CVE-2026-54299

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:

1. Have a prerendered 404 or 500 page
2. 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.

Credit

5ud0 / Tarmo Technologies

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-54299 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.4.6); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-54299? CVE-2026-54299 is a high-severity improper input validation vulnerability in astro (npm), affecting versions < 6.4.6. It is fixed in 6.4.6. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-54299? CVE-2026-54299 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of astro are affected by CVE-2026-54299? astro (npm) versions < 6.4.6 is affected.
4. Is there a fix for CVE-2026-54299? Yes. CVE-2026-54299 is fixed in 6.4.6. Upgrade to this version or later.
5. Is CVE-2026-54299 exploitable, and should I be worried? Whether CVE-2026-54299 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-54299 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-54299? Upgrade astro to 6.4.6 or later.