Astro allows unauthorized third-party images in _image endpoint

Summary

In affected versions of astro, the image optimization endpoint in projects deployed with on-demand rendering allows images from unauthorized third-party domains to be served.

Details

On-demand rendered sites built with Astro include an /_image endpoint which returns optimized versions of images.

The /_image endpoint is restricted to processing local images bundled with the site and also supports remote images from domains the site developer has manually authorized (using the `image.domains` or `image.remotePatterns` options).

However, a bug in impacted versions of astro allows an attacker to bypass the third-party domain restrictions by using a protocol-relative URL as the image source, e.g. /_image?href=//example.com/image.png.

Proof of Concept

1. Create a new minimal Astro project (astro@5.13.0).

2. Configure it to use the Node adapter (@astrojs/node@9.1.0 — newer versions are not impacted):

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

   export default defineConfig({
   	adapter: node({ mode: 'standalone' }),
   });

3. Build the site by running astro build.

4. Run the server, e.g. with astro preview.

5. Append /_image?href=//placehold.co/600x400 to the preview URL, e.g. <http://localhost:4321/_image?href=//placehold.co/600x400>

6. The site will serve the image from the unauthorized placehold.co origin.

Impact

Allows a non-authorized third-party to create URLs on an impacted site’s origin that serve unauthorized image content. In the case of SVG images, this could include the risk of cross-site scripting (XSS) if a user followed a link to a maliciously crafted SVG.