feat: require re-authentication for device pairing and SSO

app/api/auth/reauth/sso/complete/route.ts

@@ -0,0 +1,70 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { cookies } from 'next/headers';
+import { logger } from '@/lib/logger';
+import { decryptPayload } from '@/lib/auth/crypto';
+import { exchangeCodeForTokens } from '@/lib/oauth/token-exchange';
+import { setPairReauth } from '@/lib/auth/pair-reauth';
+
+// Completes the step-up re-authentication for device pairing. The user was sent
+// to the IdP with prompt=login (see /api/auth/sso/start with purpose=reauth);
+// here we verify the returned code against the pending state and exchange it to
+// confirm a fresh login actually happened, then set the short-lived pairing
+// re-auth proof cookie. We deliberately do NOT issue a login session or write
+// any refresh-token cookies — the user is already signed in; this only proves
+// recency for the pairing action.
+
+const SSO_PENDING_COOKIE = 'sso_pending';
+const SSO_PENDING_MAX_AGE_MS = 5 * 60 * 1000;
+
+export async function POST(request: NextRequest) {
+  const cookieStore = await cookies();
+  try {
+    const { code, state } = await request.json();
+    if (!code || !state) {
+      return NextResponse.json({ error: 'Missing code or state' }, { status: 400 });
+    }
+
+    const pendingCookie = cookieStore.get(SSO_PENDING_COOKIE)?.value;
+    if (!pendingCookie) {
+      return NextResponse.json({ error: 'No pending re-auth session' }, { status: 400 });
+    }
+
+    const pending = decryptPayload(pendingCookie);
+    cookieStore.delete(SSO_PENDING_COOKIE);
+    if (!pending) {
+      return NextResponse.json({ error: 'Invalid re-auth session' }, { status: 400 });
+    }
+
+    // Only honor pending sessions that were started for the reauth purpose, so
+    // a normal login code can't be redirected into setting a pairing proof.
+    if (pending.purpose !== 'reauth') {
+      return NextResponse.json({ error: 'Not a re-auth session' }, { status: 400 });
+    }
+    if (pending.state !== state) {
+      return NextResponse.json({ error: 'State mismatch' }, { status: 400 });
+    }
+    const createdAt = pending.created_at as number;
+    if (!createdAt || Date.now() - createdAt > SSO_PENDING_MAX_AGE_MS) {
+      return NextResponse.json({ error: 'Re-auth session expired' }, { status: 400 });
+    }
+
+    const codeVerifier = pending.code_verifier as string;
+    const redirectUri = pending.redirect_uri as string;
+    const pendingServerId = typeof pending.server_id === 'string' ? pending.server_id : null;
+    if (!codeVerifier || !redirectUri) {
+      return NextResponse.json({ error: 'Invalid re-auth session data' }, { status: 400 });
+    }
+
+    // A successful exchange proves the user just authenticated at the IdP (the
+    // freshness is enforced by prompt=login on the authorize request). We don't
+    // keep the resulting tokens.
+    await exchangeCodeForTokens(code, codeVerifier, redirectUri, pendingServerId);
+
+    await setPairReauth();
+    return NextResponse.json({ ok: true });
+  } catch (error) {
+    cookieStore.delete(SSO_PENDING_COOKIE);
+    logger.error('Reauth complete error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Re-authentication failed' }, { status: 401 });
+  }
+}