h3: Missing Path Segment Boundary Check in `mount()` Causes Middleware Execution on Unrelated Prefix-Matching Routes

Exploitation flow:

1. Developer mounts a sub-app at /admin with middleware that sets event.context.isAdmin = true
2. Developer defines a separate route /admin-public/info on the parent app that reads event.context.isAdmin
3. Attacker requests GET /admin-public/info
4. The /admin mount's startsWith check passes → admin middleware executes → sets isAdmin = true
5. The middleware's "restore pathname" callback fires, control returns to the parent app
6. The /admin-public/info handler sees event.context.isAdmin === true

PoC

// poc.js — demonstrates context pollution across mount boundaries
import { H3 } from "h3";

const adminApp = new H3();

// Admin middleware sets privileged context
adminApp.use(() => {}, {
  onRequest: (event) => {
    event.context.isAdmin = true;
  }
});

adminApp.get("/dashboard", (event) => {
  return { admin: true, context: event.context };
});

const app = new H3();

// Mount admin sub-app at /admin
app.mount("/admin", adminApp);

// Public route that happens to share the "/admin" prefix
app.get("/admin-public/info", (event) => {
  return {
    path: event.url.pathname,
    isAdmin: event.context.isAdmin ?? false,  // Should always be false here
  };
});

// Test with fetch
const server = Bun.serve({ port: 3000, fetch: app.fetch });

// This request should NOT trigger admin middleware, but it does
const res = await fetch("http://localhost:3000/admin-public/info");
const body = await res.json();
console.log(body);
// Actual output: { path: "/admin-public/info", isAdmin: true }
// Expected output: { path: "/admin-public/info", isAdmin: false }

server.stop();

Steps to reproduce:

# 1. Clone h3 and install
git clone https://github.com/h3js/h3 && cd h3
corepack enable && pnpm install && pnpm build

# 2. Save poc.js (above) and run
bun poc.js
# Output shows isAdmin: true — admin middleware leaked to /admin-public/info

# 3. Verify the boundary leak with additional paths:
# GET /administrator → admin middleware fires
# GET /adminstuff   → admin middleware fires
# GET /admin123     → admin middleware fires
# GET /admi         → admin middleware does NOT fire (correct)

Impact

• Context pollution across mount boundaries: Middleware registered on a mounted sub-app executes for any route sharing the string prefix, not just routes under the intended path segment tree. This can set privileged flags (isAdmin, isAuthenticated, role assignments) on requests to completely unrelated routes.
• Authorization bypass: If an application uses mount-scoped middleware to set permissive context flags and other routes check those flags, an attacker can access protected functionality by requesting a path that string-prefix-matches the mount base but routes to a different handler.
• Path mangling: The withoutBase() utility produces incorrect paths (e.g., /-public/info instead of /admin-public/info) when the input shares only a string prefix, potentially causing routing errors or further security issues in downstream path processing.
• Scope: Any h3 v2 application using mount() with a base path that is a string prefix of other routes is affected. The impact scales with how the application uses middleware-set context values.

Recommended Fix

Add a segment boundary check after the startsWith call in both locations. The character immediately following the base prefix must be /, ?, #, or the string must end exactly at the base:

Fix for src/h3.ts:127:

 mount(base: string, input: FetchHandler | FetchableObject | H3Type) {
   if ("handler" in input) {
     if (input["~middleware"].length > 0) {
       this["~middleware"].push((event, next) => {
         const originalPathname = event.url.pathname;
-        if (!originalPathname.startsWith(base)) {
+        if (!originalPathname.startsWith(base) ||
+            (originalPathname.length > base.length && originalPathname[base.length] !== "/")) {
           return next();
         }

Fix for src/utils/internal/path.ts:40:

 export function withoutBase(input: string = "", base: string = ""): string {
   if (!base || base === "/") {
     return input;
   }
   const _base = withoutTrailingSlash(base);
-  if (!input.startsWith(_base)) {
+  if (!input.startsWith(_base) ||
+      (input.length > _base.length && input[_base.length] !== "/")) {
     return input;
   }

This ensures that /admin only matches /admin, /admin/, and /admin/... — never /admin-public, /administrator, or other coincidental string-prefix matches.