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

Summary

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.

Details

Vulnerable code

internal/api/handlers/v0/auth/http.go:125-133:

func isBlockedIP(ip net.IP) bool {
    if ip == nil {
        return true
    }
    return ip.IsLoopback() || ip.IsPrivate() ||
        ip.IsLinkLocalUnicast() || ip.IsMulticast() ||
        ip.IsUnspecified() ||
        cgnatRange.Contains(ip)
}

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:

huma.Register(api, huma.Operation{
    OperationID: "exchange-http-token...",
    Method:      http.MethodPost,
    Path:        pathPrefix + "/auth/http",
    Summary:     "Exchange HTTP signature for Registry JWT",
    ...
}, func(ctx context.Context, input *HTTPTokenExchangeInput) (...) {
    response, err := handler.ExchangeToken(ctx, input.Body.Domain, ...)
    ...
})

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):

1. ValidateDomainAndTimestamp(domain, timestamp) — domain format check (no IP literal, must contain dot)
2. DecodeAndValidateSignature(signedTimestamp) — hex decode
3. keyFetcher(ctx, domain) ← SSRF dial happens here
4. VerifySignatureWithKeys(...) ← only AFTER fetch

So the SSRF dial fires before any signature verification. Attacker needs only a valid RFC3339 timestamp (±15s window) and any hex string for signedTimestamp.

PoC

Tested against main HEAD 23f4fda (make dev-compose boots Registry on localhost:8080).

Step 1 — Set up attacker DNS

Configure attacker.example with the AAAA records:

attacker-6to4.example.       AAAA  2002:a9fe:a9fe::         ; 6to4 -> 169.254.169.254
attacker-nat64.example.      AAAA  64:ff9b::a9fe:a9fe       ; NAT64 -> 169.254.169.254
attacker-rfc1918.example.    AAAA  64:ff9b::a00:0001        ; NAT64 -> 10.0.0.1

(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)

curl -i https://registry.modelcontextprotocol.io/v0/auth/http \
  -H 'Content-Type: application/json' \
  -d "{\"domain\":\"attacker-nat64.example\",\"timestamp\":\"$(date -u +%Y-%m-%dT%H:%M:%SZ)\",\"signedTimestamp\":\"00\"}"

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 — Observe

On 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:

func isBlockedIPv6Prefix(ip net.IP) bool {
    v6 := ip.To16()
    if v6 == nil || ip.To4() != nil {
        return false
    }
    // 6to4 (2002::/16)
    if v6[0] == 0x20 && v6[1] == 0x02 {
        return true
    }
    // NAT64 well-known 64:ff9b::/96
    if v6[0] == 0x00 && v6[1] == 0x64 && v6[2] == 0xff && v6[3] == 0x9b &&
       v6[4] == 0 && v6[5] == 0 && v6[6] == 0 && v6[7] == 0 {
        return true
    }
    // NAT64 RFC 8215 local-use 64:ff9b:1::/48
    if v6[0] == 0x00 && v6[1] == 0x64 && v6[2] == 0xff && v6[3] == 0x9b &&
       v6[4] == 0x00 && v6[5] == 0x01 {
        return true
    }
    // Site-local fec0::/10 (deprecated, RFC 3879 -- still honoured by some stacks)
    if v6[0] == 0xfe && (v6[1]&0xc0) == 0xc0 {
        return true
    }
    return false
}

Then extend the call site:

return ip.IsLoopback() || ip.IsPrivate() ||
    ip.IsLinkLocalUnicast() || ip.IsMulticast() ||
    ip.IsUnspecified() ||
    cgnatRange.Contains(ip) ||
    isBlockedIPv6Prefix(ip)

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.

Impact

CWE: 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 #1227

The 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.

Credit

Reported by Matteo Panzeri (GitHub: matte1782).