diff --git a/backend/src/routes/auth/__tests__/oauthHandler.test.ts b/backend/src/routes/auth/__tests__/oauthHandler.test.ts
new file mode 100644
index 00000000..d3d9f1ee
--- /dev/null
+++ b/backend/src/routes/auth/__tests__/oauthHandler.test.ts
@@ -0,0 +1,117 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('../../../config.js', () => ({
+  env: {
+    googleClientId: 'cid',
+    googleClientSecret: 'sec',
+    googleCallbackUrl: 'http://localhost/callback',
+  },
+}));
+
+vi.mock('../../../logging/logger.js', () => ({
+  appLogger: { error: vi.fn(), warn: vi.fn(), info: vi.fn() },
+}));
+
+const hashPassword = vi.fn().mockResolvedValue('hash');
+const generatePasswordResetToken = vi.fn().mockReturnValue('rand');
+const generateTokens = vi.fn().mockReturnValue({ accessToken: 'a', refreshToken: 'r' });
+const hashRefreshToken = vi.fn().mockReturnValue('rh');
+const refreshTokenExpiryMs = vi.fn().mockReturnValue(1000);
+
+vi.mock('../../../services/authService.js', () => ({
+  authService: {
+    hashPassword,
+    generatePasswordResetToken,
+    generateTokens,
+    hashRefreshToken,
+    refreshTokenExpiryMs,
+  },
+}));
+
+import { handleGoogleCallback } from '../oauthHandler.js';
+
+function mockRes() {
+  const res: any = {};
+  res.status = vi.fn().mockReturnValue(res);
+  res.json = vi.fn().mockReturnValue(res);
+  return res;
+}
+
+function mockRepo(overrides: Record<string, unknown> = {}) {
+  return {
+    findByEmail: vi.fn().mockResolvedValue(null),
+    create: vi.fn(),
+    markEmailVerified: vi.fn(),
+    findById: vi.fn(),
+    updateLastLogin: vi.fn(),
+    toSafeUser: vi.fn((u) => u),
+    storeRefreshToken: vi.fn(),
+    ...overrides,
+  } as any;
+}
+
+describe('handleGoogleCallback', () => {
+  beforeEach(() => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async (url: string) => {
+        if (String(url).includes('oauth2.googleapis.com/token')) {
+          return { ok: true, json: async () => ({ access_token: 'tok', id_token: 'id' }) } as any;
+        }
+        return {
+          ok: true,
+          json: async () => (globalThis as any).__googleUser,
+        } as any;
+      })
+    );
+  });
+
+  it('rejects unverified Google emails', async () => {
+    (globalThis as any).__googleUser = {
+      id: 'g1',
+      email: 'a@x.com',
+      name: 'A',
+      verified_email: false,
+    };
+    const res = mockRes();
+    await handleGoogleCallback({ body: { code: 'c' }, ip: '1', get: () => 'ua' } as any, res, mockRepo());
+    expect(res.status).toHaveBeenCalledWith(403);
+  });
+
+  it('blocks silent merge into unverified password account', async () => {
+    (globalThis as any).__googleUser = {
+      id: 'g1',
+      email: 'a@x.com',
+      name: 'A',
+      verified_email: true,
+    };
+    const existing = { user_id: 'u1', email: 'a@x.com', email_verified: false };
+    const res = mockRes();
+    await handleGoogleCallback(
+      { body: { code: 'c' }, ip: '1', get: () => 'ua' } as any,
+      res,
+      mockRepo({ findByEmail: vi.fn().mockResolvedValue(existing) })
+    );
+    expect(res.status).toHaveBeenCalledWith(409);
+  });
+
+  it('logs into verified existing account', async () => {
+    (globalThis as any).__googleUser = {
+      id: 'g1',
+      email: 'a@x.com',
+      name: 'A',
+      verified_email: true,
+    };
+    const existing = { user_id: 'u1', email: 'a@x.com', email_verified: true };
+    const repo = mockRepo({
+      findByEmail: vi.fn().mockResolvedValue(existing),
+      toSafeUser: vi.fn().mockReturnValue(existing),
+    });
+    const res = mockRes();
+    await handleGoogleCallback({ body: { code: 'c' }, ip: '1', get: () => 'ua' } as any, res, repo);
+    expect(repo.updateLastLogin).toHaveBeenCalledWith('u1');
+    expect(res.json).toHaveBeenCalled();
+    expect(res.status).not.toHaveBeenCalledWith(409);
+    expect(res.status).not.toHaveBeenCalledWith(403);
+  });
+});
diff --git a/backend/src/routes/auth/oauthHandler.ts b/backend/src/routes/auth/oauthHandler.ts
index 71aa2922..e2844c47 100644
--- a/backend/src/routes/auth/oauthHandler.ts
+++ b/backend/src/routes/auth/oauthHandler.ts
@@ -85,12 +85,38 @@ export async function handleGoogleCallback(
     verified_email?: boolean;
   };
 
+  // Require Google-verified email to prevent account takeover via unverified aliases.
+  if (!googleUser.verified_email) {
+    appLogger.warn(`[auth] Google OAuth rejected unverified email ${googleUser.email}`);
+    return res.status(403).json({
+      error: 'Google account email is not verified. Verify the email with Google and try again.'
+    });
+  }
+
   // Check if user exists by email
   const existingUser = await userRepository.findByEmail(googleUser.email);
   let safeUser: SafeUser;
 
   if (existingUser) {
-    // User exists, update last login
+    // Do not silently merge OAuth into a password-registered account without
+    // proof of control. Password accounts keep a password_hash; OAuth-only
+    // accounts were created with a random unusable password but are marked
+    // email_verified at creation. Require an already-verified email on the
+    // local user before linking (login), otherwise ask them to sign in with
+    // password first or contact support to link.
+    const isEmailVerified = Boolean(
+      (existingUser as { email_verified?: boolean; emailVerified?: boolean }).email_verified
+      ?? (existingUser as { emailVerified?: boolean }).emailVerified
+    );
+    if (!isEmailVerified) {
+      appLogger.warn(
+        `[auth] Google OAuth blocked silent merge into unverified password account ${googleUser.email}`
+      );
+      return res.status(409).json({
+        error:
+          'An account with this email already exists. Sign in with your password, then link Google from settings.'
+      });
+    }
     await userRepository.updateLastLogin(existingUser.user_id);
     safeUser = userRepository.toSafeUser(existingUser);
   } else {