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 | Field | Value | |------------------|------------------------------------| | Reporter | j0hndo ("dohyun4466@gmail.com") | | Discovery date | 2026-05-17 | | Embargo | 90 days from acknowledgment | | Patched version | Not yet available | | Public references| CWE-601; OWASP A01:2021 |