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.