diff --git a/modules/backend/controllers/Auth.php b/modules/backend/controllers/Auth.php
index 549d96f7ac..16e8a6b9bd 100644
--- a/modules/backend/controllers/Auth.php
+++ b/modules/backend/controllers/Auth.php
@@ -120,6 +120,10 @@ public function signout()
      */
     public function restore()
     {
+        if (BackendAuth::user()) {
+            return Backend::redirect('backend');
+        }
+
         $this->bodyClass = 'restore';
 
         try {
@@ -136,6 +140,10 @@ public function restore()
      */
     public function restore_onSubmit()
     {
+        if (BackendAuth::user()) {
+            return Backend::redirect('backend');
+        }
+
         // Force Trusted Host verification on password reset link generation
         // regardless of config to protect against host header poisoning
         $trustedHosts = Config::get('app.trustedHosts', false);
diff --git a/modules/backend/tests/controllers/AuthRestoreOracleTest.php b/modules/backend/tests/controllers/AuthRestoreOracleTest.php
new file mode 100644
index 0000000000..020464290e
--- /dev/null
+++ b/modules/backend/tests/controllers/AuthRestoreOracleTest.php
@@ -0,0 +1,172 @@
+<?php
+
+namespace Backend\Tests\Controllers;
+
+use Backend\Controllers\Auth;
+use Backend\Models\User;
+use Illuminate\Http\Request as HttpRequest;
+use Illuminate\Support\Facades\Request;
+use System\Tests\Bootstrap\PluginTestCase;
+use Winter\Storm\Database\Model;
+use Winter\Storm\Support\Facades\Config;
+use Winter\Storm\Support\Facades\Flash;
+use Winter\Storm\Support\Facades\Mail;
+
+/**
+ * The public restore form's response must not depend on whether the submitted login matches
+ * an existing account.
+ *
+ * The form is for signed-out users and has no notion of an acting user, but nothing stopped an
+ * authenticated session reaching it, and the user model it calls does apply an authorization
+ * check when one is present. Its response therefore has to be asserted with a session as well
+ * as without, for both a login that matches an account and one that does not.
+ */
+class AuthRestoreOracleTest extends PluginTestCase
+{
+    protected User $alice;
+
+    public function setUp(): void
+    {
+        parent::setUp();
+
+        Config::set('cms.backendUri', 'backend');
+        Config::set('cms.enableCsrfProtection', false);
+        Config::set('app.url', 'http://localhost');
+        // The known-login branch sends the reset mail inline, and the response it produces is
+        // exactly what is being compared, so nothing must reach a real transport
+        Mail::fake();
+        HttpRequest::setTrustedHosts([]);
+
+        $this->alice = $this->makeUser('alice');
+    }
+
+    protected function makeUser(string $login): User
+    {
+        Model::unguard();
+        $user = User::create([
+            'first_name' => ucfirst($login),
+            'last_name' => 'User',
+            'login' => $login,
+            'email' => "{$login}@test.test",
+            'password' => 'TestPassword1',
+            'password_confirmation' => 'TestPassword1',
+            'is_activated' => true,
+        ]);
+        Model::reguard();
+
+        return $user;
+    }
+
+    protected function submitRestore(string $login): array
+    {
+        Flash::forget();
+
+        Request::swap(HttpRequest::create('http://localhost/', 'POST', [
+            'postback' => 1,
+            'login' => $login,
+        ]));
+
+        try {
+            (new Auth)->restore_onSubmit();
+            $thrown = null;
+        } catch (\Throwable $ex) {
+            $thrown = get_class($ex) . ': ' . $ex->getMessage();
+        }
+
+        return [
+            'exception' => $thrown,
+            'success' => Flash::get('success'),
+            'error' => Flash::get('error'),
+        ];
+    }
+
+    /** Control: as a guest, a known and an unknown login are already indistinguishable. */
+    public function testGuestRestoreResponseIsIdenticalForKnownAndUnknownLogins(): void
+    {
+        $known = $this->submitRestore('alice');
+        $unknown = $this->submitRestore('nobody-here');
+
+        $this->assertNull($known['exception']);
+        $this->assertSame($known, $unknown);
+    }
+
+    /** Invalidation test: the guest flow still issues a code for a real account. */
+    public function testGuestRestoreStillIssuesACodeForAKnownAccount(): void
+    {
+        $this->submitRestore('alice');
+
+        $this->assertNotEmpty(User::find($this->alice->getKey())->reset_password_code);
+    }
+
+    public function testAuthenticatedRestoreDoesNotRevealAccountExistence(): void
+    {
+        $mallory = $this->makeUser('mallory');
+        $this->assertFalse($mallory->hasAccess('backend.manage_users'), 'Precondition: no user management access');
+
+        $this->actingAs($mallory);
+
+        $known = $this->submitRestore('alice');
+        $unknown = $this->submitRestore('nobody-here');
+
+        $this->assertNull($known['exception'], 'A known login must not raise where an unknown one does not');
+        $this->assertSame($known, $unknown);
+    }
+
+    /**
+     * `restore_onSubmit` is also reachable as the `onSubmit` AJAX handler for the restore
+     * action, and Controller::run() dispatches AJAX handlers before the action method - so a
+     * guard placed only in restore() would leave this route open.
+     */
+    public function testAuthenticatedRestoreAjaxHandlerDoesNotRevealAccountExistence(): void
+    {
+        $this->actingAs($this->makeUser('mallory'));
+
+        $responses = [];
+
+        foreach (['alice', 'nobody-here'] as $login) {
+            $request = HttpRequest::create('http://localhost/backend/backend/auth/restore', 'POST', [
+                'login' => $login,
+            ]);
+            $request->headers->set('X-Requested-With', 'XMLHttpRequest');
+            $request->headers->set('X-WINTER-REQUEST-HANDLER', 'onSubmit');
+            Request::swap($request);
+
+            $response = (new Auth)->run('restore', []);
+
+            $responses[$login] = [
+                'status' => $response->getStatusCode(),
+                'content' => (string) $response->getContent(),
+            ];
+        }
+
+        $this->assertSame($responses['alice'], $responses['nobody-here']);
+    }
+
+    /**
+     * The reset URL carries the account's numeric id. Neither the reset page nor a reset
+     * submission answers differently for an id that exists and one that does not, so the id in
+     * the link is not itself an enumeration oracle. Kept as a control, since it is the property
+     * the original report described.
+     */
+    public function testResetEndpointIsIdenticalForKnownAndUnknownUserIds(): void
+    {
+        $this->assertNull(\BackendAuth::findUserById(999999), 'Precondition: id 999999 is absent');
+
+        $render = [];
+
+        foreach ([$this->alice->getKey(), 999999] as $id) {
+            Flash::forget();
+            Request::swap(HttpRequest::create('http://localhost/', 'GET'));
+
+            $controller = new Auth;
+            $controller->reset($id, 'bogus-code');
+
+            $render[$id] = [
+                'flash' => Flash::all(),
+                'code' => $controller->vars['code'],
+            ];
+        }
+
+        $this->assertSame($render[$this->alice->getKey()]['flash'], $render[999999]['flash']);
+    }
+}