Astro, a popular web framework, implements a server‑side render (SSR) engine that can display custom error pages. In versions earlier than 9.5.4, when a non‑existent route triggers such a custom error page, the framework uses the Host header supplied by the client to fetch a file named /500.html. If an attacker supplies a Host header that points to an external server, the framework will perform a request to that server just to obtain the error page. By controlling the content served on that external server, the attacker can cause Astro to follow a redirect back into the internal network and read the resulting response body. This chain turns the Host header into a primitive for Server‑Side Request Forgery, enabling access to internal URLs, cloud metadata endpoints, or other services that are normally unreachable from the public internet.

The vulnerability affects the withastro:astro product in all releases prior to 9.5.4. All users running Astro versions 9.5.3 and earlier on any Node.js environment are potentially exposed.

The CVSS v3 score of 6.9 reflects a moderate severity, with a low exploitation probability (EPSS < 1%) and no current listing in the CISA KEV catalog. Attackers need the ability to send a request bearing a crafted Host header to the affected application; this could arise via direct access or through a compromised proxy that does not validate the Host header. Once the prerequisite reaches the server, the attacker can read sensitive internal content but no arbitrary code execution is described.