopback address` — proving the gate did not block. The differential error message provides a blind-SSRF oracle for probing internal services for existence / TLS port reachability.Expected behaviour after fix`isBlockedIP` should return `true` for any IPv6 address in the prefix families listed above, mirroring the n8n-mcp `isPrivateOrMappedIpv6` helper (GHSA-56c3-vfp2-5qqj patch). Reference implementation:Then extend the call site:A regression test fixture should set up a stub resolver returning each of the four prefix families and assert that `safeDialContext` returns the "private/loopback" error before any dial.ImpactCWE: **CWE-918** Server-Side Request Forgery (consistent with parent precedent GHSA-56c3-vfp2-5qqj).CVSS:3.1: matching the n8n-mcp precedent (`AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N` ~= **8.5 HIGH**). AC = High because exploitation depends on the registry host having NAT64 or 6to4 routing — the **default** on IPv6-only and dual-stack cloud network plans (GKE IPv6, AWS IPv6-only EC2, Azure IPv6 VMs with NAT64) but not on plain-IPv4 deployments. Privileges = None (the endpoint is the login flow itself).For the official `https://registry.modelcontextprotocol.io` deployment specifically, this lets an unauthenticated attacker reach any IPv4 address that is routable from the registry's outbound interface — including AWS / GCP / Azure metadata services if hosted on a cloud VM with metadata enabled, internal Kubernetes API servers, internal admin panels, etc. The 4 KiB response cap (`MaxKeyResponseSize`) limits exfiltrated content per request but does not prevent fingerprinting / oracle attacks (status-code differential, response-length differential).Self-hosters running the registry on dual-stack / IPv6-only infrastructure are equally exposed.Why this slipped past PR #1227The April 29 hardening batch (commit `1201cbd`, "security: fix open redirect and add small hardening") explicitly added `safeDialContext` to block "loopback, RFC1918, link-local, multicast, CGNAT, or IP-literal/single-label" addresses. The author correctly identified the IPv4 attack surface and the link-local cloud-metadata vector, but composed the blocklist from Go's per-class stdlib helpers — which collectively miss the IPv6 prefix families that *embed* IPv4. The same gap was caught and fixed in n8n-mcp (GHSA-56c3-vfp2-5qqj). No commits in `git log --since=2026-03-01 internal/api/handlers/v0/auth/http.go` reference 6to4 / NAT64 / site-local.CreditReported by **Matteo Panzeri** (GitHub: **matte1782**).
CVSS v4.0: 6.3
CVSS v3.1: 4