CVE-2026-54299: Astro Web Framework SSRF Vulnerability

CVE-2026-54299 Overview

CVE-2026-54299 is a high-severity input validation vulnerability in the Astro web framework. Astro server-side rendering (SSR) applications with prerendered error pages fetch those pages over HTTP at runtime when an error occurs. The framework derives the fetch URL from request.url, which obtains its origin from the incoming Host header. When the Host header is not validated against allowedDomains, an attacker can redirect the fetch to an arbitrary host and read the response. The issue affects Astro versions prior to 6.4.6 and is classified under [CWE-20: Improper Input Validation].

Critical Impact
Attackers can manipulate the Host header to coerce Astro SSR apps into fetching error pages from attacker-controlled hosts, exposing the response content to the requester.

Affected Products

Astro web framework versions prior to 6.4.6
Astro SSR applications using prerendered /404 or /500 pages with export const prerender = true
Node.js-based Astro deployments without allowedDomains configured

Discovery Timeline

2026-06-22 - CVE-2026-54299 published to the National Vulnerability Database (NVD)
2026-06-23 - Last updated in NVD database

Technical Details for CVE-2026-54299

Vulnerability Analysis

The vulnerability resides in how Astro handles runtime fetches for prerendered error pages in SSR mode. When an application defines /404 or /500 pages with export const prerender = true, Astro retrieves these static assets over HTTP when a runtime error occurs. The framework constructs the fetch URL using request.url, whose origin component is taken directly from the inbound Host header.

Without validation against an allowedDomains list, an attacker controls the destination of this server-side fetch. The attacker submits a crafted request with a malicious Host header value, causing the Astro server to issue an outbound HTTP request to the attacker-supplied host. The response from the attacker's server is then surfaced to the original requester. This pattern aligns with classic server-side request forgery behavior and information disclosure, scoped to confidentiality impact.

Root Cause

The root cause is missing input validation on the Host header before it is used to construct the origin in request.url. Astro trusts the client-supplied header value when computing the URL used to fetch prerendered error pages, violating the principle of validating untrusted input at trust boundaries.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker sends an HTTP request to a vulnerable Astro SSR endpoint that triggers an error condition, supplying a Host header pointing to an attacker-controlled domain. The Astro runtime then fetches what it expects to be a local prerendered error page from the attacker's origin and returns the attacker-supplied content in the response chain.

No verified public proof-of-concept code is available. Refer to the GitHub Security Advisory GHSA-2pvr-wf23-7pc7 for additional technical detail.

Detection Methods for CVE-2026-54299

Indicators of Compromise

Inbound HTTP requests containing Host header values that do not match the application's expected hostnames or allowedDomains configuration.
Outbound HTTP fetches from the Astro server process to unexpected external hosts when serving /404 or /500 routes.
Server logs showing error-triggering requests immediately followed by outbound fetches to unfamiliar domains.

Detection Strategies

Inspect reverse proxy and web server logs for Host header mismatches relative to the application's canonical domain list.
Correlate error page responses with the source Host header value to identify anomalous patterns.
Monitor egress traffic from Astro Node.js processes for connections to non-allowlisted destinations.

Monitoring Recommendations

Enable verbose request logging that captures the Host header on all SSR endpoints.
Alert on outbound HTTP requests originating from the Astro runtime to domains outside the organization's allowlist.
Track the version of the Astro framework deployed across environments and flag any instance running a version below 6.4.6.

How to Mitigate CVE-2026-54299

Immediate Actions Required

Upgrade Astro to version 6.4.6 or later across all SSR deployments.
Configure the allowedDomains option to explicitly list valid hostnames for the application.
Audit applications using prerendered /404 or /500 pages with export const prerender = true for exposure.

Patch Information

The vulnerability is fixed in Astro 6.4.6. The maintainers published remediation guidance in the Astro Security Advisory GHSA-2pvr-wf23-7pc7. Operators should review their astro.config.mjs after upgrading to ensure allowedDomains is correctly defined.

Workarounds

Enforce Host header validation at an upstream reverse proxy or load balancer, rejecting requests whose Host value does not match the canonical application domain.
Remove export const prerender = true from /404 and /500 error pages until the framework is upgraded.
Restrict outbound network egress from the Astro Node.js process to known internal destinations only.

bash

# Example reverse proxy Host header allowlist (nginx)
server {
    listen 443 ssl;
    server_name app.example.com;

    if ($host !~* ^(app\.example\.com)$) {
        return 421;
    }

    location / {
        proxy_pass http://astro_upstream;
        proxy_set_header Host $host;
    }
}