feat: block disposable email signups (#2883)

Reject disposable / throwaway email providers (mailinator, yopmail,
10minutemail, ...) across all signup paths: email/password, Google,
Microsoft, personal OIDC and organisation OIDC. Backed by the
mailchecker package (offline, ~55k domains, subdomain-aware).

Exposes a SIGNUP_DISPOSABLE_EMAIL error code so the signup form and
SSO redirect alert can show a dedicated message instead of the
generic 'signup disabled' one.

packages/auth/server/lib/errors/error-codes.ts

@@ -18,6 +18,7 @@ export const AuthenticationErrorCode = {
   // TwoFactorMissingCredentials: 'TWO_FACTOR_MISSING_CREDENTIALS',
   InvalidTwoFactorCode: 'INVALID_TWO_FACTOR_CODE',
   SignupDisabled: 'SIGNUP_DISABLED',
+  SignupDisposableEmail: 'SIGNUP_DISPOSABLE_EMAIL',
   // IncorrectTwoFactorBackupCode: 'INCORRECT_TWO_FACTOR_BACKUP_CODE',
   // IncorrectIdentityProvider: 'INCORRECT_IDENTITY_PROVIDER',
   // IncorrectPassword: 'INCORRECT_PASSWORD',

packages/auth/server/lib/utils/handle-oauth-callback-url.ts

@@ -1,5 +1,9 @@
 import { NEXT_PUBLIC_WEBAPP_URL } from '@documenso/lib/constants/app';
-import { isEmailDomainAllowedForSignup, isSignupEnabledForProvider } from '@documenso/lib/constants/auth';
+import {
+  isDisposableEmail,
+  isEmailDomainAllowedForSignup,
+  isSignupEnabledForProvider,
+} from '@documenso/lib/constants/auth';
 import { AppError, AppErrorCode } from '@documenso/lib/errors/app-error';
 import { onCreateUserHook } from '@documenso/lib/server-only/user/create-user';
 import { deletedServiceAccountEmail } from '@documenso/lib/server-only/user/service-accounts/deleted-account';
@@ -132,6 +136,15 @@ export const handleOAuthCallbackUrl = async (options: HandleOAuthCallbackUrlOpti
     return c.redirect(errorUrl.toString(), 302);
   }
 
+  // Reject disposable / throwaway email providers for new SSO users.
+  if (isDisposableEmail(email)) {
+    const errorUrl = new URL('/signin', NEXT_PUBLIC_WEBAPP_URL());
+
+    errorUrl.searchParams.set('error', AuthenticationErrorCode.SignupDisposableEmail);
+
+    return c.redirect(errorUrl.toString(), 302);
+  }
+
   // Handle new user.
   const createdUser = await prisma.$transaction(async (tx) => {
     const user = await tx.user.create({

packages/auth/server/lib/utils/handle-oauth-organisation-callback-url.ts

@@ -1,5 +1,5 @@
 import { sendOrganisationAccountLinkConfirmationEmail } from '@documenso/ee/server-only/lib/send-organisation-account-link-confirmation-email';
-import { isSignupEnabledForProvider } from '@documenso/lib/constants/auth';
+import { isDisposableEmail, isSignupEnabledForProvider } from '@documenso/lib/constants/auth';
 import { AppError } from '@documenso/lib/errors/app-error';
 import { onCreateUserHook } from '@documenso/lib/server-only/user/create-user';
 import { formatOrganisationLoginUrl } from '@documenso/lib/utils/organisation-authentication-portal';
@@ -74,6 +74,15 @@ export const handleOAuthOrganisationCallbackUrl = async (options: HandleOAuthOrg
       return c.redirect(errorUrl.toString(), 302);
     }
 
+    // Reject disposable / throwaway email providers for new SSO users.
+    if (isDisposableEmail(email)) {
+      const errorUrl = new URL(formatOrganisationLoginUrl(orgUrl));
+
+      errorUrl.searchParams.set('error', AuthenticationErrorCode.SignupDisposableEmail);
+
+      return c.redirect(errorUrl.toString(), 302);
+    }
+
     userToLink = await prisma.user.create({
       data: {
         email: email,

packages/auth/server/routes/email-password.ts

@@ -1,4 +1,8 @@
-import { isEmailDomainAllowedForSignup, isSignupEnabledForProvider } from '@documenso/lib/constants/auth';
+import {
+  isDisposableEmail,
+  isEmailDomainAllowedForSignup,
+  isSignupEnabledForProvider,
+} from '@documenso/lib/constants/auth';
 import { EMAIL_VERIFICATION_STATE } from '@documenso/lib/constants/email';
 import { AppError } from '@documenso/lib/errors/app-error';
 import { jobsClient } from '@documenso/lib/jobs/client';
@@ -214,6 +218,12 @@ export const emailPasswordRoute = new Hono<HonoAuthContext>()
       });
     }
 
+    if (isDisposableEmail(email)) {
+      throw new AppError(AuthenticationErrorCode.SignupDisposableEmail, {
+        statusCode: 400,
+      });
+    }
+
     const user = await createUser({ name, email, password, signature }).catch((err) => {
       console.error(err);
       throw err;

packages/lib/constants/auth.ts

@@ -1,3 +1,4 @@
+import MailChecker from 'mailchecker';
 import { z } from 'zod';
 
 import { env } from '../utils/env';
@@ -121,6 +122,48 @@ export const isEmailDomainAllowedForSignup = (email: string): boolean => {
   return allowedDomains.includes(emailDomain);
 };
 
+/**
+ * Check if the given email belongs to a known disposable / throwaway provider
+ * (e.g. mailinator, yopmail, 10minutemail, ...).
+ *
+ * Backed by the `mailchecker` package which bundles a static list of 55k+
+ * disposable domains. The check is offline and synchronous.
+ *
+ * Matching also covers subdomains (e.g. `foo.mailinator.com` resolves to
+ * `mailinator.com`).
+ *
+ * Returns `true` when the email is disposable and should be rejected.
+ * Email format validation is intentionally NOT performed here — that is
+ * handled by Zod upstream.
+ */
+export const isDisposableEmail = (email: string): boolean => {
+  const domain = email.toLowerCase().split('@').pop();
+
+  if (!domain) {
+    return false;
+  }
+
+  const blacklist = MailChecker.blacklist();
+
+  let currentDomain: string | undefined = domain;
+
+  while (currentDomain) {
+    if (blacklist.has(currentDomain)) {
+      return true;
+    }
+
+    const nextDot = currentDomain.indexOf('.');
+
+    if (nextDot === -1) {
+      break;
+    }
+
+    currentDomain = currentDomain.slice(nextDot + 1);
+  }
+
+  return false;
+};
+
 /**
  * Check if signup is enabled for the given provider.
  * The master switch takes precedence over the per-provider flags.

packages/lib/package.json

@@ -51,6 +51,7 @@
     "konva": "^10.0.9",
     "kysely": "0.29.2",
     "luxon": "^3.7.2",
+    "mailchecker": "^6.0.20",
     "nanoid": "^5.1.6",
     "oslo": "^0.17.0",
     "p-map": "^7.0.4",