feat: require re-authentication for device pairing and SSO

app/(main)/[locale]/auth/callback/page.tsx

@@ -32,6 +32,42 @@ function OAuthCallbackInner() {
       return;
     }
 
+    // Step-up re-auth for device pairing: the QR generator sent the user here
+    // via prompt=login. Don't create a login session — just confirm the fresh
+    // auth (sets the short-lived pairing proof cookie) and bounce back to the
+    // Security settings, where the QR generation auto-resumes.
+    let pairReauthResume = false;
+    try {
+      pairReauthResume = sessionStorage.getItem("pair_reauth_resume") === "1";
+    } catch { /* sessionStorage unavailable */ }
+    if (pairReauthResume && state) {
+      try { sessionStorage.removeItem("pair_reauth_resume"); } catch { /* ignore */ }
+      (async () => {
+        try {
+          const res = await apiFetch("/api/auth/reauth/sso/complete", {
+            method: "POST",
+            headers: { "Content-Type": "application/json" },
+            credentials: "include",
+            body: JSON.stringify({ code, state }),
+          });
+          if (!res.ok) {
+            setError("token_exchange_failed");
+            return;
+          }
+          try {
+            sessionStorage.setItem("pair_reauth_done", "1");
+            // Land back on the Security tab (readPersistedTab reads this key).
+            sessionStorage.setItem("settings-deep-link-tab", "security");
+          } catch { /* ignore */ }
+          const prefix = getPathPrefix(params.locale as string);
+          router.push(`${prefix}/${params.locale}/settings`);
+        } catch {
+          setError("token_exchange_failed");
+        }
+      })();
+      return;
+    }
+
     const savedState = sessionStorage.getItem("oauth_state");
 
     if (savedState) {

app/api/auth/pair/create/route.ts

@@ -5,6 +5,7 @@ import { refreshTokenCookieName, refreshTokenServerCookieName } from '@/lib/oaut
 import { buildOAuthParams, getRequiredConfig, getTokenEndpoint } from '@/lib/oauth/token-exchange';
 import { getCookieOptions } from '@/lib/oauth/cookie-config';
 import { createPairing } from '@/lib/auth/pairing-store';
+import { hasValidPairReauth } from '@/lib/auth/pair-reauth';
 import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 
 // Desktop side of the cross-device QR login. The caller must be a signed-in
@@ -22,6 +23,13 @@ import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 export async function POST(request: NextRequest) {
   const cookieStore = await cookies();
   try {
+    // Step-up gate: minting a pairing code grants new-device access, so it
+    // requires a recent fresh IdP re-authentication (see the reauth SSO flow).
+    // The client turns this 401 into a re-auth redirect, then retries.
+    if (!(await hasValidPairReauth())) {
+      return NextResponse.json({ error: 'reauth_required' }, { status: 401 });
+    }
+
     const body = await request.json().catch(() => ({}));
     const slot =
       typeof body.slot === 'number' && body.slot >= 0 && body.slot < MAX_ACCOUNT_SLOTS

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 });
+  }
+}

app/api/auth/sso/start/route.ts

@@ -30,8 +30,14 @@ export async function POST(request: NextRequest) {
       server_id: bodyServerId,
       mobile_redirect_uri: rawMobileRedirectUri,
       mobile_state: rawMobileState,
+      purpose: rawPurpose,
     } = await request.json();
 
+    // `reauth` drives the step-up flow for device pairing: it forces a fresh
+    // IdP login (prompt=login) and the /reauth/sso/complete handler sets the
+    // short-lived pairing re-auth proof instead of logging the user in again.
+    const isReauth = rawPurpose === 'reauth';
+
     if (!redirect_uri || typeof redirect_uri !== 'string') {
       return NextResponse.json({ error: 'Missing redirect_uri' }, { status: 400 });
     }
@@ -85,6 +91,7 @@ export async function POST(request: NextRequest) {
       ...(serverId ? { server_id: serverId } : {}),
       ...(mobileRedirectUri ? { mobile_redirect_uri: mobileRedirectUri } : {}),
       ...(mobileState ? { mobile_state: mobileState } : {}),
+      ...(isReauth ? { purpose: 'reauth' } : {}),
     };
 
     const encrypted = encryptPayload(pendingData);
@@ -109,6 +116,14 @@ export async function POST(request: NextRequest) {
       authUrl.searchParams.set('ui_locales', locale);
     }
 
+    // Force a fresh credential entry for step-up re-auth. prompt=login and
+    // max_age=0 both ask the IdP to re-authenticate even if it has an active
+    // session; honoring them depends on the IdP supporting these OIDC params.
+    if (isReauth) {
+      authUrl.searchParams.set('prompt', 'login');
+      authUrl.searchParams.set('max_age', '0');
+    }
+
     return NextResponse.json({
       authorize_url: authUrl.toString(),
       state,