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The Express.js framework provides the express.json() middleware for parsing JSON request bodies. By default, without a specified size limit, this middleware accepts arbitrarily large JSON payloads. An attacker can exploit this by sending HTTP POST requests with excessively large JSON bodies to endpoints using express.json(). The server then allocates memory to parse the entire payload, leading to excessive memory consumption. If the payload is large enough, it can exhaust available system memory, degrade performance, or crash the Node.js process. This results in a Denial of Service (DoS), rendering the application unavailable. The vulnerability stems from misconfiguration, not a flaw in Express itself, as developers often omit the limit option. Applications exposed to untrusted clients, such as public APIs, are particularly at risk. The attack is simple to execute and can be performed with basic tools like curl, making it a significant threat if left unmitigated.
Platform: Express.js
Version: All versions
Vulnerability: Denial of Service
Severity: Critical
Date: Not disclosed
Prediction: Configuration fix required
What Undercode Say:
Analytics:
grep -r “express.json()” .
app.use(express.json({ limit: “100kb” }));
curl -X POST -H “Content-Type: application/json” –data-binary @large.json http://localhost:3000
How Exploit:
Send large JSON payloads to unlimited express.json() endpoints.
Protection from this CVE:
Set express.json() limit.
Use reverse proxy limits.
Implement rate limiting.
Impact:
Denial of Service.
Memory exhaustion.
Performance degradation.
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Sources:
Reported By: github.com
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