Summary
Astro SSR apps with prerendered error pages (/404 or /500 using export const prerender = true) fetch those pages over HTTP at runtime when an error occurs. The URL for this fetch is derived from request.url, which in turn gets its origin from the incoming Host header. When the Host header is not validated against allowedDomains, an attacker can point the fetch at an arbitrary host and read the response.
Who is affected
This affects SSR deployments that:
- Have a prerendered 404 or 500 page
- Use
createRequestFromNodeRequest from astro/app/node with app.render() without overriding prerenderedErrorPageFetch — this includes custom servers built on the public API and third-party adapters
Not affected:
@astrojs/node >= 9.5.4 (reads error pages from disk)@astrojs/cloudflare (uses the ASSETS binding)- The dev server (renders error pages in-process)
How it works
createRequestFromNodeRequest builds request.url from the raw Host / :authority header. The allowedDomains option is accepted but only gates X-Forwarded-For — it does not constrain the URL origin. (The public createRequest does fall back to localhost for unvalidated hosts; this internal builder did not.)
When app.render() encounters a 404 or 500 with a prerendered error route, default-handler.ts constructs the error page URL using the origin from request.url and fetches it via prerenderedErrorPageFetch, which defaults to global fetch. The response body is served to the client.
An attacker sends a request with Host: attacker-host:port, triggers an error (e.g., requesting a nonexistent path for a 404), and receives the response from the attacker-controlled host reflected back.
Remediation
The error page fetch origin is now validated against allowedDomains before use. When the host is validated, the original origin is preserved. Otherwise, it falls back to localhost. The fetch is also wrapped in a try/catch so that connection failures degrade gracefully to a plain error response.
Credit
5ud0 / Tarmo Technologies
References
5ud0er Reporter
Summary
Astro SSR apps with prerendered error pages (
/404or/500usingexport const prerender = true) fetch those pages over HTTP at runtime when an error occurs. The URL for this fetch is derived fromrequest.url, which in turn gets its origin from the incomingHostheader. When theHostheader is not validated againstallowedDomains, an attacker can point the fetch at an arbitrary host and read the response.Who is affected
This affects SSR deployments that:
createRequestFromNodeRequestfromastro/app/nodewithapp.render()without overridingprerenderedErrorPageFetch— this includes custom servers built on the public API and third-party adaptersNot affected:
@astrojs/node>= 9.5.4 (reads error pages from disk)@astrojs/cloudflare(uses the ASSETS binding)How it works
createRequestFromNodeRequestbuildsrequest.urlfrom the rawHost/:authorityheader. TheallowedDomainsoption is accepted but only gatesX-Forwarded-For— it does not constrain the URL origin. (The publiccreateRequestdoes fall back tolocalhostfor unvalidated hosts; this internal builder did not.)When
app.render()encounters a 404 or 500 with a prerendered error route,default-handler.tsconstructs the error page URL using the origin fromrequest.urland fetches it viaprerenderedErrorPageFetch, which defaults to globalfetch. The response body is served to the client.An attacker sends a request with
Host: attacker-host:port, triggers an error (e.g., requesting a nonexistent path for a 404), and receives the response from the attacker-controlled host reflected back.Remediation
The error page fetch origin is now validated against
allowedDomainsbefore use. When the host is validated, the original origin is preserved. Otherwise, it falls back tolocalhost. The fetch is also wrapped in a try/catch so that connection failures degrade gracefully to a plain error response.Credit
5ud0 / Tarmo Technologies
References