diff --git a/src/CoreBundle/Controller/ExceptionController.php b/src/CoreBundle/Controller/ExceptionController.php
index ffed9abf73a..d2e4ead0f0e 100644
--- a/src/CoreBundle/Controller/ExceptionController.php
+++ b/src/CoreBundle/Controller/ExceptionController.php
@@ -6,6 +6,7 @@
 
 namespace Chamilo\CoreBundle\Controller;
 
+use Chamilo\CoreBundle\Helpers\AccessUrlHelper;
 use Exception;
 use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
 use Symfony\Component\ErrorHandler\Exception\FlattenException;
@@ -13,6 +14,7 @@
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Exception\HttpException;
 use Symfony\Component\Routing\Attribute\Route;
+use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
 
 class ExceptionController extends AbstractController
 {
@@ -91,4 +93,33 @@ public function error(Request $request): Response
             'exception' => $exception,
         ]);
     }
+
+    #[Route(path: '/error/undefined-url', name: 'undefined_url_error')]
+    public function undefinedUrlError(
+        Request $request,
+        UrlGeneratorInterface $urlGenerator,
+        AccessUrlHelper $accessUrlHelper
+    ): Response {
+        $host = $request->getHost();
+
+        $accessUrl = $accessUrlHelper->getFirstAccessUrl();
+        $themeHost = rtrim($accessUrl?->getUrl() ?? '', '/');
+        $themeName = 'chamilo';
+
+        $cssUrl = $themeHost . $urlGenerator->generate('theme_asset', [
+                'name' => $themeName,
+                'path' => 'colors.css',
+            ], UrlGeneratorInterface::ABSOLUTE_PATH);
+
+        $logoUrl = $themeHost . $urlGenerator->generate('theme_asset', [
+                'name' => $themeName,
+                'path' => 'images/header-logo.svg',
+            ], UrlGeneratorInterface::ABSOLUTE_PATH);
+
+        return $this->render('@ChamiloCore/Exception/undefined_url.html.twig', [
+            'host' => $host,
+            'cssUrl' => $cssUrl,
+            'logoUrl' => $logoUrl,
+        ]);
+    }
 }
diff --git a/src/CoreBundle/EventListener/AccessUrlValidationListener.php b/src/CoreBundle/EventListener/AccessUrlValidationListener.php
new file mode 100644
index 00000000000..3689a6cde04
--- /dev/null
+++ b/src/CoreBundle/EventListener/AccessUrlValidationListener.php
@@ -0,0 +1,78 @@
+<?php
+
+declare(strict_types=1);
+
+/* For licensing terms, see /license.txt */
+
+namespace Chamilo\CoreBundle\EventListener;
+
+use Chamilo\CoreBundle\Helpers\AccessUrlHelper;
+use Symfony\Component\HttpFoundation\RedirectResponse;
+use Symfony\Component\HttpKernel\Event\RequestEvent;
+use Symfony\Component\Routing\RouterInterface;
+
+/**
+ * This event listener checks whether the current request is using a valid, registered AccessUrl.
+ * If not, it redirects to a custom error page (/error/undefined-url).
+ */
+class AccessUrlValidationListener
+{
+    public function __construct(
+        private readonly AccessUrlHelper $accessUrlHelper,
+        private readonly RouterInterface $router
+    ) {}
+
+    public function onKernelRequest(RequestEvent $event): void
+    {
+        // Only handle the main request, not subrequests.
+        if (!$event->isMainRequest()) {
+            return;
+        }
+
+        $request = $event->getRequest();
+        $path = $request->getPathInfo();
+
+        // Prevent infinite redirection loop to the same error page
+        if (str_starts_with($path, '/error/undefined-url')) {
+            return;
+        }
+
+        // Skip validation for static assets and special files
+        $excludedPrefixes = [
+            '/build/',                // Webpack build assets
+            '/theme/',                // Legacy theme assets
+            '/favicon.ico',           // Favicon
+            '/robots.txt',            // Robots file
+            '/apple-touch-icon.png',  // Mobile shortcut icon
+        ];
+
+        foreach ($excludedPrefixes as $prefix) {
+            if (str_starts_with($path, $prefix)) {
+                return;
+            }
+        }
+
+        // Skip validation if multi-URL is not enabled
+        if (!$this->accessUrlHelper->isMultiple()) {
+            return;
+        }
+
+        try {
+            // Try to get the current AccessUrl from the request
+            $accessUrl = $this->accessUrlHelper->getCurrent();
+
+            // If it's null, throw an exception to trigger redirection
+            if (null === $accessUrl) {
+                throw new \RuntimeException('AccessUrl not defined');
+            }
+        } catch (\Throwable $e) {
+            // Log host and URI for debugging
+            $host = $request->getHost();
+            $uri = $request->getUri();
+
+            // Redirect to custom error page
+            $url = $this->router->generate('undefined_url_error');
+            $event->setResponse(new RedirectResponse($url));
+        }
+    }
+}
diff --git a/src/CoreBundle/Helpers/AuthenticationConfigHelper.php b/src/CoreBundle/Helpers/AuthenticationConfigHelper.php
index 5725945ec6f..c69b0795f2a 100644
--- a/src/CoreBundle/Helpers/AuthenticationConfigHelper.php
+++ b/src/CoreBundle/Helpers/AuthenticationConfigHelper.php
@@ -71,7 +71,7 @@ public function getAuthSources(?AccessUrl $url)
             ? $this->parameterBag->get('authentication')
             : [];
 
-        if (isset($authentication[$urlId->getId()])) {
+        if ($urlId && isset($authentication[$urlId->getId()])) {
             return $authentication[$urlId->getId()];
         }
 
diff --git a/src/CoreBundle/Helpers/SettingsManagerHelper.php b/src/CoreBundle/Helpers/SettingsManagerHelper.php
index a5e29573194..e932dce9e71 100644
--- a/src/CoreBundle/Helpers/SettingsManagerHelper.php
+++ b/src/CoreBundle/Helpers/SettingsManagerHelper.php
@@ -31,7 +31,7 @@ public function getOverride(string $name, ?AccessUrl $accessUrl = null): mixed
 
         $accessUrl ??= $this->accessUrlHelper->getCurrent();
 
-        if (isset($settingsOverrides[$accessUrl->getId()][$name])) {
+        if ($accessUrl && isset($settingsOverrides[$accessUrl->getId()][$name])) {
             return $settingsOverrides[$accessUrl->getId()][$name];
         }
 
diff --git a/src/CoreBundle/Helpers/ThemeHelper.php b/src/CoreBundle/Helpers/ThemeHelper.php
index c38f61849da..ebc94193931 100644
--- a/src/CoreBundle/Helpers/ThemeHelper.php
+++ b/src/CoreBundle/Helpers/ThemeHelper.php
@@ -6,6 +6,7 @@
 
 namespace Chamilo\CoreBundle\Helpers;
 
+use Chamilo\CoreBundle\Entity\AccessUrl;
 use Chamilo\CoreBundle\Settings\SettingsManager;
 use Chamilo\CourseBundle\Settings\SettingsCourseManager;
 use League\Flysystem\FilesystemException;
@@ -46,9 +47,12 @@ public function getVisualTheme(): string
             return $visualTheme;
         }
 
+        $visualTheme = null;
         $accessUrl = $this->accessUrlHelper->getCurrent();
 
-        $visualTheme = $accessUrl->getActiveColorTheme()?->getColorTheme()->getSlug();
+        if ($accessUrl instanceof AccessUrl) {
+            $visualTheme = $accessUrl->getActiveColorTheme()?->getColorTheme()->getSlug();
+        }
 
         if ('true' == $this->settingsManager->getSetting('profile.user_selected_theme')) {
             $visualTheme = $this->userHelper->getCurrent()?->getTheme();
diff --git a/src/CoreBundle/Resources/config/services.yml b/src/CoreBundle/Resources/config/services.yml
index 5ab88f49c86..73b153d7ea0 100644
--- a/src/CoreBundle/Resources/config/services.yml
+++ b/src/CoreBundle/Resources/config/services.yml
@@ -146,3 +146,10 @@ services:
       arguments:
         $registry: '@doctrine'
         $em: '@doctrine.orm.entity_manager'
+
+    Chamilo\CoreBundle\EventListener\AccessUrlValidationListener:
+      arguments:
+        $accessUrlHelper: '@Chamilo\CoreBundle\Helpers\AccessUrlHelper'
+        $router: '@router'
+      tags:
+        - { name: kernel.event_listener, event: kernel.request, method: onKernelRequest, priority: 255 }
diff --git a/src/CoreBundle/Resources/views/Exception/undefined_url.html.twig b/src/CoreBundle/Resources/views/Exception/undefined_url.html.twig
new file mode 100644
index 00000000000..2e5454d4a0c
--- /dev/null
+++ b/src/CoreBundle/Resources/views/Exception/undefined_url.html.twig
@@ -0,0 +1,42 @@
+{% extends '@ChamiloCore/Layout/layout_one_col.html.twig' %}
+
+{% block chamilo_head %}
+    {{ parent() }}
+    <link rel="stylesheet" href="{{ cssUrl }}">
+{% endblock %}
+
+{% block content %}
+    <div class="text-center my-10">
+        {% if logoUrl %}
+            <img
+                    src="{{ logoUrl }}"
+                    alt="Logo"
+                    class="mx-auto h-20 mb-6"
+            />
+        {% endif %}
+
+        <div style="background-color: rgb(var(--color-danger-base)); color: rgb(var(--color-danger-button-text));"
+             class="mx-auto p-6 rounded-lg shadow-md md:w-3/4 lg:w-1/2">
+            <p style="font-size: 1.5rem; font-weight: bold;" class="mb-4">
+                {{ 'Oops! URL not registered.'|trans }}
+            </p>
+
+            <p style="font-size: 1.2rem;" class="mb-4">
+                {{ 'The address "%host%" is not configured in this portal instance.'|trans({'%host%': host}) }}
+            </p>
+
+            <p style="font-size: 1rem;" class="mb-6">
+                {{ 'If the problem persists, please contact support.'|trans }}
+            </p>
+
+            <a href="/"
+               class="inline-block px-6 py-2 rounded"
+               style="
+                   background-color: rgb(var(--color-primary-base));
+                   color: rgb(var(--color-primary-button-alternative-text));
+               ">
+                {{ 'Go to home page'|trans }}
+            </a>
+        </div>
+    </div>
+{% endblock %}