Impact

The Express server uses express.json() without a size limit, which can allow attackers to send extremely large request bodies. This may lead to excessive memory usage, degraded performance, or process crashes, resulting in a Denial of Service (DoS). Any application using the JSON parser without limits and exposed to untrusted clients is affected.

Patches

This issue is not a flaw in Express itself but in configuration. Users should set a request-size limit when enabling the JSON body parser. For example:
app.use(express.json({ limit: "100kb" }));

Workarounds

Users can mitigate the issue without upgrading by:

• Adding a limit option to the JSON parser
• Implementing rate limiting at the application or reverse-proxy level
• Rejecting unusually large requests before parsing
• Using a reverse proxy (such as NGINX) to enforce maximum request body sizes

References

• Express documentation for body-parsing configuration
• CWE-400: Uncontrolled Resource Consumption