diff --git a/app/(main)/[locale]/auth/callback/page.tsx b/app/(main)/[locale]/auth/callback/page.tsx
index 3a51a93af..fafb122bc 100644
--- a/app/(main)/[locale]/auth/callback/page.tsx
+++ b/app/(main)/[locale]/auth/callback/page.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { Suspense, useEffect, useState } from "react";
+import { Suspense, useEffect, useRef, useState } from "react";
 import { useRouter, useSearchParams } from "next/navigation";
 import { useTranslations } from "next-intl";
 import { useAuthStore } from "@/stores/auth-store";
@@ -18,8 +18,14 @@ function OAuthCallbackInner() {
   const t = useTranslations("login");
   const { loginWithOAuth, loginWithServerSso } = useAuthStore();
   const [error, setError] = useState<string | null>(null);
+  // The authorization code is single-use. React's development double effect
+  // ran this twice: the second pass no longer saw the pairing re-auth flag
+  // (the first had removed it) and spent the code on the login flow instead.
+  const handledRef = useRef(false);
 
   useEffect(() => {
+    if (handledRef.current) return;
+    handledRef.current = true;
     const code = searchParams.get("code");
     const state = searchParams.get("state");
     const errorParam = searchParams.get("error");
diff --git a/app/api/auth/sso/complete/route.ts b/app/api/auth/sso/complete/route.ts
index b26c73501..0bcd565ea 100644
--- a/app/api/auth/sso/complete/route.ts
+++ b/app/api/auth/sso/complete/route.ts
@@ -63,6 +63,14 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: 'SSO session expired. Please try again.' }, { status: 400 });
     }
 
+    // A step-up for device pairing completes at /api/auth/reauth/sso/complete
+    // only; here it would sign this browser in as whoever answered the prompt.
+    if (pending.purpose === 'reauth') {
+      logger.warn('SSO complete: refused a pairing re-auth session');
+      cookieStore.delete(SSO_PENDING_COOKIE);
+      return NextResponse.json({ error: 'Not a login session' }, { 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;
diff --git a/lib/__tests__/sso-complete-refuses-reauth.test.ts b/lib/__tests__/sso-complete-refuses-reauth.test.ts
new file mode 100644
index 000000000..1edc3e069
--- /dev/null
+++ b/lib/__tests__/sso-complete-refuses-reauth.test.ts
@@ -0,0 +1,77 @@
+// @vitest-environment node
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { NextRequest } from 'next/server';
+
+// A pairing step-up (purpose `reauth`) completes at
+// /api/auth/reauth/sso/complete only. Handed to the login route, its code
+// would sign this browser in as whoever answered the provider's prompt.
+
+vi.mock('@/lib/auth/session-secret', () => ({
+  getSessionSecret: () => 's'.repeat(64),
+  hasSessionSecret: () => true,
+}));
+vi.mock('@/lib/logger', () => ({
+  logger: { warn: () => {}, error: () => {}, info: () => {}, debug: () => {} },
+}));
+
+const jar = new Map<string, string>();
+const cookieStore = {
+  get: (name: string) => (jar.has(name) ? { name, value: jar.get(name)! } : undefined),
+  set: (name: string, value: string) => { jar.set(name, value); },
+  delete: (arg: string | { name: string }) => { jar.delete(typeof arg === 'string' ? arg : arg.name); },
+  getAll: () => [...jar].map(([name, value]) => ({ name, value })),
+};
+vi.mock('next/headers', () => ({ cookies: async () => cookieStore }));
+
+const exchangeCodeForTokens = vi.fn();
+vi.mock('@/lib/oauth/token-exchange', () => ({
+  exchangeCodeForTokens: (...args: unknown[]) => exchangeCodeForTokens(...args),
+  getTokenEndpoint: async () => 'https://idp.example.net/token',
+  getRequiredConfig: () => ({ clientId: 'webmail', serverUrl: 'https://mail.example.org', discoveryUrl: 'https://idp.example.net' }),
+}));
+
+import { encryptPayload } from '@/lib/auth/crypto';
+import { POST } from '@/app/api/auth/sso/complete/route';
+
+function pending(extra: Record<string, unknown> = {}) {
+  jar.set('sso_pending', encryptPayload({
+    state: 'state-1',
+    code_verifier: 'verifier-1',
+    redirect_uri: 'https://webmail.example/en/auth/callback',
+    created_at: Date.now(),
+    ...extra,
+  }, 'sso-pending'));
+}
+
+async function complete() {
+  const res = await POST(new NextRequest('https://webmail.example/api/auth/sso/complete', {
+    method: 'POST',
+    headers: { 'content-type': 'application/json', 'sec-fetch-site': 'same-origin' },
+    body: JSON.stringify({ code: 'code-1', state: 'state-1' }),
+  }));
+  return { status: res.status, body: await res.json() };
+}
+
+beforeEach(() => {
+  jar.clear();
+  exchangeCodeForTokens.mockReset().mockResolvedValue({ access_token: 'at', refresh_token: 'rt', expires_in: 3600 });
+});
+
+describe('/api/auth/sso/complete', () => {
+  it('refuses a pairing re-auth and spends nothing', async () => {
+    pending({ purpose: 'reauth', slot: 0 });
+    const res = await complete();
+    expect(res.status).toBe(400);
+    expect(exchangeCodeForTokens).not.toHaveBeenCalled();
+    expect(jar.has('jmap_rt')).toBe(false);
+    expect(jar.has('sso_pending')).toBe(false);
+  });
+
+  it('still completes an ordinary login', async () => {
+    pending();
+    const res = await complete();
+    expect(res.status).toBe(200);
+    expect(exchangeCodeForTokens).toHaveBeenCalledOnce();
+    expect(jar.get('jmap_rt')).toBe('rt');
+  });
+});