CVE-2026-53607: @apostrophecms/file pretty-URL Vulnerable to Unauthenticated SSRF via Host header

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).