Waku has an Open Redirect via `unstable_redirect` Helper

Summary

The unstable_redirect() helper exported from waku/router/server (packages/waku/src/router/define-router.tsx:156–161) accepts an arbitrary string and reflects it unchanged into the HTTP Location response header with no URL validation, scheme restriction, or path-only enforcement. Any application that passes user-controlled input to this helper — the natural pattern documented in the JSDoc and official fixtures — is vulnerable to open redirect attacks. An attacker who convinces a victim to click a crafted link can silently redirect the browser to an arbitrary external domain, enabling phishing, credential harvesting, and OAuth token theft. Additionally, scheme-relative URLs (//evil.example/) bypass naive https?://-only allow-list filters that developers might add as ad-hoc mitigations.

Dynamic PoC confirmed against waku 1.0.0-beta.0 (commit 8e9f542) in an isolated Docker environment. Two independent dynamic runs produced identical results.

Root Cause

packages/waku/src/router/define-router.tsx:156–161:

export function unstable_redirect(
  location: string, // only URL `pathname` is supported.
  status: 303 | 307 | 308 = 307,
): never {
  throw createCustomError('Redirect', { status, location });
}

The JSDoc comment states "only URL pathname is supported", but this constraint is expressed as documentation only — the function performs no validation. The location value propagates via createCustomError (custom-errors.ts:22–26) into an error digest, is recovered by getErrorInfo in the request handler (handler.ts:79–89), and reflected directly into headers.location of the outgoing Response with no sanitization:

if (info?.location) {
  headers.location = info.location;   // handler.ts:87 — unvalidated reflection
}
return new Response(body, { status, headers });

Trigger (one-line summary)

Any developer-supplied user input passed to unstable_redirect() is reflected unchanged into the HTTP Location header, enabling navigation to an attacker-controlled domain.

Affected Entry Surfaces

• unstable_redirect(location, status?) — waku/router/server public export
• Any page/route component that passes searchParams, query, or other user- controlled strings to unstable_redirect (the standard post-login or callback redirect pattern)
• All waku adapters (Node.js, Cloudflare Workers, Vercel Edge, Deno) share the same handler.ts reflection path; cross-runtime CRLF parity is unaudited (see note below)

Additional Defense-in-Depth Concern

On Node.js, CRLF injection via the Location header is rejected by node:_http_outgoing.setHeader (ERR_INVALID_CHAR). This defense is platform-specific and not present in the waku source. Cloudflare Workers, Deno Deploy, and other edge runtimes have not been verified to offer equivalent protection. A cross-runtime audit is recommended.

Suggested Fix Outline

Validate the location argument inside unstable_redirect before the error is thrown: reject any value that does not begin with a single / (no //), and reject any value containing control characters (\x00–\x1f). An opt-in allow-list for intentional cross-origin redirects can be provided via a framework configuration option.

Disclosure