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](https://docs.astro.build/en/reference/configuration-reference/#imagedomains) or [image.remotePatterns](https://docs.astro.build/en/reference/configuration-reference/#imageremotepatterns) 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 ([email protected]).
2. Configure it to use the Node adapter (@astrojs/[email protected] — newer versions are not impacted):
```js
// 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.
