Security researcher Simon Koeck has discovered a critical vulnerability in h3, the underlying HTTP framework used by the popular Nuxt.js web framework and the Nitro server engine. The flaw allows attackers to perform HTTP request smuggling by exploiting a case-sensitivity error in how the framework processes header data.
At the core of the issue is a function within h3 that parses request bodies. When checking for chunked transfer encoding, the code uses a case-sensitive search for the string "chunked." According to RFC 7230, however, HTTP header values must be treated as case-insensitive. By sending a header like "Transfer-Encoding: ChunKed," an attacker can trick the h3 framework into ignoring the request body entirely.
The mechanics of the smuggling attack
When an application is deployed behind a reverse proxy, the discrepancy between how the proxy and the backend interpret the request creates a security gap. A standard proxy will correctly identify "ChunKed" as valid chunked encoding and forward the full request. Because h3 fails to recognize the encoding, it treats the request as having no body and returns an immediate response.
This creates a desynchronization between the proxy and the server. The data that h3 ignored remains in the connection buffer, where it is misinterpreted as the beginning of a subsequent HTTP request. This allows an attacker to inject, or "smuggle," a second, unauthorized request into the system.
Koeck confirmed the behavior by comparing h3 against the Express framework. While Express correctly identified the malformed request and waited for the remaining data, h3 processed the request instantly, failing to recognize the body.
This vulnerability poses significant risks to modern web deployments. Because the smuggled request is effectively hidden from the initial inspection, it can bypass Web Application Firewalls (WAFs) and authentication proxies. In a shared keep-alive connection environment, this can lead to response poisoning, where sensitive data or session tokens meant for one user are delivered to another.
"The worst part: none of this leaves an obvious trace," Koeck noted. Because the smuggled request is treated as a new, legitimate-looking transaction by the backend, the attack often avoids traditional logging and monitoring systems. Developers using Nuxt and Nitro are advised to ensure their environments are updated to mitigate the risk of desynchronization.