feat(protect): off_origin match type for open-redirect detection (#109)

Add an `off_origin` response-phase primitive: true (→ block) when a 3xx redirect's Location
header points to a different origin than the request's own Host. Relative / same-origin
Locations never match; lenient when the request Host is unknown. This is the first capability
unlocked by threading the request into the response phase (#106).

Also derive the request Host from the URL in reqContextFromFetch — a fetch Request doesn't
expose a Host header, and origin-comparing response rules (open-redirect, later CORS) need it.

Not a default rule (many apps redirect off-site legitimately); it's authored and route-scoped
via `when`. +6 tests; 632 pass; typecheck clean.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: patchstackdave

src/protect/engine/engine.js

@@ -202,6 +202,26 @@ function hostFromUrl(value) {
   }
 }
 
+// Open-redirect primitive (response phase): a 3xx whose Location header points to a DIFFERENT origin
+// than the request's own Host. A relative Location (same-origin) never matches. Needs the request
+// Host, which the response phase threads in via reqCtx. Lenient: no Location, no request Host, or a
+// same-origin / relative target → not flagged (so it can't false-positive without the signal).
+function isOffOriginRedirect(resolver) {
+  try {
+    const status = Number(resolver.resolve('response.status')[0] ?? 0);
+    if (status < 300 || status >= 400) return false;
+    const location = resolver.resolve('response.header.location')[0];
+    if (!location) return false;
+    const target = hostFromUrl(String(location)); // null for a relative (same-origin) Location
+    if (target === null) return false;
+    const host = String(resolver.resolve('server.HTTP_HOST')[0] ?? '').toLowerCase();
+    if (!host) return false;
+    return target !== host;
+  } catch {
+    return false;
+  }
+}
+
 // `matchObj` is the full match object; needed by types that read sibling fields
 // (array_key_value reads `key`/`match`). Optional so direct callers/tests can keep
 // using the (type, value, matchVal) signature.
@@ -450,6 +470,12 @@ export class RuleEngine {
       return isCrossOrigin(resolver);
     }
 
+    // `off_origin` (response phase): true (→ block) when a 3xx redirects to a different origin than
+    // the request Host — the open-redirect primitive. Like cross_origin, it needs the whole resolver.
+    if (match && match.type === 'off_origin') {
+      return isOffOriginRedirect(resolver);
+    }
+
     // `parameter` may be an array (e.g. ["get.action","post.action"]) — the rule_v2 format
     // uses these pervasively to mean "any of these sources". Resolve each and OR the
     // candidate values together. (A bare string resolves as a single source.)

src/protect/runtime.js

@@ -243,7 +243,11 @@ export async function createProtection(options = {}) {
   const reqContextFromFetch = (request) => {
     try {
       const u = new URL(request.url);
-      return { method: request.method, originalUrl: u.pathname + u.search, headers: headerObject(request.headers) };
+      const headers = headerObject(request.headers);
+      // A fetch Request doesn't expose the Host header (it's set at send time), so derive it from the
+      // URL — response rules that compare origins (open-redirect / CORS) need the request Host.
+      if (!headers.host) headers.host = u.host;
+      return { method: request.method, originalUrl: u.pathname + u.search, headers };
     } catch {
       return undefined;
     }

tests/protect/open-redirect.test.ts

@@ -0,0 +1,60 @@
+import { describe, expect, it } from 'vitest';
+import { createProtection } from '../../src/protect/runtime.js';
+
+// `off_origin` (response phase): flags a 3xx whose Location points to a different origin than the
+// request Host — the open-redirect primitive, enabled by threading the request into the response
+// phase. Not a default (many apps redirect off-site legitimately); authored + route-scoped.
+
+const emptyBundle = { firewall: [], whitelists: [], whitelist_keys: {} };
+const rule = (when?: any) => ({
+  phase: 'response',
+  category: 'open-redirect',
+  action: 'block',
+  ...(when ? { when } : {}),
+  rule_v2: [{ match: { type: 'off_origin' } }],
+});
+const redirect = (location: string, status = 302) => new Response(null, { status, headers: { location } });
+const req = (url: string) => new Request(url);
+const setup = (when?: any) =>
+  createProtection({ rules: emptyBundle, responseRules: [rule(when)], mode: 'block' });
+
+describe('off_origin — open-redirect detection', () => {
+  it('blocks a 3xx that redirects to a different origin', async () => {
+    const p: any = await setup();
+    const out = await p.screenResponse(redirect('https://evil.com/x'), req('https://app.example.com/go'));
+    expect(out.status).toBe(500); // redirect withheld
+  });
+
+  it('allows a same-origin absolute redirect', async () => {
+    const p: any = await setup();
+    const out = await p.screenResponse(redirect('https://app.example.com/dashboard'), req('https://app.example.com/go'));
+    expect(out.status).toBe(302);
+    expect(out.headers.get('location')).toBe('https://app.example.com/dashboard');
+  });
+
+  it('allows a relative (same-origin) redirect', async () => {
+    const p: any = await setup();
+    const out = await p.screenResponse(redirect('/dashboard'), req('https://app.example.com/go'));
+    expect(out.status).toBe(302);
+  });
+
+  it('does not flag a non-3xx response that carries a Location header', async () => {
+    const p: any = await setup();
+    const out = await p.screenResponse(redirect('https://evil.com/x', 200), req('https://app.example.com/go'));
+    expect(out.status).toBe(200);
+  });
+
+  it('is lenient with no request context (no Host to compare against)', async () => {
+    const p: any = await setup();
+    const out = await p.screenResponse(redirect('https://evil.com/x')); // no request passed
+    expect(out.status).toBe(302);
+  });
+
+  it('honours `when` route scope — blocks on the scoped route only', async () => {
+    const p: any = await setup({ path: '/go' });
+    const onScope = await p.screenResponse(redirect('https://evil.com/x'), req('https://app.example.com/go'));
+    expect(onScope.status).toBe(500);
+    const offScope = await p.screenResponse(redirect('https://evil.com/x'), req('https://app.example.com/elsewhere'));
+    expect(offScope.status).toBe(302);
+  });
+});