fix: verify credentials upstream before minting identity cookies (GHSA-wxcm-j4jc-9fxq)

lib/__tests__/auth-routes-credential-check.test.ts

@@ -0,0 +1,308 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+/**
+ * GHSA-wxcm-j4jc-9fxq: /api/auth/session and /api/auth/stalwart-context used
+ * to mint the encrypted identity cookies for any username when the requested
+ * server was the admin-configured one, without checking the credential
+ * upstream. Routes that trust the cookie's username claim without contacting
+ * the mail server (settings sync, plugin-approval attribution) then accepted
+ * the forged identity. Both routes must now verify upstream and bind the
+ * username to the credential before writing anything.
+ */
+
+vi.mock('next/server', () => ({
+  NextResponse: {
+    json: (data: unknown, init?: { status?: number }) => ({
+      json: async () => data,
+      status: init?.status ?? 200,
+    }),
+  },
+}));
+
+vi.mock('@/lib/logger', () => ({
+  logger: { warn: vi.fn(), error: vi.fn(), debug: vi.fn(), info: vi.fn() },
+}));
+
+const cookieSet = vi.fn();
+vi.mock('next/headers', () => ({
+  cookies: async () => ({
+    get: () => undefined,
+    set: cookieSet,
+    delete: vi.fn(),
+    getAll: () => [],
+  }),
+}));
+
+const config: Record<string, unknown> = {};
+vi.mock('@/lib/admin/config-manager', () => ({
+  configManager: {
+    ensureLoaded: async () => {},
+    get: (key: string, fallback: unknown) => (key in config ? config[key] : fallback),
+  },
+}));
+
+const encryptSession = vi.fn((..._args: unknown[]) => 'encrypted-session');
+vi.mock('@/lib/auth/crypto', () => ({
+  encryptSession: (...args: unknown[]) => encryptSession(...args),
+  decryptSession: () => null,
+  decryptPayload: () => null,
+  encryptPayload: () => 'encrypted',
+}));
+
+const setStalwartAuthContext = vi.fn();
+const setStalwartAuthContextInStore = vi.fn();
+vi.mock('@/lib/stalwart/auth-context', () => ({
+  setStalwartAuthContext: (...args: unknown[]) => setStalwartAuthContext(...args),
+  setStalwartAuthContextInStore: (...args: unknown[]) => setStalwartAuthContextInStore(...args),
+  clearStalwartAuthContextInStore: vi.fn(),
+}));
+
+const recordLogin = vi.fn();
+vi.mock('@/lib/telemetry/login-tracker', () => ({
+  recordLogin: (...args: unknown[]) => recordLogin(...args),
+}));
+
+const SERVER = 'https://mail.example.org';
+const HOST = 'webmail.example';
+
+/** The mail server as the routes see it: who exists, and which token is whose. */
+const PASSWORDS: Record<string, string> = {
+  'alice@example.org': 'alice-secret',
+  'mallory@example.org': 'mallory-secret',
+  // OAuth/SSO login name that differs from the primary identity e-mail.
+  'lrath': 'linus-secret',
+};
+const TOKENS: Record<string, string> = {
+  'tok-mallory': 'mallory@example.org',
+  'tok-linus': 'lrath',
+};
+const IDENTITY_EMAILS: Record<string, string[]> = {
+  'mallory@example.org': ['mallory@example.org'],
+  'lrath': ['linus@example.org'],
+};
+
+function json(data: unknown, status = 200) {
+  return new Response(JSON.stringify(data), { status, headers: { 'content-type': 'application/json' } });
+}
+
+function authenticate(init?: RequestInit): string | null {
+  const auth = new Headers(init?.headers).get('authorization') || '';
+  const basic = /^Basic\s+(\S+)$/i.exec(auth);
+  if (basic) {
+    const [user, pass] = Buffer.from(basic[1], 'base64').toString('utf8').split(':');
+    return PASSWORDS[user] === pass ? user : null;
+  }
+  const bearer = /^Bearer\s+(\S+)$/i.exec(auth);
+  if (bearer) return TOKENS[bearer[1]] ?? null;
+  return null;
+}
+
+let fetchSpy: ReturnType<typeof vi.spyOn>;
+let identityGetCalls: number;
+
+function installFakeServer() {
+  identityGetCalls = 0;
+  fetchSpy = vi.spyOn(globalThis, 'fetch').mockImplementation(async (input, init) => {
+    const url = String(input);
+    const user = authenticate(init);
+    if (!user) {
+      return json({ type: 'about:blank', status: 401, title: 'Unauthorized' }, 401);
+    }
+    if (url === `${SERVER}/.well-known/jmap`) {
+      return json({
+        // Advertised on a hostname this process cannot resolve, like Stalwart does.
+        apiUrl: 'https://stwtest.local/jmap/',
+        username: user,
+        primaryAccounts: { 'urn:ietf:params:jmap:mail': 'c' },
+        accounts: { c: { name: user } },
+      });
+    }
+    if (url === `${SERVER}/jmap/` && init?.method === 'POST') {
+      identityGetCalls++;
+      const list = (IDENTITY_EMAILS[user] ?? []).map((email, i) => ({ id: `i${i}`, email, name: user }));
+      return json({ methodResponses: [['Identity/get', { accountId: 'c', list, notFound: [] }, '0']] });
+    }
+    return json({ error: `unexpected ${init?.method ?? 'GET'} ${url}` }, 404);
+  });
+}
+
+type HeaderMap = Record<string, string>;
+const SAME_ORIGIN_FETCH: HeaderMap = {
+  host: HOST,
+  origin: `https://${HOST}`,
+  'content-type': 'application/json',
+  'sec-fetch-site': 'same-origin',
+  'sec-fetch-mode': 'cors',
+  'sec-fetch-dest': 'empty',
+};
+
+function mockRequest(body: unknown) {
+  return {
+    method: 'POST',
+    json: async () => body,
+    headers: { get: (k: string) => SAME_ORIGIN_FETCH[k.toLowerCase()] ?? null },
+    nextUrl: { searchParams: new URLSearchParams(), origin: `https://${HOST}` },
+  };
+}
+
+type RouteResponse = { status: number; json: () => Promise<{ error?: string; ok?: boolean }> };
+
+async function postSession(body: unknown) {
+  const { POST } = await import('@/app/api/auth/session/route');
+  const res = (await POST(mockRequest(body) as unknown as Parameters<typeof POST>[0])) as unknown as RouteResponse;
+  return { status: res.status, body: await res.json() };
+}
+
+async function postContext(body: unknown) {
+  const { POST } = await import('@/app/api/auth/stalwart-context/route');
+  const res = (await POST(mockRequest(body) as unknown as Parameters<typeof POST>[0])) as unknown as RouteResponse;
+  return { status: res.status, body: await res.json() };
+}
+
+function basic(user: string, pass: string): string {
+  return `Basic ${Buffer.from(`${user}:${pass}`).toString('base64')}`;
+}
+
+beforeEach(() => {
+  for (const k of Object.keys(config)) delete config[k];
+  config.jmapServerUrl = SERVER;
+  cookieSet.mockClear();
+  encryptSession.mockClear();
+  setStalwartAuthContext.mockClear();
+  setStalwartAuthContextInStore.mockClear();
+  recordLogin.mockClear();
+  vi.resetModules();
+  installFakeServer();
+});
+
+afterEach(() => {
+  fetchSpy.mockRestore();
+});
+
+describe('POST /api/auth/session (GHSA-wxcm-j4jc-9fxq)', () => {
+  it('refuses to mint cookies for a wrong password on the configured server', async () => {
+    const { status, body } = await postSession({
+      serverUrl: SERVER, username: 'alice@example.org', password: 'TOTALLY-WRONG', slot: 0,
+    });
+    expect(status).toBe(401);
+    expect(body.error).toMatch(/authentication failed/i);
+    expect(cookieSet).not.toHaveBeenCalled();
+    expect(setStalwartAuthContextInStore).not.toHaveBeenCalled();
+    expect(recordLogin).not.toHaveBeenCalled();
+  });
+
+  it('refuses to mint cookies for a user that does not exist upstream', async () => {
+    const { status } = await postSession({
+      serverUrl: SERVER, username: 'nobody@example.org', password: 'x', slot: 0,
+    });
+    expect(status).toBe(401);
+    expect(cookieSet).not.toHaveBeenCalled();
+    expect(setStalwartAuthContextInStore).not.toHaveBeenCalled();
+  });
+
+  it('writes no cookie when the upstream server cannot be reached', async () => {
+    fetchSpy.mockRejectedValue(new TypeError('fetch failed'));
+    const { status } = await postSession({
+      serverUrl: SERVER, username: 'alice@example.org', password: 'alice-secret', slot: 0,
+    });
+    expect(status).toBe(502);
+    expect(cookieSet).not.toHaveBeenCalled();
+    expect(setStalwartAuthContextInStore).not.toHaveBeenCalled();
+  });
+
+  it('still mints cookies for the right password after checking upstream', async () => {
+    const { status, body } = await postSession({
+      serverUrl: SERVER, username: 'alice@example.org', password: 'alice-secret', slot: 0,
+    });
+    expect(status).toBe(200);
+    expect(body.ok).toBe(true);
+    expect(fetchSpy).toHaveBeenCalledWith(
+      `${SERVER}/.well-known/jmap`,
+      expect.objectContaining({ headers: { Authorization: basic('alice@example.org', 'alice-secret') } }),
+    );
+    expect(cookieSet).toHaveBeenCalledTimes(1);
+    expect(setStalwartAuthContextInStore).toHaveBeenCalledWith(
+      expect.anything(), 0, expect.objectContaining({ username: 'alice@example.org', serverUrl: SERVER }),
+    );
+  });
+});
+
+describe('POST /api/auth/stalwart-context (GHSA-wxcm-j4jc-9fxq)', () => {
+  it('refuses a Basic credential the server rejects', async () => {
+    const { status } = await postContext({
+      serverUrl: SERVER, username: 'alice@example.org', authHeader: basic('alice@example.org', 'nope'), slot: 0,
+    });
+    expect(status).toBe(401);
+    expect(setStalwartAuthContext).not.toHaveBeenCalled();
+  });
+
+  it('refuses a Basic credential whose user part differs from the claimed username, before any fetch', async () => {
+    const { status, body } = await postContext({
+      serverUrl: SERVER, username: 'alice@example.org', authHeader: basic('mallory@example.org', 'mallory-secret'), slot: 0,
+    });
+    expect(status).toBe(400);
+    expect(body.error).toMatch(/does not match/i);
+    expect(fetchSpy).not.toHaveBeenCalled();
+    expect(setStalwartAuthContext).not.toHaveBeenCalled();
+  });
+
+  it('refuses a valid Bearer token whose owner is not the claimed username', async () => {
+    const { status, body } = await postContext({
+      serverUrl: SERVER, username: 'alice@example.org', authHeader: 'Bearer tok-mallory', slot: 0,
+    });
+    expect(status).toBe(403);
+    expect(body.error).toMatch(/does not match/i);
+    expect(identityGetCalls).toBe(1);
+    expect(setStalwartAuthContext).not.toHaveBeenCalled();
+  });
+
+  it('refuses a Bearer token when the identity lookup fails, rather than trusting the claim', async () => {
+    fetchSpy.mockImplementation(async (input: RequestInfo | URL) => {
+      if (String(input).endsWith('/.well-known/jmap')) {
+        return json({ apiUrl: `${SERVER}/jmap/`, username: 'lrath', primaryAccounts: { 'urn:ietf:params:jmap:mail': 'c' }, accounts: {} });
+      }
+      return json({ error: 'boom' }, 500);
+    });
+    const { status } = await postContext({
+      serverUrl: SERVER, username: 'linus@example.org', authHeader: 'Bearer tok-linus', slot: 0,
+    });
+    expect(status).toBe(502);
+    expect(setStalwartAuthContext).not.toHaveBeenCalled();
+  });
+
+  it('accepts a Bearer token whose session username is the claim, without an identity lookup', async () => {
+    const { status } = await postContext({
+      serverUrl: SERVER, username: 'mallory@example.org', authHeader: 'Bearer tok-mallory', slot: 1,
+    });
+    expect(status).toBe(200);
+    expect(identityGetCalls).toBe(0);
+    expect(setStalwartAuthContext).toHaveBeenCalledWith(1, expect.objectContaining({ username: 'mallory@example.org' }));
+  });
+
+  it('accepts an OAuth login registered under the primary identity e-mail via Identity/get on the reachable host', async () => {
+    const { status } = await postContext({
+      serverUrl: SERVER, username: 'linus@example.org', authHeader: 'Bearer tok-linus', slot: 0,
+    });
+    expect(status).toBe(200);
+    expect(identityGetCalls).toBe(1);
+    // apiUrl was advertised on stwtest.local; the lookup must go to the verified origin.
+    expect(fetchSpy).toHaveBeenCalledWith(`${SERVER}/jmap/`, expect.objectContaining({ method: 'POST' }));
+    expect(setStalwartAuthContext).toHaveBeenCalledWith(0, expect.objectContaining({ username: 'linus@example.org' }));
+  });
+
+  it('accepts the bare local part of the session username (Stalwart canonicalizes it)', async () => {
+    const { status } = await postContext({
+      serverUrl: SERVER, username: 'lrath@corp.example', authHeader: 'Bearer tok-linus', slot: 0,
+    });
+    expect(status).toBe(200);
+    expect(identityGetCalls).toBe(0);
+  });
+
+  it('still mints the context for a Basic credential the server accepts', async () => {
+    const { status } = await postContext({
+      serverUrl: SERVER, username: 'alice@example.org', authHeader: basic('alice@example.org', 'alice-secret'), slot: 2,
+    });
+    expect(status).toBe(200);
+    expect(setStalwartAuthContext).toHaveBeenCalledWith(2, expect.objectContaining({ username: 'alice@example.org', serverUrl: SERVER }));
+  });
+});

lib/__tests__/auth-routes-same-origin.test.ts

@@ -1,4 +1,4 @@
-import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 
 /**
  * GHSA-qvr9-m8cq-7wvg: the unauthenticated /api/auth/* routes write
@@ -150,12 +150,26 @@ describe('isSameOriginRequest', () => {
 });
 
 describe('POST /api/auth/session (GHSA-qvr9-m8cq-7wvg session fixation)', () => {
+  let fetchSpy: ReturnType<typeof vi.spyOn>;
+
   beforeEach(() => {
     for (const k of Object.keys(config)) delete config[k];
     config.jmapServerUrl = TRUSTED;
     cookieSet.mockClear();
     setStalwartAuthContextInStore.mockClear();
     vi.resetModules();
+    // The route now verifies the credential upstream even for the configured
+    // server (GHSA-wxcm-j4jc-9fxq); stand in for a server that accepts it.
+    fetchSpy = vi.spyOn(globalThis, 'fetch').mockResolvedValue(
+      new Response(
+        JSON.stringify({ apiUrl: `${TRUSTED}/jmap/`, username: SESSION_BODY.username, accounts: {} }),
+        { status: 200, headers: { 'content-type': 'application/json' } },
+      ),
+    );
+  });
+
+  afterEach(() => {
+    fetchSpy.mockRestore();
   });
 
   it('refuses a cross-site form POST before reading the body and writes no cookie', async () => {

lib/__tests__/verify-jmap-identity.test.ts

@@ -0,0 +1,181 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+/**
+ * verifyJmapIdentity binds a username claim to a verified credential
+ * (GHSA-wxcm-j4jc-9fxq). The route-level behaviour for admin-configured
+ * servers is covered in auth-routes-credential-check.test.ts; this file
+ * covers the matching rules and the user-chosen (untrusted) endpoint path,
+ * which must keep every hop on the rebinding-safe fetch.
+ */
+
+const lookup = vi.fn();
+const guardedFetch = vi.fn();
+
+vi.mock('@/lib/security/url-guard', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('@/lib/security/url-guard')>();
+  return {
+    ...actual,
+    fetchPublicUrl: (...args: unknown[]) => guardedFetch(...args),
+  };
+});
+
+vi.mock('node:dns/promises', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('node:dns/promises')>();
+  return {
+    ...actual,
+    default: { ...actual, lookup: (...args: unknown[]) => lookup(...args) },
+    lookup: (...args: unknown[]) => lookup(...args),
+  };
+});
+
+function json(data: unknown, status = 200) {
+  return new Response(JSON.stringify(data), { status, headers: { 'content-type': 'application/json' } });
+}
+
+function basic(user: string, pass: string): string {
+  return `Basic ${Buffer.from(`${user}:${pass}`).toString('base64')}`;
+}
+
+async function load() {
+  return import('@/lib/auth/verify-jmap-auth');
+}
+
+describe('usernameMatchesSession', () => {
+  it('matches exact and case-different login names', async () => {
+    const { usernameMatchesSession } = await load();
+    expect(usernameMatchesSession('usera@example.org', 'usera@example.org')).toBe(true);
+    expect(usernameMatchesSession('UserA@Example.org', 'usera@example.org')).toBe(true);
+  });
+
+  it('matches a bare local part against the canonical e-mail login on either side', async () => {
+    const { usernameMatchesSession } = await load();
+    expect(usernameMatchesSession('usera', 'usera@example.org')).toBe(true);
+    expect(usernameMatchesSession('linus@example.org', 'linus')).toBe(true);
+  });
+
+  it('rejects different users, different domains and missing session usernames', async () => {
+    const { usernameMatchesSession } = await load();
+    expect(usernameMatchesSession('alice@example.org', 'mallory@example.org')).toBe(false);
+    expect(usernameMatchesSession('alice@example.org', 'alice@other.example')).toBe(false);
+    expect(usernameMatchesSession('alice', 'alicia@example.org')).toBe(false);
+    expect(usernameMatchesSession('alice@example.org', undefined)).toBe(false);
+    expect(usernameMatchesSession('alice@example.org', '')).toBe(false);
+  });
+});
+
+describe('verifyJmapIdentity', () => {
+  let fetchSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    lookup.mockReset();
+    guardedFetch.mockReset();
+    vi.resetModules();
+    fetchSpy = vi.spyOn(globalThis, 'fetch');
+  });
+
+  afterEach(() => {
+    fetchSpy.mockRestore();
+  });
+
+  it('rejects an empty username before any fetch', async () => {
+    const { verifyJmapIdentity } = await load();
+    await expect(verifyJmapIdentity('https://example.com', 'Bearer x', '', { trusted: true }))
+      .rejects.toMatchObject({ status: 400 });
+    expect(fetchSpy).not.toHaveBeenCalled();
+  });
+
+  it('rejects a Basic credential for another user before any fetch', async () => {
+    const { verifyJmapIdentity } = await load();
+    await expect(
+      verifyJmapIdentity('https://example.com', basic('mallory@example.org', 'pw'), 'alice@example.org', { trusted: true }),
+    ).rejects.toMatchObject({ status: 400 });
+    expect(fetchSpy).not.toHaveBeenCalled();
+  });
+
+  it('surfaces the upstream 401 for a Basic credential the server rejects', async () => {
+    fetchSpy.mockResolvedValueOnce(json({ status: 401 }, 401));
+    const { verifyJmapIdentity } = await load();
+    await expect(
+      verifyJmapIdentity('https://mail.internal', basic('alice@example.org', 'wrong'), 'alice@example.org', { trusted: true }),
+    ).rejects.toMatchObject({ status: 401, upstreamStatus: 401 });
+  });
+
+  it('accepts a Basic credential the server accepts without inspecting Session.username', async () => {
+    // Stalwart canonicalizes the login; a Basic caller who typed an alias is
+    // still proven by the server accepting exactly that user:password pair.
+    fetchSpy.mockResolvedValueOnce(json({ apiUrl: 'https://mail.internal/jmap/', username: 'alice', accounts: {} }));
+    const { verifyJmapIdentity } = await load();
+    await expect(
+      verifyJmapIdentity('https://mail.internal/', basic('a.smith@example.org', 'pw'), 'a.smith@example.org', { trusted: true }),
+    ).resolves.toBe('https://mail.internal');
+    expect(fetchSpy).toHaveBeenCalledTimes(1);
+  });
+
+  it('on a user-chosen endpoint, runs the identity lookup through the guarded fetch on the verified origin', async () => {
+    lookup.mockResolvedValue([{ address: '93.184.216.34', family: 4 }]);
+    guardedFetch
+      .mockResolvedValueOnce(json({
+        apiUrl: 'https://public.example.com:8443/jmap/',
+        username: 'linus',
+        primaryAccounts: { 'urn:ietf:params:jmap:mail': 'c' },
+        accounts: {},
+      }))
+      .mockResolvedValueOnce(json({
+        methodResponses: [['Identity/get', { accountId: 'c', list: [{ id: 'i0', email: 'Linus.Rath@example.com' }] }, '0']],
+      }));
+    const { verifyJmapIdentity } = await load();
+    await expect(
+      verifyJmapIdentity('https://example.com', 'Bearer tok', 'linus.rath@example.com', { trusted: false }),
+    ).resolves.toBe('https://example.com');
+    expect(guardedFetch).toHaveBeenCalledTimes(2);
+    expect(guardedFetch).toHaveBeenLastCalledWith(
+      'https://example.com/jmap/',
+      expect.objectContaining({ method: 'POST' }),
+    );
+    const body = JSON.parse((guardedFetch.mock.calls[1][1] as { body: string }).body);
+    expect(body.methodCalls[0][0]).toBe('Identity/get');
+    expect(body.methodCalls[0][1]).toEqual({ accountId: 'c', ids: null });
+    expect(fetchSpy).not.toHaveBeenCalled();
+  });
+
+  it('rejects a Bearer claim that neither Session.username nor any identity confirms', async () => {
+    fetchSpy
+      .mockResolvedValueOnce(json({
+        apiUrl: 'https://mail.internal/jmap/',
+        username: 'mallory@example.org',
+        primaryAccounts: { 'urn:ietf:params:jmap:mail': 'c' },
+        accounts: {},
+      }))
+      .mockResolvedValueOnce(json({
+        methodResponses: [['Identity/get', { accountId: 'c', list: [{ id: 'i0', email: 'mallory@example.org' }] }, '0']],
+      }));
+    const { verifyJmapIdentity } = await load();
+    await expect(
+      verifyJmapIdentity('https://mail.internal', 'Bearer tok', 'alice@example.org', { trusted: true }),
+    ).rejects.toMatchObject({ status: 403 });
+  });
+
+  it('rejects a Bearer claim when the session has no primary mail account to look identities up on', async () => {
+    fetchSpy.mockResolvedValueOnce(json({ apiUrl: 'https://mail.internal/jmap/', username: 'mallory', accounts: {} }));
+    const { verifyJmapIdentity } = await load();
+    await expect(
+      verifyJmapIdentity('https://mail.internal', 'Bearer tok', 'alice@example.org', { trusted: true }),
+    ).rejects.toMatchObject({ status: 403 });
+    expect(fetchSpy).toHaveBeenCalledTimes(1);
+  });
+
+  it('treats a malformed Identity/get response as a failure, not as a match', async () => {
+    fetchSpy
+      .mockResolvedValueOnce(json({
+        apiUrl: 'https://mail.internal/jmap/',
+        username: 'mallory',
+        primaryAccounts: { 'urn:ietf:params:jmap:mail': 'c' },
+        accounts: {},
+      }))
+      .mockResolvedValueOnce(new Response('not json', { status: 200 }));
+    const { verifyJmapIdentity } = await load();
+    await expect(
+      verifyJmapIdentity('https://mail.internal', 'Bearer tok', 'alice@example.org', { trusted: true }),
+    ).rejects.toMatchObject({ status: 502 });
+  });
+});

lib/auth/verify-jmap-auth.ts

@@ -87,11 +87,196 @@ export function assertBasicAuthMatchesUsername(authHeader: string, claimedUserna
   }
 }
 
+export interface VerifiedJmapSession {
+  apiUrl: string;
+  /** Canonical login name the server reports for the credential (RFC 8620 §2). */
+  username?: string;
+  primaryAccounts?: Record<string, string>;
+  accounts: Record<string, unknown>;
+}
+
+export interface VerifyJmapAuthOptions {
+  trusted?: boolean;
+}
+
+/**
+ * Verify `authHeader` against `serverUrl` and return the normalized URL.
+ * Callers that need to bind a username claim to the credential must use
+ * {@link verifyJmapIdentity} instead: a valid credential alone says nothing
+ * about *which* user it belongs to.
+ */
 export async function verifyJmapAuth(
   serverUrl: string,
   authHeader: string,
-  options: { trusted?: boolean } = {},
+  options: VerifyJmapAuthOptions = {},
 ): Promise<string> {
+  const { serverUrl: normalizedServerUrl } = await fetchVerifiedSession(serverUrl, authHeader, options);
+  return normalizedServerUrl;
+}
+
+function usernamesEqual(a: string, b: string): boolean {
+  return a.localeCompare(b, undefined, { sensitivity: 'accent' }) === 0;
+}
+
+/**
+ * Does the claimed login name denote the server-reported one? Stalwart
+ * canonicalizes the login it echoes in `Session.username` (`UserA@x` and the
+ * bare local part `usera` both come back as `usera@x`), so besides a
+ * case-insensitive match we accept a bare local part on either side.
+ */
+export function usernameMatchesSession(claimed: string, sessionUsername: string | undefined): boolean {
+  if (!sessionUsername) return false;
+  if (usernamesEqual(claimed, sessionUsername)) return true;
+  const claimedAt = claimed.indexOf('@');
+  const sessionAt = sessionUsername.indexOf('@');
+  if (claimedAt < 0 && sessionAt > 0) {
+    return usernamesEqual(claimed, sessionUsername.slice(0, sessionAt));
+  }
+  if (sessionAt < 0 && claimedAt > 0) {
+    return usernamesEqual(claimed.slice(0, claimedAt), sessionUsername);
+  }
+  return false;
+}
+
+/**
+ * Verify `authHeader` upstream AND prove that `claimedUsername` is the user
+ * that credential authenticates (GHSA-wxcm-j4jc-9fxq). The identity cookies
+ * minted from this result are consumed by routes that trust the cookie's
+ * username claim without contacting the mail server (settings sync, plugin
+ * approval attribution), so an unbound claim is a cross-user forgery.
+ *
+ *  - Basic: the credential's user part must equal the claim; the upstream
+ *    session fetch then proves that user's password is right.
+ *  - Bearer: the token is opaque, so the claim is checked against the
+ *    server's `Session.username`, and failing that against the e-mail
+ *    addresses of the account's sending identities (OAuth/SSO logins register
+ *    under the primary identity's e-mail, which may differ from the login).
+ *
+ * Returns the normalized server URL.
+ */
+export async function verifyJmapIdentity(
+  serverUrl: string,
+  authHeader: string,
+  claimedUsername: string,
+  options: VerifyJmapAuthOptions = {},
+): Promise<string> {
+  if (!claimedUsername) {
+    throw new JmapAuthVerificationError('Missing username', 400);
+  }
+  validateProxyAuthHeader(authHeader);
+  const isBasic = /^Basic\s/i.test(authHeader);
+  if (isBasic) {
+    assertBasicAuthMatchesUsername(authHeader, claimedUsername);
+  }
+
+  const { serverUrl: normalizedServerUrl, session } = await fetchVerifiedSession(serverUrl, authHeader, options);
+  if (isBasic) {
+    return normalizedServerUrl;
+  }
+
+  if (usernameMatchesSession(claimedUsername, session.username)) {
+    return normalizedServerUrl;
+  }
+
+  const identityEmails = await fetchIdentityEmails(normalizedServerUrl, authHeader, session, options);
+  if (identityEmails.some((email) => usernamesEqual(email, claimedUsername))) {
+    return normalizedServerUrl;
+  }
+
+  throw new JmapAuthVerificationError('Username does not match credentials', 403);
+}
+
+/**
+ * Rebase the session's advertised `apiUrl` onto the origin we verified
+ * against: the advertised public hostname may not resolve from this process
+ * (mirrors `rebaseApiUrl` in `@/lib/stalwart/jmap-api`).
+ */
+function rebaseApiUrl(apiUrl: string, normalizedServerUrl: string): string {
+  const base = new URL(normalizedServerUrl);
+  const api = new URL(apiUrl, `${normalizedServerUrl}/`);
+  return new URL(api.pathname + api.search, base.origin).toString();
+}
+
+async function fetchIdentityEmails(
+  normalizedServerUrl: string,
+  authHeader: string,
+  session: VerifiedJmapSession,
+  options: VerifyJmapAuthOptions,
+): Promise<string[]> {
+  const accountId = session.primaryAccounts?.['urn:ietf:params:jmap:mail'];
+  if (typeof accountId !== 'string' || !accountId) return [];
+
+  let apiUrl: string;
+  try {
+    apiUrl = rebaseApiUrl(session.apiUrl, normalizedServerUrl);
+  } catch {
+    throw new JmapAuthVerificationError('Invalid JMAP session response', 502);
+  }
+
+  const controller = new AbortController();
+  const timeout = setTimeout(() => controller.abort(), VERIFY_TIMEOUT_MS);
+  try {
+    if (!options.trusted && !(await isPublicHttpUrl(apiUrl))) {
+      throw new JmapAuthVerificationError('Server URL is not allowed', 400);
+    }
+    const requestInit = {
+      method: 'POST',
+      headers: { Authorization: authHeader, 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        using: ['urn:ietf:params:jmap:core', 'urn:ietf:params:jmap:mail'],
+        methodCalls: [['Identity/get', { accountId, ids: null }, '0']],
+      }),
+      signal: controller.signal,
+    };
+    // No redirect handling here: a redirect on the POST would downgrade it to
+    // a GET and can only come from a misconfigured proxy - treat as failure.
+    const response = options.trusted
+      ? await fetch(apiUrl, { ...requestInit, redirect: 'manual' })
+      : await fetchPublicUrl(apiUrl, requestInit);
+    if (!response.ok) {
+      throw new JmapAuthVerificationError(
+        response.status === 401 || response.status === 403
+          ? 'Authentication failed'
+          : 'Failed to verify JMAP identity',
+        response.status === 401 || response.status === 403 ? 401 : 502,
+        response.status,
+      );
+    }
+    const body = await response.json().catch(() => null) as { methodResponses?: unknown } | null;
+    const responses = Array.isArray(body?.methodResponses) ? body.methodResponses : null;
+    if (!responses) {
+      throw new JmapAuthVerificationError('Invalid JMAP response', 502);
+    }
+    const emails: string[] = [];
+    for (const entry of responses) {
+      if (!Array.isArray(entry) || entry[0] !== 'Identity/get') continue;
+      const list = (entry[1] as { list?: unknown })?.list;
+      if (!Array.isArray(list)) continue;
+      for (const identity of list) {
+        const email = (identity as { email?: unknown })?.email;
+        if (typeof email === 'string' && email) emails.push(email);
+      }
+    }
+    return emails;
+  } catch (error) {
+    if (error instanceof JmapAuthVerificationError) throw error;
+    if (error instanceof DisallowedUrlError) {
+      throw new JmapAuthVerificationError('Server URL is not allowed', 400);
+    }
+    if (error instanceof Error && error.name === 'AbortError') {
+      throw new JmapAuthVerificationError('JMAP identity verification timed out', 504);
+    }
+    throw new JmapAuthVerificationError('Failed to verify JMAP identity', 502);
+  } finally {
+    clearTimeout(timeout);
+  }
+}
+
+async function fetchVerifiedSession(
+  serverUrl: string,
+  authHeader: string,
+  options: VerifyJmapAuthOptions,
+): Promise<{ serverUrl: string; session: VerifiedJmapSession }> {
   const normalizedServerUrl = normalizeJmapServerUrl(serverUrl);
   validateProxyAuthHeader(authHeader);
 
@@ -152,12 +337,24 @@ export async function verifyJmapAuth(
       );
     }
 
-    const session = await response.json().catch(() => null) as { apiUrl?: unknown; accounts?: unknown } | null;
+    const session = await response.json().catch(() => null) as
+      { apiUrl?: unknown; accounts?: unknown; username?: unknown; primaryAccounts?: unknown } | null;
     if (!session || typeof session.apiUrl !== 'string' || typeof session.accounts !== 'object' || session.accounts === null) {
       throw new JmapAuthVerificationError('Invalid JMAP session response', 502);
     }
 
-    return normalizedServerUrl;
+    return {
+      serverUrl: normalizedServerUrl,
+      session: {
+        apiUrl: session.apiUrl,
+        username: typeof session.username === 'string' ? session.username : undefined,
+        primaryAccounts:
+          session.primaryAccounts && typeof session.primaryAccounts === 'object'
+            ? (session.primaryAccounts as Record<string, string>)
+            : undefined,
+        accounts: session.accounts as Record<string, unknown>,
+      },
+    };
   } catch (error) {
     if (error instanceof JmapAuthVerificationError) {
       throw error;