CVE-2026-53607

ApostropheCMS is an open-source Node.js content management system. In versions up to and including 4.30.0, 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. 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 neutralized by cuid uniqueness, but blind-SSRF residuals remain (network-topology mapping via response-code / timing differences and verbose proxy/WAF 404 body disclosure). As of time of publication, no known patched versions exist.

ApostropheCMS is an open-source Node.js content management system. In versions up to and including 4.30.0, 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. 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 neutralized by cuid uniqueness, but blind-SSRF residuals remain (network-topology mapping via response-code / timing differences and verbose proxy/WAF 404 body disclosure). As of time of publication, no known patched versions exist.

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 [email protected] (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…

ApostropheCMS is an open-source Node.js content management system. In versions up to and including 4.30.0, 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. 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 neutralized by cuid uniqueness, but blind-SSRF residuals remain (network-topology mapping via response-code / timing differences and verbose proxy/WAF 404 body disclosure). As of time of publication, no known patched versions exist.

ApostropheCMS is an open-source Node.js content management system. In versions up to and including 4.30.0, 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. 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 neutralized by cuid uniqueness, but blind-SSRF residuals remain (network-topology mapping via response-code / timing differences and verbose proxy/WAF 404 body disclosure). As of time of publication, no known patched versions exist.

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 [email protected] (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…