fix(login): redirect unknown users to external IdP after domain discovery when enumeration protection is active (#12581)

# Which Problems Are Solved

A `login_hint` that resolves an organization via domain discovery but
matches no
existing user is no longer automatically redirected to the
organization's external
IdP when enumeration protection (`ignoreUnknownUsernames`) is active on
the request
context.

#12369 combined the unknown-user IdP redirect into a single condition,
but gated it
with `&& !ignoreUnknownUsernames`:

```ts
if ((!effectiveLoginSettings?.allowLocalAuthentication || discoveredOrganization) && !ignoreUnknownUsernames) {
```

Since the flag is resolved from the pre-discovery context (the instance
default
settings — during domain discovery there is no organization context
yet), enabling
enumeration protection instance-wide disables the redirect even for
IdP-only
organizations (`allowLocalAuthentication: false`). The flow falls
through to
`preventUserEnumeration()` and strands the user on a decoy Password
screen for an
account that does not exist; the Back button re-enters the auto-submit
loop and
dead-ends identically. This breaks silent-SSO first logins where the
external IdP
is supposed to create the account on first authentication (Login V2).

The guard is also counterproductive on its own terms: existing users of
the same
org still auto-redirect to the IdP, so the divergent treatment acts as
an
enumeration oracle — redirect means the user exists, password screen
means it
doesn't. And it protects nothing in practice, since the IdP remains
reachable
manually via the account switcher → login-name screen → IdP button.

# How the Problems Are Solved

Split the condition by what known users of the resolved organization
actually
experience, so unknown usernames always mimic the known-user flow:

```ts
if (
  !effectiveLoginSettings?.allowLocalAuthentication ||
  (discoveredOrganization && !ignoreUnknownUsernames)
) {
```

- **Local authentication disabled (IdP-only org):** redirect unknown
usernames to
the single allowed IdP regardless of `ignoreUnknownUsernames`. Known
users are
auto-redirected there, so the identical redirect is precisely what keeps
unknown
usernames indistinguishable — and it restores first-login account
creation via
  the external IdP.
- **Local authentication enabled:** known (password) users see the
password screen,
so the decoy password screen remains the indistinguishable behavior and
enumeration protection keeps gating the discovery-based IdP redirect, as
  introduced by #12369.

Adds regression tests for both cases: domain discovery into an IdP-only
org with
`ignoreUnknownUsernames: true` expects the IdP redirect; discovery into
a
local-auth org expects the decoy `/password` redirect with no IdP flow
started.

apps/login/src/lib/server/loginname.test.ts

@@ -546,6 +546,81 @@ describe("sendLoginname", () => {
       expect(result?.redirect).toContain("organization=org123");
     });
 
+    test("should redirect to IDP via domain discovery for IdP-only org even when ignoreUnknownUsernames is true", async () => {
+      // Regression test: enumeration protection must not divert unknown users of
+      // an IdP-only org (allowLocalAuthentication: false) to the decoy password
+      // screen — known users of that org are auto-redirected to the IdP, so the
+      // redirect is the indistinguishable (and functional) behavior.
+      mockGetLoginSettings
+        .mockResolvedValueOnce({
+          // context (instance default) settings
+          ignoreUnknownUsernames: true,
+          allowLocalAuthentication: true,
+        })
+        .mockResolvedValueOnce({
+          // discovered org settings: IdP-only
+          allowDomainDiscovery: true,
+          allowRegister: false,
+          allowLocalAuthentication: false,
+          ignoreUnknownUsernames: true,
+        });
+
+      mockGetOrgsByDomain.mockResolvedValue({
+        result: [{ id: "discovered-org-sso", name: "Enterprise Org" }],
+      });
+
+      mockGetActiveIdentityProviders.mockResolvedValue({
+        identityProviders: [{ id: "idp-entra", type: "OIDC", options: { isCreationAllowed: true } }],
+      });
+      mockStartIdentityProviderFlow.mockResolvedValue({ url: "https://entra.example.com/auth" });
+
+      const result = await sendLoginname({
+        loginName: "[email protected]",
+        requestId: "req123",
+      });
+
+      expect(result).toEqual({ redirect: "https://entra.example.com/auth" });
+
+      expect(mockGetActiveIdentityProviders).toHaveBeenCalledWith({
+        serviceConfig: { baseUrl: "https://api.example.com" },
+        orgId: "discovered-org-sso",
+      });
+    });
+
+    test("should keep decoy password screen when ignoreUnknownUsernames is true and discovered org allows local authentication", async () => {
+      // With local authentication enabled, known (password) users see the
+      // password screen, so the decoy is what keeps unknown users
+      // indistinguishable — the discovery-based IdP redirect stays gated.
+      mockGetLoginSettings
+        .mockResolvedValueOnce({
+          // context (instance default) settings
+          ignoreUnknownUsernames: true,
+          allowLocalAuthentication: true,
+        })
+        .mockResolvedValueOnce({
+          // discovered org settings: local auth enabled
+          allowDomainDiscovery: true,
+          allowRegister: false,
+          allowLocalAuthentication: true,
+          ignoreUnknownUsernames: true,
+        });
+
+      mockGetOrgsByDomain.mockResolvedValue({
+        result: [{ id: "discovered-org-local", name: "Password Org" }],
+      });
+
+      const result = await sendLoginname({
+        loginName: "[email protected]",
+        requestId: "req123",
+      });
+
+      expect(result).toBeDefined();
+      expect(result?.redirect).toMatch(/^\/password\?/);
+      expect(result?.redirect).toContain("loginName=newuser%40company.com");
+      expect(result?.redirect).toContain("organization=discovered-org-local");
+      expect(mockStartIdentityProviderFlow).not.toHaveBeenCalled();
+    });
+
     test("should return error when user not found and no registration allowed", async () => {
       mockGetLoginSettings.mockResolvedValue({
         allowRegister: false,

apps/login/src/lib/server/loginname.ts

@@ -562,8 +562,19 @@ export async function sendLoginname(command: SendLoginnameCommand) {
   // and must not prevent authentication via an external IdP.
   // Fixes: https://github.com/zitadel/zitadel/issues/12021
   // Fixes: https://github.com/zitadel/zitadel/issues/12023
+  //
+  // Enumeration protection (ignoreUnknownUsernames) must NOT gate the redirect
+  // when local authentication is disabled: in an IdP-only org, known users are
+  // auto-redirected to the IdP, so sending unknown usernames to the very same
+  // IdP is what keeps them indistinguishable. Diverting unknown users to the
+  // decoy password screen instead would both break silent SSO for first-time
+  // users (the IdP is meant to create the account on first login) and act as an
+  // enumeration oracle (redirect = user exists, password screen = unknown).
+  // Conversely, when local authentication is enabled, the decoy password screen
+  // is what known (password) users see, so enumeration protection keeps gating
+  // the discovery-based redirect there.
 
-  if ((!effectiveLoginSettings?.allowLocalAuthentication || discoveredOrganization) && !ignoreUnknownUsernames) {
+  if (!effectiveLoginSettings?.allowLocalAuthentication || (discoveredOrganization && !ignoreUnknownUsernames)) {
     const resp = await redirectUserToIDP(undefined, discoveredOrganization);
     if (resp) {
       logger.debug("Redirecting to IDP", { organization: discoveredOrganization });