diff --git a/lib/__tests__/stalwart-password-login-redirect.test.ts b/lib/__tests__/stalwart-password-login-redirect.test.ts
new file mode 100644
index 000000000..710fd7403
--- /dev/null
+++ b/lib/__tests__/stalwart-password-login-redirect.test.ts
@@ -0,0 +1,125 @@
+// @vitest-environment node
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import http from 'node:http';
+import type { AddressInfo } from 'node:net';
+
+vi.mock('@/lib/logger', () => ({
+  logger: { warn: () => {}, error: () => {}, info: () => {}, debug: () => {} },
+}));
+
+import { stalwartPasswordLogin } from '@/lib/auth/stalwart-password-login';
+
+// The password login posts the password, the authorization code, the PKCE
+// verifier and the client secret to an admin-configured server. A redirect
+// from that server (or whatever sits in front of it) must not carry them to
+// another origin.
+
+interface Received { path: string; body: string }
+
+const servers: http.Server[] = [];
+
+async function serve(handler: (req: http.IncomingMessage, body: string, res: http.ServerResponse) => void) {
+  const received: Received[] = [];
+  const server = http.createServer((req, res) => {
+    let body = '';
+    req.on('data', (chunk) => { body += chunk; });
+    req.on('end', () => {
+      received.push({ path: req.url ?? '', body });
+      handler(req, body, res);
+    });
+  });
+  servers.push(server);
+  await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve));
+  const { port } = server.address() as AddressInfo;
+  return { origin: `http://127.0.0.1:${port}`, received };
+}
+
+function stalwart(req: http.IncomingMessage, _body: string, res: http.ServerResponse) {
+  res.setHeader('Content-Type', 'application/json');
+  res.end(JSON.stringify(req.url?.endsWith('/api/auth')
+    ? { type: 'authenticated', client_code: 'code-1' }
+    : { access_token: 'at', refresh_token: 'rt', expires_in: 3600 }));
+}
+
+function login(serverUrl: string) {
+  return stalwartPasswordLogin({
+    serverUrl,
+    username: 'alice@example.org',
+    password: 'hunter2',
+    clientId: 'webmail',
+    clientSecret: 'admin-secret',
+    redirectUri: 'https://webmail.example/en/auth/callback',
+    trusted: true,
+  });
+}
+
+afterEach(async () => {
+  await Promise.all(servers.splice(0).map((s) => new Promise((resolve) => s.close(resolve))));
+});
+
+describe('stalwartPasswordLogin redirects', () => {
+  it('does not re-post the password to another origin', async () => {
+    const other = await serve(stalwart);
+    const mail = await serve((req, _body, res) => {
+      res.statusCode = 307;
+      res.setHeader('Location', `${other.origin}${req.url}`);
+      res.end();
+    });
+
+    const result = await login(mail.origin);
+
+    expect(result.ok).toBe(false);
+    expect(other.received).toEqual([]);
+  });
+
+  it('does not send the code and client secret to another origin', async () => {
+    const other = await serve(stalwart);
+    const mail = await serve((req, body, res) => {
+      if (req.url?.endsWith('/auth/token')) {
+        res.statusCode = 308;
+        res.setHeader('Location', `${other.origin}/auth/token`);
+        res.end();
+        return;
+      }
+      stalwart(req, body, res);
+    });
+
+    const result = await login(mail.origin);
+
+    expect(result).toMatchObject({ ok: false, error: 'token_exchange_failed' });
+    expect(other.received).toEqual([]);
+  });
+
+  it('follows a redirect that stays on the same origin', async () => {
+    const mail = await serve((req, body, res) => {
+      if (!req.url?.startsWith('/stalwart/')) {
+        res.statusCode = 308;
+        res.setHeader('Location', `/stalwart${req.url}`);
+        res.end();
+        return;
+      }
+      stalwart(req, body, res);
+    });
+
+    const result = await login(mail.origin);
+
+    expect(result).toMatchObject({ ok: true, tokens: { access_token: 'at' } });
+    expect(mail.received.map((r) => r.path)).toEqual([
+      '/api/auth', '/stalwart/api/auth', '/auth/token', '/stalwart/auth/token',
+    ]);
+    expect(mail.received[1].body).toContain('hunter2');
+  });
+});
+
+describe('isAllowedLoginRedirect', () => {
+  it('allows the same origin and an http to https upgrade of the same host only', async () => {
+    const { isAllowedLoginRedirect } = await import('@/lib/auth/stalwart-password-login');
+    expect(isAllowedLoginRedirect('https://mail.example/api/auth', 'https://mail.example/s/api/auth')).toBe(true);
+    expect(isAllowedLoginRedirect('http://mail.example/api/auth', 'https://mail.example/api/auth')).toBe(true);
+    expect(isAllowedLoginRedirect('http://mail.example:8080/api/auth', 'https://mail.example/api/auth')).toBe(true);
+    expect(isAllowedLoginRedirect('https://mail.example/api/auth', 'http://mail.example/api/auth')).toBe(false);
+    expect(isAllowedLoginRedirect('https://mail.example/api/auth', 'https://other.example/api/auth')).toBe(false);
+    expect(isAllowedLoginRedirect('https://mail.example/api/auth', 'https://mail.example:8443/api/auth')).toBe(false);
+    expect(isAllowedLoginRedirect('http://mail.example/api/auth', 'http://mail.example.evil/api/auth')).toBe(false);
+  });
+});
diff --git a/lib/auth/stalwart-password-login.ts b/lib/auth/stalwart-password-login.ts
index 7bfe227e5..0fe7125a6 100644
--- a/lib/auth/stalwart-password-login.ts
+++ b/lib/auth/stalwart-password-login.ts
@@ -96,12 +96,36 @@ async function upstreamDetail(response: Response, trusted: boolean): Promise<str
   return (await response.text()).substring(0, 500);
 }
 
+/** A redirect that stays on the same origin, or upgrades the same host to https. */
+export function isAllowedLoginRedirect(from: string, to: string): boolean {
+  const a = new URL(from);
+  const b = new URL(to);
+  if (a.origin === b.origin) return true;
+  return a.hostname === b.hostname && a.protocol === 'http:' && b.protocol === 'https:';
+}
+
+/**
+ * fetch for an admin-configured server. The requests carry the password, the
+ * authorization code, the PKCE verifier and the client secret, so a redirect
+ * is followed once and only within the same host; anywhere else is an error.
+ */
+async function fetchConfiguredServer(input: string, init: RequestInit): Promise<Response> {
+  const first = await fetch(input, { ...init, redirect: 'manual' });
+  if (first.status < 300 || first.status >= 400) return first;
+  const location = first.headers.get('location');
+  const next = location ? new URL(location, input).toString() : null;
+  if (!next || !isAllowedLoginRedirect(input, next)) {
+    throw new Error(`${input} redirects to another host`);
+  }
+  return fetch(next, { ...init, redirect: 'error' });
+}
+
 export async function stalwartPasswordLogin(
   opts: StalwartPasswordLoginOptions,
 ): Promise<StalwartPasswordLoginResult> {
   const base = trimUrl(opts.serverUrl);
   const upstreamFetch: typeof fetch = opts.trusted
-    ? fetch
+    ? ((input, init) => fetchConfiguredServer(String(input), init ?? {}))
     : ((input, init) => fetchPublicUrl(String(input), init as Parameters<typeof fetchPublicUrl>[1]) as unknown as Promise<Response>);
 
   // PKCE proves the token exchange originates from the same client that