diff --git a/code/AdminController.php b/code/AdminController.php
new file mode 100644
index 000000000..f47dd401d
--- /dev/null
+++ b/code/AdminController.php
@@ -0,0 +1,359 @@
+<?php
+
+namespace SilverStripe\Admin;
+
+use BadMethodCallException;
+use InvalidArgumentException;
+use SilverStripe\Control\ContentNegotiator;
+use SilverStripe\Control\Controller;
+use SilverStripe\Control\HTTPRequest;
+use SilverStripe\Control\HTTPResponse;
+use SilverStripe\Control\HTTPResponse_Exception;
+use SilverStripe\Control\Middleware\HTTPCacheControlMiddleware;
+use SilverStripe\Forms\HTMLEditor\HTMLEditorConfig;
+use SilverStripe\i18n\i18n;
+use SilverStripe\Security\Permission;
+use SilverStripe\Security\Security;
+use SilverStripe\Versioned\Versioned;
+use SilverStripe\View\SSViewer;
+
+/**
+ * The base class for all controllers routed using the /admin/* route.
+ *
+ * This class is automatically routed via the AdminRootController.
+ * It's responsible for ensuring permissions are respected.
+ */
+abstract class AdminController extends Controller
+{
+    /**
+     * The current url segment attached to the controller
+     */
+    private static ?string $url_segment = null;
+
+    /**
+     * Used by {@link AdminRootController} to augment Director route rules for subclasses of AdminController
+     */
+    private static string $url_rule = '/$Action/$ID/$OtherID';
+
+    /**
+     * Priority order for routing rules. If two controllers match a given request, the one with a higher
+     * priority will handle the request.
+     */
+    private static int $url_priority = 50;
+
+    /**
+     * Codes which are required from the current user to view this controller.
+     *
+     * If multiple codes are provided, all of them are required.
+     * All CMS controllers require "CMS_ACCESS_LeftAndMain" as a baseline check,
+     * and fall back to "CMS_ACCESS_<class>" if no permissions are defined here.
+     * See {@link canView()} for more details on permission checks.
+     */
+    private static string|array $required_permission_codes = [];
+
+    /**
+     * The configuration passed to the supporting JS for each CMS section includes a 'name' key
+     * that by default matches the FQCN of the current class. This setting allows you to change
+     * the key if necessary (for example, if you are overloading CMSMain or another core class
+     * and want to keep the core JS - which depends on the core class names - functioning, you
+     * would need to set this to the FQCN of the class you are overloading).
+     *
+     * See getClientConfig()
+     */
+    private static ?string $section_name = null;
+
+    /**
+     * Get list of required permissions for accessing this controller.
+     * If false, no permission is required.
+     */
+    public static function getRequiredPermissions(): array|string|false
+    {
+        if (static::class === AdminController::class) {
+            throw new BadMethodCallException('getRequiredPermissions should be called on a subclass');
+        }
+        // If the user is accessing LeftAndMain directly, only generic permissions are required.
+        if (static::class === LeftAndMain::class) {
+            return 'CMS_ACCESS';
+        }
+        $codes = static::config()->get('required_permission_codes');
+        // allow explicit FALSE to disable subclass check
+        if ($codes === false) {
+            return false;
+        }
+        if ($codes) {
+            return $codes;
+        }
+        // Fallback if no explicit permission was declared
+        return 'CMS_ACCESS_' . static::class;
+    }
+
+    public function canView($member = null)
+    {
+        if (!$member && $member !== false) {
+            $member = Security::getCurrentUser();
+        }
+
+        // don't allow unauthenticated users
+        if (!$member) {
+            return false;
+        }
+
+        // alternative extended checks
+        if ($this->hasMethod('alternateAccessCheck')) {
+            $alternateAllowed = $this->alternateAccessCheck($member);
+            if ($alternateAllowed === false) {
+                return false;
+            }
+        }
+
+        // Check for "Access to all CMS sections" permission
+        if (Permission::checkMember($member, 'CMS_ACCESS_LeftAndMain')) {
+            return true;
+        }
+
+        // Check for permission to access this specific controller
+        $codes = static::getRequiredPermissions();
+        // allow explicit FALSE to disable subclass check
+        if ($codes === false) {
+            return true;
+        }
+        foreach ((array) $codes as $code) {
+            if (!Permission::checkMember($member, $code)) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    public function Link($action = null)
+    {
+        // LeftAndMain methods have a top-level uri access
+        if (static::class === LeftAndMain::class) {
+            $segment = '';
+        } else {
+            // Get url_segment
+            $segment = static::config()->get('url_segment');
+            if (!$segment) {
+                throw new BadMethodCallException(
+                    sprintf('AdminController subclasses (%s) must have url_segment', static::class)
+                );
+            }
+        }
+
+        $link = Controller::join_links(
+            AdminRootController::admin_url(),
+            $segment,
+            "$action"
+        );
+        $this->extend('updateLink', $link);
+        return $link;
+    }
+
+    /**
+     * Overloaded redirection logic to trigger a fake redirect on ajax requests.
+     * While this violates HTTP principles, its the only way to work around the
+     * fact that browsers handle HTTP redirects opaquely, no intervention via JS is possible.
+     * In isolation, that's not a problem - but combined with history.pushState()
+     * it means we would request the same redirection URL twice if we want to update the URL as well.
+     * See LeftAndMain.js for the required jQuery ajaxComplete handlers.
+     */
+    public function redirect(string $url, int $code = 302): HTTPResponse
+    {
+        if ($this->getRequest()->isAjax()) {
+            $response = $this->getResponse();
+            $response->addHeader('X-ControllerURL', $url);
+            if ($this->getRequest()->getHeader('X-Pjax') && !$response->getHeader('X-Pjax')) {
+                $response->addHeader('X-Pjax', $this->getRequest()->getHeader('X-Pjax'));
+            }
+            $newResponse = new LeftAndMain_HTTPResponse(
+                $response->getBody(),
+                $response->getStatusCode(),
+                $response->getStatusDescription()
+            );
+            foreach ($response->getHeaders() as $k => $v) {
+                $newResponse->addHeader($k, $v);
+            }
+            $newResponse->setIsFinished(true);
+            $this->setResponse($newResponse);
+            // Actual response will be re-requested by client
+            return $newResponse;
+        } else {
+            return parent::redirect($url, $code);
+        }
+    }
+
+    /**
+     * Returns configuration required by the client app
+     */
+    public function getClientConfig(): array
+    {
+        // Allows the section name to be overridden in config
+        $name = static::config()->get('section_name') ?: static::class;
+        // Trim leading/trailing slash to make it easier to concatenate URL
+        // and use in routing definitions.
+        $url = trim($this->Link(), '/');
+        $clientConfig = [
+            'name' => $name,
+            'url' => $url,
+            'reactRoutePath' => preg_replace('/^' . preg_quote(AdminRootController::admin_url(), '/') . '/', '', $url),
+        ];
+        $this->extend('updateClientConfig', $clientConfig);
+        return $clientConfig;
+    }
+
+    protected function init()
+    {
+        parent::init();
+
+        HTTPCacheControlMiddleware::singleton()->disableCache();
+
+        SSViewer::setRewriteHashLinksDefault(false);
+        ContentNegotiator::setEnabled(false);
+
+        // set language
+        $member = Security::getCurrentUser();
+        if (!empty($member->Locale)) {
+            i18n::set_locale($member->Locale);
+        }
+
+        // Don't allow access if the request hasn't finished being handled and the user can't access this controller
+        if (!$this->canView() && !$this->getResponse()->isFinished()) {
+            // Allow subclasses and extensions to redirect somewhere more appropriate
+            $this->invokeWithExtensions('onInitPermissionFailure');
+
+            // If we're redirecting away, just let that happen
+            if ($this->getResponse()->isRedirect()) {
+                return;
+            }
+
+            if (Security::getCurrentUser()) {
+                $this->getRequest()->getSession()->clear("BackURL");
+            }
+
+            // if no alternate menu items have matched, return a permission error
+            $messageSet = [
+                'default' => _t(
+                    __CLASS__ . '.PERMDEFAULT',
+                    "You must be logged in to access the administration area."
+                ),
+                'alreadyLoggedIn' => _t(
+                    __CLASS__ . '.PERMALREADY',
+                    "I'm sorry, but you can't access that part of the CMS."
+                ),
+                'logInAgain' => _t(
+                    __CLASS__ . '.PERMAGAIN',
+                    "You have been logged out of the CMS."
+                ),
+            ];
+
+            $this->suppressAdminErrorContext = true;
+            Security::permissionFailure($this, $messageSet);
+            return;
+        }
+
+        // Don't continue if there's already been a redirection request.
+        if ($this->getResponse()->isRedirect()) {
+            return;
+        }
+
+        $this->extend('onInit');
+
+        // Load the editor with original user themes before overwriting
+        // them with admin themes
+        $themes = HTMLEditorConfig::getThemes();
+        if (empty($themes)) {
+            HTMLEditorConfig::setThemes(SSViewer::get_themes());
+        }
+
+        // Assign default cms theme and replace user-specified themes
+        // This ensures any templates rendered use appropriate templates and resources
+        // instead of the front-end ones
+        SSViewer::set_themes(LeftAndMain::config()->uninherited('admin_themes'));
+
+        // Set the current reading mode
+        Versioned::set_stage(Versioned::DRAFT);
+
+        // Set default reading mode to suppress ?stage=Stage querystring params in CMS
+        Versioned::set_default_reading_mode(Versioned::get_reading_mode());
+    }
+
+    /**
+     * Get a data value from JSON in body of the POST request, ensuring it exists
+     * Will only read from the root node of the JSON body
+     */
+    protected function getPostedJsonValue(HTTPRequest $request, string $key): mixed
+    {
+        $data = json_decode($request->getBody(), true);
+        if (!array_key_exists($key, $data)) {
+            $this->jsonError(400);
+        }
+        return $data[$key];
+    }
+
+    /**
+     * Creates a successful json response
+     */
+    protected function jsonSuccess(int $statusCode, ?array $data = null): HTTPResponse
+    {
+        if ($statusCode < 200 || $statusCode >= 300) {
+            throw new InvalidArgumentException("Status code $statusCode must be between 200 and 299");
+        }
+        if (is_null($data)) {
+            $body = '';
+        } else {
+            $body = json_encode($data, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
+        }
+        return $this->getResponse()
+            ->addHeader('Content-Type', 'application/json')
+            ->setStatusCode($statusCode)
+            ->setBody($body);
+    }
+
+    /**
+     * Throw an error HTTPResponse encoded as json
+     *
+     * @throws HTTPResponse_Exception which interrupts request handling with the appropriate response
+     */
+    protected function jsonError(int $errorCode, string $errorMessage = ''): void
+    {
+        // Build error from message
+        $error = [
+            'type' => 'error',
+            'code' => $errorCode,
+        ];
+        if ($errorMessage) {
+            $error['value'] = $errorMessage;
+        } else {
+            $messageDefault = match ($errorCode) {
+                400 => 'Sorry, it seems there was something wrong with the request.',
+                401 => 'Sorry, it seems you are not authorised to access this section or object.',
+                403 => 'Sorry, it seems the action you were trying to perform is forbidden.',
+                404 => 'Sorry, it seems you were trying to access a section or object that doesn\'t exist.',
+                500 => 'Sorry, it seems there was an internal server error.',
+                503 => 'Sorry, it seems the service is temporarily unavailable.',
+                default => 'Error',
+            };
+            /** @phpstan-ignore translation.key (we need the key to be dynamic here) */
+            $error['value'] = _t(__CLASS__ . ".ErrorMessage{$errorCode}", $messageDefault);
+        }
+
+        // Support explicit error handling with status = error, or generic message handling
+        // with a message of type = error
+        $result = [
+            'status' => 'error',
+            'errors' => [$error]
+        ];
+        $response = HTTPResponse::create(json_encode($result), $errorCode)
+            ->addHeader('Content-Type', 'application/json');
+
+        // Call a handler method such as onBeforeHTTPError404
+        $this->extend("onBeforeJSONError{$errorCode}", $request, $response);
+
+        // Call a handler method such as onBeforeHTTPError, passing 404 as the first arg
+        $this->extend('onBeforeJSONError', $errorCode, $request, $response);
+
+        // Throw a new exception
+        throw new HTTPResponse_Exception($response);
+    }
+}
diff --git a/code/AdminRootController.php b/code/AdminRootController.php
index 32fade435..0a5a7c6f0 100644
--- a/code/AdminRootController.php
+++ b/code/AdminRootController.php
@@ -10,24 +10,41 @@
 use SilverStripe\Core\Injector\Injector;
 use SilverStripe\View\TemplateGlobalProvider;
 
+/**
+ * Controller in charge of managing AdminController routing.
+ *
+ * Adds AdminController classes to the Director routing rules and redirects to an
+ * appropriate controller if a user navigates directly to /admin
+ */
 class AdminRootController extends Controller implements TemplateGlobalProvider
 {
-
     /**
      * Fallback admin URL in case this cannot be infered from Director.rules
+     */
+    private static string $url_base = 'admin';
+
+    /**
+     * The LeftAndMain child that will be used as the initial panel to display if none is selected (i.e. if you
+     * visit /admin)
+     */
+    private static string $default_panel = SecurityAdmin::class;
+
+    /**
+     * Holds an array of url_pattern => controller k/v pairs, the same as Director::rules. However this is built
+     * dynamically from introspecting on all the classes that derive from LeftAndMain.
+     *
+     * Don't access this directly - always access via the rules() accessor, which will build this array
+     * the first time it's accessed
      *
-     * @var string
-     * @config
+     * @internal
      */
-    private static $url_base = 'admin';
+    private static ?array $adminRules = null;
 
     /**
      * Convenience function to return the admin route config.
      * Looks for the {@link Director::$rules} for the current admin Controller.
-     *
-     * @return string
      */
-    public static function get_admin_route()
+    public static function get_admin_route(): string
     {
         $rules = Director::config()->get('rules');
         $adminRoute = array_search(__CLASS__, $rules ?? []);
@@ -36,44 +53,22 @@ public static function get_admin_route()
 
     /**
      * Returns the root admin URL for the site with trailing slash
-     *
-     * @return string
      */
-    public static function admin_url(string $action = '')
+    public static function admin_url(string $action = ''): string
     {
         return Controller::join_links(AdminRootController::get_admin_route(), $action);
     }
 
-    /**
-     * @var string
-     * @config
-     * The LeftAndMain child that will be used as the initial panel to display if none is selected (i.e. if you
-     * visit /admin)
-     */
-    private static $default_panel = SecurityAdmin::class;
-
-    /**
-     * @var array
-     * @internal
-     *
-     * Holds an array of url_pattern => controller k/v pairs, the same as Director::rules. However this is built
-     * dynamically from introspecting on all the classes that derive from LeftAndMain.
-     *
-     * Don't access this directly - always access via the rules() accessor below, which will build this array
-     * the first time it's accessed
-     */
-    private static $adminRules = null;
-
     /**
      * Gets a list of url_pattern => controller k/v pairs for each LeftAndMain derived controller
      */
-    public static function rules()
+    public static function rules(): array
     {
         if (AdminRootController::$adminRules === null) {
             AdminRootController::$adminRules = [];
 
             // Map over the array calling add_rule_for_controller on each
-            $classes = CMSMenu::get_cms_classes(null, true, CMSMenu::URL_PRIORITY);
+            $classes = CMSMenu::get_cms_classes(AdminController::class, true, CMSMenu::URL_PRIORITY);
             array_map([__CLASS__, 'add_rule_for_controller'], $classes ?? []);
         }
         return AdminRootController::$adminRules;
@@ -81,10 +76,8 @@ public static function rules()
 
     /**
      * Add the appropriate k/v pair to AdminRootController::$rules for the given controller.
-     *
-     * @param string $controllerClass Name of class
      */
-    protected static function add_rule_for_controller($controllerClass)
+    protected static function add_rule_for_controller(string $controllerClass): void
     {
         $config = Config::forClass($controllerClass);
         $urlSegment = $config->get('url_segment');
@@ -118,7 +111,7 @@ public function handleRequest(HTTPRequest $request): HTTPResponse
         // Otherwise
         $rules = AdminRootController::rules();
         foreach ($rules as $pattern => $controller) {
-            if (($arguments = $request->match($pattern, true)) !== false) {
+            if ($request->match($pattern, true) !== false) {
                 /** @var LeftAndMain $controllerObj */
                 $controllerObj = Injector::inst()->create($controller);
                 return $controllerObj->handleRequest($request);
@@ -131,10 +124,10 @@ public function handleRequest(HTTPRequest $request): HTTPResponse
     }
 
     /**
-     * @return array Returns an array of strings of the method names of methods on the call that should be exposed
+     * Returns an array of strings of the method names of methods on the call that should be exposed
      * as global variables in the templates.
      */
-    public static function get_template_global_variables()
+    public static function get_template_global_variables(): array
     {
         return [
             'adminURL' => 'admin_url'
diff --git a/code/LeftAndMain.php b/code/LeftAndMain.php
index 9eb920f7b..a55ea69de 100644
--- a/code/LeftAndMain.php
+++ b/code/LeftAndMain.php
@@ -2,19 +2,16 @@
 
 namespace SilverStripe\Admin;
 
-use BadMethodCallException;
 use InvalidArgumentException;
 use LogicException;
 use ReflectionClass;
 use SilverStripe\CMS\Controllers\CMSMain;
 use SilverStripe\Admin\Navigator\SilverStripeNavigator;
-use SilverStripe\Control\ContentNegotiator;
 use SilverStripe\Control\Controller;
 use SilverStripe\Control\Director;
 use SilverStripe\Control\HTTPRequest;
 use SilverStripe\Control\HTTPResponse;
 use SilverStripe\Control\HTTPResponse_Exception;
-use SilverStripe\Control\Middleware\HTTPCacheControlMiddleware;
 use SilverStripe\Control\PjaxResponseNegotiator;
 use SilverStripe\Core\ClassInfo;
 use SilverStripe\Core\Config\Config;
@@ -43,13 +40,11 @@
 use SilverStripe\Model\List\SS_List;
 use SilverStripe\Core\Validation\ValidationException;
 use SilverStripe\Core\Validation\ValidationResult;
-use SilverStripe\Security\Member;
 use SilverStripe\Security\Permission;
 use SilverStripe\Security\PermissionProvider;
 use SilverStripe\Security\Security;
 use SilverStripe\Security\SecurityToken;
 use SilverStripe\SiteConfig\SiteConfig;
-use SilverStripe\Versioned\Versioned;
 use SilverStripe\Model\ArrayData;
 use SilverStripe\View\Requirements;
 use SilverStripe\View\SSViewer;
@@ -61,7 +56,7 @@
  * This is essentially an abstract class which should be subclassed.
  * See {@link CMSMain} for a good example.
  */
-class LeftAndMain extends Controller implements PermissionProvider
+class LeftAndMain extends AdminController implements PermissionProvider
 {
 
     /**
@@ -77,20 +72,6 @@ class LeftAndMain extends Controller implements PermissionProvider
      */
     private static $client_debugging = true;
 
-    /**
-     * The current url segment attached to the LeftAndMain instance
-     *
-     * @config
-     * @var string
-     */
-    private static $url_segment = null;
-
-    /**
-     * @config
-     * @var string Used by {@link AdminRootController} to augment Director route rules for sub-classes of LeftAndMain
-     */
-    private static $url_rule = '/$Action/$ID/$OtherID';
-
     /**
      * @config
      * @var string
@@ -109,12 +90,6 @@ class LeftAndMain extends Controller implements PermissionProvider
      */
     private static $menu_priority = 0;
 
-    /**
-     * @config
-     * @var int
-     */
-    private static $url_priority = 50;
-
     /**
      * A subclass of {@link DataObject}.
      *
@@ -187,18 +162,6 @@ class LeftAndMain extends Controller implements PermissionProvider
      */
     private static $admin_themes = [];
 
-    /**
-     * Codes which are required from the current user to view this controller.
-     * If multiple codes are provided, all of them are required.
-     * All CMS controllers require "CMS_ACCESS_LeftAndMain" as a baseline check,
-     * and fall back to "CMS_ACCESS_<class>" if no permissions are defined here.
-     * See {@link canView()} for more details on permission checks.
-     *
-     * @config
-     * @var array
-     */
-    private static $required_permission_codes;
-
     /**
      * Namespace for session info, e.g. current record.
      * Defaults to the current class name, but can be amended to share a namespace in case
@@ -281,16 +244,35 @@ class LeftAndMain extends Controller implements PermissionProvider
     private static $frame_options = 'SAMEORIGIN';
 
     /**
-     * The configuration passed to the supporting JS for each CMS section includes a 'name' key
-     * that by default matches the FQCN of the current class. This setting allows you to change
-     * the key if necessary (for example, if you are overloading CMSMain or another core class
-     * and want to keep the core JS - which depends on the core class names - functioning, you
-     * would need to set this to the FQCN of the class you are overloading).
+     * The urls used for the links in the Help dropdown in the backend
+     *
+     * Set this to `false` via yml config if you do not want to show any help links
      *
      * @config
-     * @var string|null
+     * @var array
      */
-    private static $section_name = null;
+    private static $help_links = [
+        'CMS User help' => 'https://userhelp.silverstripe.org/en/5',
+        'Developer docs' => 'https://docs.silverstripe.org/en/5/',
+        'Community' => 'https://www.silverstripe.org/',
+        'Feedback' => 'https://www.silverstripe.org/give-feedback/',
+    ];
+
+    /**
+     * The href for the anchor on the Silverstripe logo
+     *
+     * @config
+     * @var string
+     */
+    private static $application_link = '//www.silverstripe.org/';
+
+    /**
+     * The application name
+     *
+     * @config
+     * @var string
+     */
+    private static $application_name = 'Silverstripe';
 
     /**
      * @var PjaxResponseNegotiator
@@ -302,6 +284,13 @@ class LeftAndMain extends Controller implements PermissionProvider
      */
     protected $versionProvider;
 
+    /**
+     * Cached search filter instance for current search
+     *
+     * @var LeftAndMain_SearchFilter
+     */
+    protected $searchFilterCache = null;
+
     /**
      * Gets the combined configuration of all LeftAndMain subclasses required by the client app.
      *
@@ -312,7 +301,7 @@ class LeftAndMain extends Controller implements PermissionProvider
     public function getCombinedClientConfig()
     {
         $combinedClientConfig = ['sections' => []];
-        $cmsClassNames = CMSMenu::get_cms_classes(LeftAndMain::class, true, CMSMenu::URL_PRIORITY);
+        $cmsClassNames = CMSMenu::get_cms_classes(AdminController::class, true, CMSMenu::URL_PRIORITY);
 
         // append LeftAndMain to the list as well
         $cmsClassNames[] = LeftAndMain::class;
@@ -336,40 +325,20 @@ public function getCombinedClientConfig()
         return json_encode($combinedClientConfig);
     }
 
-    /**
-     * Returns configuration required by the client app
-     *
-     * @return array
-     */
-    public function getClientConfig()
+    public function getClientConfig(): array
     {
-        // Allows the section name to be overridden in config
-        $name = $this->config()->get('section_name');
-        $url = trim($this->Link() ?? '', '/');
-
-        if (!$name) {
-            $name = static::class;
-        }
-
-        $clientConfig = [
-            // Trim leading/trailing slash to make it easier to concatenate URL
-            // and use in routing definitions.
-            'name' => $name,
-            'url' => $url,
-            'reactRoutePath' => preg_replace('/^' . preg_quote(AdminRootController::admin_url(), '/') . '/', '', $url),
-            'form' => [
+        // Add WYSIWYG link form schema before extensions are applied
+        $this->beforeExtending('updateClientConfig', function (array &$clientConfig): void {
+            $clientConfig['form'] = [
                 'EditorExternalLink' => [
                     'schemaUrl' => $this->Link('methodSchema/Modals/EditorExternalLink'),
                 ],
                 'EditorEmailLink' => [
                     'schemaUrl' => $this->Link('methodSchema/Modals/EditorEmailLink'),
                 ],
-            ],
-        ];
-
-        $this->extend('updateClientConfig', $clientConfig);
-
-        return $clientConfig;
+            ];
+        });
+        return parent::getClientConfig();
     }
 
     /**
@@ -425,87 +394,9 @@ public function schema(HTTPRequest $request): HTTPResponse
     }
 
     /**
-     * Get a data value from JSON in body of the POST request, ensuring it exists
-     * Will only read from the root node of the JSON body
-     */
-    protected function getPostedJsonValue(HTTPRequest $request, string $key): mixed
-    {
-        $data = json_decode($request->getBody(), true);
-        if (!array_key_exists($key, $data)) {
-            $this->jsonError(400);
-        }
-        return $data[$key];
-    }
-
-    /**
-     * Creates a successful json response
-     */
-    protected function jsonSuccess(int $statusCode, ?array $data = null): HTTPResponse
-    {
-        if ($statusCode < 200 || $statusCode >= 300) {
-            throw new InvalidArgumentException("Status code $statusCode must be between 200 and 299");
-        }
-        if (is_null($data)) {
-            $body = '';
-        } else {
-            $body = json_encode($data, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
-        }
-        return $this->getResponse()
-            ->addHeader('Content-Type', 'application/json')
-            ->setStatusCode($statusCode)
-            ->setBody($body);
-    }
-
-    /**
-     * Return an error HTTPResponse encoded as json
-     *
-     * @param int $errorCode
-     * @param string $errorMessage
-     * @return HTTPResponse
-     * @throws HTTPResponse_Exception
+     * Get the form schema from a given method.
+     * The method must return a Form.
      */
-    public function jsonError($errorCode, $errorMessage = null)
-    {
-        // Build error from message
-        $error = [
-            'type' => 'error',
-            'code' => $errorCode,
-        ];
-        if ($errorMessage) {
-            $error['value'] = $errorMessage;
-        } else {
-            $messageDefault = match ($errorCode) {
-                400 => 'Sorry, it seems there was something wrong with the request.',
-                401 => 'Sorry, it seems you are not authorised to access this section or object.',
-                403 => 'Sorry, it seems the action you were trying to perform is forbidden.',
-                404 => 'Sorry, it seems you were trying to access a section or object that doesn\'t exist.',
-                500 => 'Sorry, it seems there was an internal server error.',
-                503 => 'Sorry, it seems the service is temporarily unavailable.',
-                default => 'Error',
-            };
-            /** @phpstan-ignore translation.key (we need the key to be dynamic here) */
-            $error['value'] = _t(__CLASS__ . ".ErrorMessage{$errorCode}", $messageDefault);
-        }
-
-        // Support explicit error handling with status = error, or generic message handling
-        // with a message of type = error
-        $result = [
-            'status' => 'error',
-            'errors' => [$error]
-        ];
-        $response = HTTPResponse::create(json_encode($result), $errorCode)
-            ->addHeader('Content-Type', 'application/json');
-
-        // Call a handler method such as onBeforeHTTPError404
-        $this->extend("onBeforeJSONError{$errorCode}", $request, $response);
-
-        // Call a handler method such as onBeforeHTTPError, passing 404 as the first arg
-        $this->extend('onBeforeJSONError', $errorCode, $request, $response);
-
-        // Throw a new exception
-        throw new HTTPResponse_Exception($response);
-    }
-
     public function methodSchema(HTTPRequest $request): HTTPResponse
     {
         $method = $request->param('Method');
@@ -574,258 +465,136 @@ protected function getSchemaResponse($schemaID, $form = null, ValidationResult $
     }
 
     /**
-     * @param Member $member
-     * @return bool
+     * Try to redirect to an appropriate admin section if we can't access this one
      */
-    public function canView($member = null)
+    public function onInitPermissionFailure(): void
     {
-        if (!$member && $member !== false) {
-            $member = Security::getCurrentUser();
-        }
-
-        // cms menus only for logged-in members
-        if (!$member) {
-            return false;
-        }
-
-        // alternative extended checks
-        if ($this->hasMethod('alternateAccessCheck')) {
-            $alternateAllowed = $this->alternateAccessCheck($member);
-            if ($alternateAllowed === false) {
-                return false;
-            }
-        }
-
-        // Check for "CMS admin" permission
-        if (Permission::checkMember($member, "CMS_ACCESS_LeftAndMain")) {
-            return true;
-        }
-
-        // Check for LeftAndMain sub-class permissions
-        $codes = $this->getRequiredPermissions();
-        if ($codes === false) { // allow explicit FALSE to disable subclass check
-            return true;
-        }
-        foreach ((array)$codes as $code) {
-            if (!Permission::checkMember($member, $code)) {
-                return false;
+        $menu = $this->MainMenu();
+        foreach ($menu as $candidate) {
+            if ($candidate->Link &&
+                $candidate->Link != $this->Link()
+                && $candidate->MenuItem->controller
+                && singleton($candidate->MenuItem->controller)->canView()
+            ) {
+                $this->redirect($candidate->Link);
+                return;
             }
         }
-
-        return true;
-    }
-
-    /**
-     * Get list of required permissions
-     *
-     * @return array|string|bool Code, array of codes, or false if no permission required
-     */
-    public static function getRequiredPermissions()
-    {
-        $class = get_called_class();
-        // If the user is accessing LeftAndMain directly, only generic permissions are required.
-        if ($class === LeftAndMain::class) {
-            return 'CMS_ACCESS';
-        }
-        $code = Config::inst()->get($class, 'required_permission_codes');
-        if ($code === false) {
-            return false;
-        }
-        if ($code) {
-            return $code;
-        }
-        return 'CMS_ACCESS_' . $class;
+        $this->suppressAdminErrorContext = true;
     }
 
-    /**
-     * @uses CMSMenu
-     */
     protected function init()
     {
-        parent::init();
-
-        HTTPCacheControlMiddleware::singleton()->disableCache();
-
-        SSViewer::setRewriteHashLinksDefault(false);
-        ContentNegotiator::setEnabled(false);
-
-        // set language
-        $member = Security::getCurrentUser();
-        if (!empty($member->Locale)) {
-            i18n::set_locale($member->Locale);
-        }
-
-        // Allow customisation of the access check by a extension
-        // Also all the canView() check to execute Controller::redirect()
-        if (!$this->canView() && !$this->getResponse()->isFinished()) {
-            // When access /admin/, we should try a redirect to another part of the admin rather than be locked out
-            $menu = $this->MainMenu();
-            foreach ($menu as $candidate) {
-                if ($candidate->Link &&
-                    $candidate->Link != $this->Link()
-                    && $candidate->MenuItem->controller
-                    && singleton($candidate->MenuItem->controller)->canView()
-                ) {
-                    $this->redirect($candidate->Link);
-                    return;
-                }
+        $this->beforeExtending('onInit', function () {
+            // Audit logging hook
+            if (empty($_REQUEST['executeForm']) && !$this->getRequest()->isAjax()) {
+                $this->extend('accessedCMS');
             }
 
+            // Set the members html editor config
             if (Security::getCurrentUser()) {
-                $this->getRequest()->getSession()->clear("BackURL");
+                HTMLEditorConfig::set_active_identifier(Security::getCurrentUser()->getHtmlEditorConfigForCMS());
             }
 
-            // if no alternate menu items have matched, return a permission error
-            $messageSet = [
-                'default' => _t(
-                    __CLASS__ . '.PERMDEFAULT',
-                    "You must be logged in to access the administration area; please enter your credentials below."
-                ),
-                'alreadyLoggedIn' => _t(
-                    __CLASS__ . '.PERMALREADY',
-                    "I'm sorry, but you can't access that part of the CMS.  If you want to log in as someone else, do"
-                    . " so below."
-                ),
-                'logInAgain' => _t(
-                    __CLASS__ . '.PERMAGAIN',
-                    "You have been logged out of the CMS.  If you would like to log in again, enter a username and"
-                    . " password below."
-                ),
-            ];
-
-            $this->suppressAdminErrorContext = true;
-            Security::permissionFailure($this, $messageSet);
-            return;
-        }
-
-        // Don't continue if there's already been a redirection request.
-        if ($this->redirectedTo()) {
-            return;
-        }
-
-        // Audit logging hook
-        if (empty($_REQUEST['executeForm']) && !$this->getRequest()->isAjax()) {
-            $this->extend('accessedCMS');
-        }
-
-        // Set the members html editor config
-        if (Security::getCurrentUser()) {
-            HTMLEditorConfig::set_active_identifier(Security::getCurrentUser()->getHtmlEditorConfigForCMS());
-        }
-
-        // Set default values in the config if missing.  These things can't be defined in the config
-        // file because insufficient information exists when that is being processed
-        $htmlEditorConfig = HTMLEditorConfig::get_active();
-        $htmlEditorConfig->setOption('language', TinyMCEConfig::get_tinymce_lang());
-        $langUrl = TinyMCEConfig::get_tinymce_lang_url();
-        if ($langUrl) {
-            $htmlEditorConfig->setOption('language_url', $langUrl);
-        }
-
-        Requirements::customScript("
-            window.ss = window.ss || {};
-            window.ss.config = " . $this->getCombinedClientConfig() . ";
-        ");
+            // Set default values in the config if missing.  These things can't be defined in the config
+            // file because insufficient information exists when that is being processed
+            $htmlEditorConfig = HTMLEditorConfig::get_active();
+            $htmlEditorConfig->setOption('language', TinyMCEConfig::get_tinymce_lang());
+            $langUrl = TinyMCEConfig::get_tinymce_lang_url();
+            if ($langUrl) {
+                $htmlEditorConfig->setOption('language_url', $langUrl);
+            }
 
-        Requirements::javascript('silverstripe/admin: client/dist/js/vendor.js');
-        Requirements::javascript('silverstripe/admin: client/dist/js/bundle.js');
+            Requirements::customScript("
+                window.ss = window.ss || {};
+                window.ss.config = " . $this->getCombinedClientConfig() . ";
+            ");
+
+            Requirements::javascript('silverstripe/admin: client/dist/js/vendor.js');
+            Requirements::javascript('silverstripe/admin: client/dist/js/bundle.js');
+
+            // Bootstrap components
+            Requirements::javascript('silverstripe/admin: thirdparty/popper/popper.min.js');
+            Requirements::javascript('silverstripe/admin: thirdparty/bootstrap/js/dist/util.js');
+            Requirements::javascript('silverstripe/admin: thirdparty/bootstrap/js/dist/collapse.js');
+            Requirements::javascript('silverstripe/admin: thirdparty/bootstrap/js/dist/tooltip.js');
+            Requirements::customScript(
+                "window.jQuery('body').tooltip({ selector: '[data-toggle=tooltip]' });",
+                'bootstrap.tooltip-boot'
+            );
 
-        // Bootstrap components
-        Requirements::javascript('silverstripe/admin: thirdparty/popper/popper.min.js');
-        Requirements::javascript('silverstripe/admin: thirdparty/bootstrap/js/dist/util.js');
-        Requirements::javascript('silverstripe/admin: thirdparty/bootstrap/js/dist/collapse.js');
-        Requirements::javascript('silverstripe/admin: thirdparty/bootstrap/js/dist/tooltip.js');
-        Requirements::customScript(
-            "window.jQuery('body').tooltip({ selector: '[data-toggle=tooltip]' });",
-            'bootstrap.tooltip-boot'
-        );
+            Requirements::css('silverstripe/admin: client/dist/styles/bundle.css');
+            Requirements::add_i18n_javascript('silverstripe/admin:client/lang');
+            Requirements::add_i18n_javascript('silverstripe/admin:client/dist/moment-locales', false);
 
-        Requirements::css('silverstripe/admin: client/dist/styles/bundle.css');
-        Requirements::add_i18n_javascript('silverstripe/admin:client/lang');
-        Requirements::add_i18n_javascript('silverstripe/admin:client/dist/moment-locales', false);
+            if (LeftAndMain::config()->uninherited('session_keepalive_ping')) {
+                Requirements::javascript('silverstripe/admin: client/dist/js/LeftAndMain.Ping.js');
+            }
 
-        if (LeftAndMain::config()->uninherited('session_keepalive_ping')) {
-            Requirements::javascript('silverstripe/admin: client/dist/js/LeftAndMain.Ping.js');
-        }
+            // Custom requirements
+            $extraJs = $this->config()->get('extra_requirements_javascript');
 
-        // Custom requirements
-        $extraJs = $this->config()->get('extra_requirements_javascript');
+            if ($extraJs) {
+                foreach ($extraJs as $file => $config) {
+                    if (is_numeric($file)) {
+                        $file = $config;
+                        $config = [];
+                    }
 
-        if ($extraJs) {
-            foreach ($extraJs as $file => $config) {
-                if (is_numeric($file)) {
-                    $file = $config;
-                    $config = [];
+                    Requirements::javascript($file, $config);
                 }
-
-                Requirements::javascript($file, $config);
             }
-        }
 
-        $extraI18n = $this->config()->get('extra_requirements_i18n');
-        if ($extraI18n) {
-            foreach ($extraI18n as $dir => $return) {
-                if (is_numeric($dir)) {
-                    $dir = $return;
-                    $return = false;
+            $extraI18n = $this->config()->get('extra_requirements_i18n');
+            if ($extraI18n) {
+                foreach ($extraI18n as $dir => $return) {
+                    if (is_numeric($dir)) {
+                        $dir = $return;
+                        $return = false;
+                    }
+                    Requirements::add_i18n_javascript($dir, $return);
                 }
-                Requirements::add_i18n_javascript($dir, $return);
             }
-        }
 
-        $extraCss = $this->config()->get('extra_requirements_css');
-
-        if ($extraCss) {
-            foreach ($extraCss as $file => $config) {
-                if (is_numeric($file)) {
-                    $file = $config;
-                    $config = [];
-                }
-                $media = null;
-                if (isset($config['media'])) {
-                     $media = $config['media'];
-                     unset($config['media']);
-                }
-
-                Requirements::css($file, $media, $config);
-            }
-        }
+            $extraCss = $this->config()->get('extra_requirements_css');
 
-        $extraThemedCss = $this->config()->get('extra_requirements_themedCss');
+            if ($extraCss) {
+                foreach ($extraCss as $file => $config) {
+                    if (is_numeric($file)) {
+                        $file = $config;
+                        $config = [];
+                    }
+                    $media = null;
+                    if (isset($config['media'])) {
+                        $media = $config['media'];
+                        unset($config['media']);
+                    }
 
-        if ($extraThemedCss) {
-            foreach ($extraThemedCss as $file => $config) {
-                if (is_numeric($file)) {
-                    $file = $config;
-                    $config = [];
-                }
-                $media = null;
-                if (isset($config['media'])) {
-                     $media = $config['media'];
-                     unset($config['media']);
+                    Requirements::css($file, $media, $config);
                 }
-
-                Requirements::themedCSS($file, $media, $config);
             }
-        }
-
-        $this->extend('onInit');
 
-        // Load the editor with original user themes before overwriting
-        // them with admin themes
-        $themes = HTMLEditorConfig::getThemes();
-        if (empty($themes)) {
-            HTMLEditorConfig::setThemes(SSViewer::get_themes());
-        }
+            $extraThemedCss = $this->config()->get('extra_requirements_themedCss');
 
-        // Assign default cms theme and replace user-specified themes
-        SSViewer::set_themes(LeftAndMain::config()->uninherited('admin_themes'));
+            if ($extraThemedCss) {
+                foreach ($extraThemedCss as $file => $config) {
+                    if (is_numeric($file)) {
+                        $file = $config;
+                        $config = [];
+                    }
+                    $media = null;
+                    if (isset($config['media'])) {
+                        $media = $config['media'];
+                        unset($config['media']);
+                    }
 
-        // Set the current reading mode
-        Versioned::set_stage(Versioned::DRAFT);
+                    Requirements::themedCSS($file, $media, $config);
+                }
+            }
+        });
 
-        // Set default reading mode to suppress ?stage=Stage querystring params in CMS
-        Versioned::set_default_reading_mode(Versioned::get_reading_mode());
+        // Run parent init and process the onInit extension hook
+        parent::init();
     }
 
     public function afterHandleRequest()
@@ -891,39 +660,6 @@ public function handleRequest(HTTPRequest $request): HTTPResponse
         return $response;
     }
 
-    /**
-     * Overloaded redirection logic to trigger a fake redirect on ajax requests.
-     * While this violates HTTP principles, its the only way to work around the
-     * fact that browsers handle HTTP redirects opaquely, no intervention via JS is possible.
-     * In isolation, that's not a problem - but combined with history.pushState()
-     * it means we would request the same redirection URL twice if we want to update the URL as well.
-     * See LeftAndMain.js for the required jQuery ajaxComplete handlers.
-     */
-    public function redirect(string $url, int $code = 302): HTTPResponse
-    {
-        if ($this->getRequest()->isAjax()) {
-            $response = $this->getResponse();
-            $response->addHeader('X-ControllerURL', $url);
-            if ($this->getRequest()->getHeader('X-Pjax') && !$response->getHeader('X-Pjax')) {
-                $response->addHeader('X-Pjax', $this->getRequest()->getHeader('X-Pjax'));
-            }
-            $newResponse = new LeftAndMain_HTTPResponse(
-                $response->getBody(),
-                $response->getStatusCode(),
-                $response->getStatusDescription()
-            );
-            foreach ($response->getHeaders() as $k => $v) {
-                $newResponse->addHeader($k, $v);
-            }
-            $newResponse->setIsFinished(true);
-            $this->setResponse($newResponse);
-            // Actual response will be re-requested by client
-            return $newResponse;
-        } else {
-            return parent::redirect($url, $code);
-        }
-    }
-
     public function index(HTTPRequest $request): HTTPResponse
     {
         return $this->getResponseNegotiator()->respond($request);
@@ -940,41 +676,6 @@ public function ShowSwitchView()
         return false;
     }
 
-
-    //------------------------------------------------------------------------------------------//
-    // Main controllers
-
-    /**
-     * You should implement a Link() function in your subclass of LeftAndMain,
-     * to point to the URL of that particular controller.
-     *
-     * @param string $action
-     * @return string
-     */
-    public function Link($action = null)
-    {
-        // LeftAndMain methods have a top-level uri access
-        if (static::class === LeftAndMain::class) {
-            $segment = '';
-        } else {
-            // Get url_segment
-            $segment = $this->config()->get('url_segment');
-            if (!$segment) {
-                throw new BadMethodCallException(
-                    sprintf('LeftAndMain subclasses (%s) must have url_segment', static::class)
-                );
-            }
-        }
-
-        $link = Controller::join_links(
-            AdminRootController::admin_url(),
-            $segment,
-            "$action"
-        );
-        $this->extend('updateLink', $link);
-        return $link;
-    }
-
     /**
      * Get menu title for this section (translated)
      *
@@ -1272,13 +973,6 @@ public function Breadcrumbs($unlinked = false)
         return $items;
     }
 
-    /**
-     * Cached search filter instance for current search
-     *
-     * @var LeftAndMain_SearchFilter
-     */
-    protected $searchFilterCache = null;
-
     /**
      * Gets the current search filter for this request, if available
      *
@@ -1861,21 +1555,6 @@ public function SiteConfig()
         return class_exists(SiteConfig::class) ? SiteConfig::current_site_config() : null;
     }
 
-    /**
-     * The urls used for the links in the Help dropdown in the backend
-     *
-     * Set this to `false` via yml config if you do not want to show any help links
-     *
-     * @config
-     * @var array
-     */
-    private static $help_links = [
-        'CMS User help' => 'https://userhelp.silverstripe.org/en/5',
-        'Developer docs' => 'https://docs.silverstripe.org/en/5/',
-        'Community' => 'https://www.silverstripe.org/',
-        'Feedback' => 'https://www.silverstripe.org/give-feedback/',
-    ];
-
     /**
      * Returns help_links in a format readable by a template
      * @return ArrayList
@@ -1901,14 +1580,6 @@ public function getHelpLinks()
         return ArrayList::create($formattedLinks);
     }
 
-    /**
-     * The href for the anchor on the Silverstripe logo
-     *
-     * @config
-     * @var string
-     */
-    private static $application_link = '//www.silverstripe.org/';
-
     /**
      * @return string
      */
@@ -1917,14 +1588,6 @@ public function ApplicationLink()
         return $this->config()->get('application_link');
     }
 
-    /**
-     * The application name
-     *
-     * @config
-     * @var string
-     */
-    private static $application_name = 'Silverstripe';
-
     /**
      * Get the application name.
      *
diff --git a/code/SudoModeController.php b/code/SudoModeController.php
index 0a5376d5e..7140c3144 100644
--- a/code/SudoModeController.php
+++ b/code/SudoModeController.php
@@ -15,12 +15,10 @@
 /**
  * Responsible for checking and verifying whether sudo mode is enabled
  */
-class SudoModeController extends LeftAndMain
+class SudoModeController extends AdminController
 {
     private static string $url_segment = 'sudomode';
 
-    private static bool $ignore_menuitem = true;
-
     private static array $allowed_actions = [
         'check',
         'activate',
@@ -43,7 +41,7 @@ class SudoModeController extends LeftAndMain
      */
     private static bool $required_permission_codes = false;
 
-    public function getClientConfig()
+    public function getClientConfig(): array
     {
         $request = Injector::inst()->get(HTTPRequest::class);
 
@@ -91,7 +89,7 @@ public function activate(HTTPRequest $request): HTTPResponse
             return $this->jsonResponse([
                 'result' => false,
                 'message' => _t(__CLASS__ . '.INVALID', 'Incorrect password'),
-            ]);
+            ], 400);
         }
 
         // Activate sudo mode and return successful result
diff --git a/lang/en.yml b/lang/en.yml
index a5b066f6d..0bad86de6 100644
--- a/lang/en.yml
+++ b/lang/en.yml
@@ -1,4 +1,12 @@
 en:
+  SilverStripe\Admin\AdminController:
+    ErrorMessage: 'Sorry, it seems there was a {errorcode} error.'
+    ErrorMessage400: 'Sorry, it seems there was something wrong with the request.'
+    ErrorMessage401: 'Sorry, it seems you are not authorised to access this section or object.'
+    ErrorMessage403: 'Sorry, it seems the action you were trying to perform is forbidden.'
+    ErrorMessage404: "Sorry, it seems you were trying to access a section or object that doesn't exist."
+    ErrorMessage500: 'Sorry, it seems there was an internal server error.'
+    ErrorMessage503: 'Sorry, it seems the service is temporarily unavailable.'
   SilverStripe\Admin\CMSProfileController:
     CANTEDIT: "You don't have permission to do that"
     MENUTITLE: 'My Profile'
diff --git a/tests/php/AdminControllerTest.php b/tests/php/AdminControllerTest.php
new file mode 100644
index 000000000..e69868489
--- /dev/null
+++ b/tests/php/AdminControllerTest.php
@@ -0,0 +1,236 @@
+<?php
+
+namespace SilverStripe\Admin\Tests;
+
+use SilverStripe\Dev\FunctionalTest;
+use SilverStripe\Security\Member;
+use SilverStripe\Control\HTTPResponse_Exception;
+use ReflectionObject;
+use InvalidArgumentException;
+use PHPUnit\Framework\Attributes\DataProvider;
+use SilverStripe\Admin\CMSProfileController;
+use SilverStripe\Admin\SecurityAdmin;
+use SilverStripe\Admin\Tests\LeftAndMainTest\DualPermissionAdminController;
+use SilverStripe\Admin\Tests\LeftAndMainTest\TestAdminController;
+
+class AdminControllerTest extends FunctionalTest
+{
+    protected static $fixture_file = 'AdminControllerTest.yml';
+
+    public static function provideCanView(): array
+    {
+        return [
+            'logged out' => [
+                'fixtureName' => null,
+                'allowed' => [],
+            ],
+            'no-access user' => [
+                'fixtureName' => 'noaccessuser',
+                'allowed' => [],
+            ],
+            'restricted CMS user' => [
+                'fixtureName' => 'securityonlyuser',
+                'allowed' => [
+                    CMSProfileController::class,
+                    SecurityAdmin::class,
+                ],
+            ],
+            'all cms sections user' => [
+                'fixtureName' => 'allcmssectionsuser',
+                'allowed' => [
+                    TestAdminController::class,
+                    DualPermissionAdminController::class,
+                    CMSProfileController::class,
+                    SecurityAdmin::class,
+                ],
+            ],
+            'admin user' => [
+                'fixtureName' => 'admin',
+                'allowed' => [
+                    TestAdminController::class,
+                    DualPermissionAdminController::class,
+                    CMSProfileController::class,
+                    SecurityAdmin::class,
+                ],
+            ],
+            'permission1 user (missing permission2)' => [
+                'fixtureName' => 'permission1user',
+                'allowed' => [],
+            ],
+            'permission2 user (missing permission1)' => [
+                'fixtureName' => 'permission2user',
+                'allowed' => [],
+            ],
+            'permission1 and 2 user user' => [
+                'fixtureName' => 'permission1and2user',
+                'allowed' => [
+                    DualPermissionAdminController::class,
+                ],
+            ],
+        ];
+    }
+
+    #[DataProvider('provideCanView')]
+    public function testCanView(?string $fixtureName, array $allowed): void
+    {
+        $allControllers = [
+            CMSProfileController::class,
+            SecurityAdmin::class,
+            TestAdminController::class,
+            DualPermissionAdminController::class,
+        ];
+        $this->logOut();
+        $user = null;
+        if ($fixtureName !== null) {
+            $user = $this->objFromFixture(Member::class, $fixtureName);
+            $this->logInAs($user);
+        }
+
+        foreach ($allowed as $allowedClass) {
+            $this->assertTrue($allowedClass::singleton()->canView(), 'Tried to access ' . $allowedClass);
+        }
+        $disallowed = array_diff($allControllers, $allowed);
+        foreach ($disallowed as $disallowedClass) {
+            $this->assertFalse($disallowedClass::singleton()->canView(), 'Tried to access ' . $disallowedClass);
+        }
+    }
+
+    public static function provideJsonSuccess(): array
+    {
+        return [
+            [
+                'statusCode' => 201,
+                'data' => null,
+                'expectedBody' => '',
+                'expectedException' => '',
+            ],
+            [
+                'statusCode' => 200,
+                'data' => [],
+                'expectedBody' => '[]',
+                'expectedException' => '',
+            ],
+            [
+                'statusCode' => 200,
+                'data' => [1, "two", 3.3],
+                'expectedBody' => '[1,"two",3.3]',
+                'expectedException' => '',
+            ],
+            [
+                'statusCode' => 200,
+                'data' => ['foo' => 'bar', 'quotes' => '"something"', 'array' => [1, 2, 3]],
+                'expectedBody' => '{"foo":"bar","quotes":"\"something\"","array":[1,2,3]}',
+                'expectedException' => '',
+            ],
+            [
+                'statusCode' => 200,
+                'data' => ['unicode' => ['one' => 'ōōō', 'two' => '℅℅℅', 'three' => '👍👍👍']],
+                'expectedBody' => '{"unicode":{"one":"ōōō","two":"℅℅℅","three":"👍👍👍"}}',
+                'expectedException' => '',
+            ],
+            [
+                'statusCode' => 199,
+                'data' => [],
+                'expectedBody' => '',
+                'expectedException' => InvalidArgumentException::class,
+            ],
+            [
+                'statusCode' => 302,
+                'data' => [],
+                'expectedBody' => '',
+                'expectedException' => InvalidArgumentException::class,
+            ],
+        ];
+    }
+
+    #[DataProvider('provideJsonSuccess')]
+    public function testJsonSuccess(
+        int $statusCode,
+        ?array $data,
+        string $expectedBody,
+        string $expectedException
+    ): void {
+        $controller = new TestAdminController();
+        $refelectionObject = new ReflectionObject($controller);
+        $method = $refelectionObject->getMethod('jsonSuccess');
+        $method->setAccessible(true);
+        if ($expectedException) {
+            $this->expectException($expectedException);
+        }
+        $response = $method->invoke($controller, $statusCode, $data);
+        $this->assertSame('application/json', $response->getHeader('Content-type'));
+        $this->assertSame($statusCode, $response->getStatusCode());
+        $this->assertSame($expectedBody, $response->getBody());
+    }
+
+    public static function provideJsonError(): array
+    {
+        return [
+            [
+                'statusCode' => 400,
+                'errorMessage' => '',
+                'expectedValue' => 'Sorry, it seems there was something wrong with the request.',
+            ],
+            [
+                'statusCode' => 401,
+                'errorMessage' => '',
+                'expectedValue' => 'Sorry, it seems you are not authorised to access this section or object.',
+            ],
+            [
+                'statusCode' => 403,
+                'errorMessage' => '',
+                'expectedValue' => 'Sorry, it seems the action you were trying to perform is forbidden.',
+            ],
+            [
+                'statusCode' => 404,
+                'errorMessage' => '',
+                'expectedValue' => 'Sorry, it seems you were trying to access a section or object that doesn\'t exist.',
+            ],
+            [
+                'statusCode' => 500,
+                'errorMessage' => '',
+                'expectedValue' => 'Sorry, it seems there was an internal server error.',
+            ],
+            [
+                'statusCode' => 503,
+                'errorMessage' => '',
+                'expectedValue' => 'Sorry, it seems the service is temporarily unavailable.',
+            ],
+            [
+                'statusCode' => 418,
+                'errorMessage' => '',
+                'expectedValue' => 'Error',
+            ],
+            [
+                'statusCode' => 400,
+                'errorMessage' => 'Test custom error message',
+                'expectedValue' => 'Test custom error message',
+            ],
+        ];
+    }
+
+    #[DataProvider('provideJsonError')]
+    public function testJsonError(
+        int $statusCode,
+        string $errorMessage,
+        ?string $expectedValue,
+    ): void {
+        $controller = new TestAdminController();
+        $refelectionObject = new ReflectionObject($controller);
+        $method = $refelectionObject->getMethod('jsonError');
+        $method->setAccessible(true);
+        $this->expectException(HTTPResponse_Exception::class);
+        $expectedMessage = json_encode((object) [
+            'status' => 'error',
+            'errors' => [
+                (object) [
+                    'type' => 'error',
+                    'code' => $statusCode,
+                    'value' => $expectedValue,
+                ],
+            ],
+        ]);
+        $this->expectExceptionMessage($expectedMessage);
+        $method->invoke($controller, $statusCode, $errorMessage);
+    }
+}
diff --git a/tests/php/LeftAndMainTest.yml b/tests/php/AdminControllerTest.yml
similarity index 62%
rename from tests/php/LeftAndMainTest.yml
rename to tests/php/AdminControllerTest.yml
index 1b3bbfaa9..045bb34c0 100644
--- a/tests/php/LeftAndMainTest.yml
+++ b/tests/php/AdminControllerTest.yml
@@ -9,6 +9,10 @@ SilverStripe\Security\Group:
     Title: allcmssections
   rooteditusers:
     Title: rooteditusers
+  permission1:
+    Title: permission1
+  permission2:
+    Title: permission2
 SilverStripe\Security\Member:
   admin:
     Email: admin@example.com
@@ -23,6 +27,17 @@ SilverStripe\Security\Member:
   rootedituser:
     Email: rootedituser@test.com
     Groups: =>SilverStripe\Security\Group.rooteditusers
+  noaccessuser:
+    Email: noaccessuser@test.com
+  permission1user:
+    Email: permission1user@test.com
+    Groups: =>SilverStripe\Security\Group.permission1
+  permission2user:
+    Email: permission2user@test.com
+    Groups: =>SilverStripe\Security\Group.permission2
+  permission1and2user:
+    Email: permission1and2user@test.com
+    Groups: =>SilverStripe\Security\Group.permission1,=>SilverStripe\Security\Group.permission2
 SilverStripe\Security\Permission:
   admin:
     Code: ADMIN
@@ -36,3 +51,9 @@ SilverStripe\Security\Permission:
   allcmssections2:
     Code: CMS_ACCESS_LeftAndMain
     GroupID: =>SilverStripe\Security\Group.rooteditusers
+  permission1:
+    Code: PERMISSION1
+    GroupID: =>SilverStripe\Security\Group.permission1
+  permission2:
+    Code: PERMISSION2
+    GroupID: =>SilverStripe\Security\Group.permission2
diff --git a/tests/php/AdminControllerTest/DualPermissionAdminController.php b/tests/php/AdminControllerTest/DualPermissionAdminController.php
new file mode 100644
index 000000000..34fece675
--- /dev/null
+++ b/tests/php/AdminControllerTest/DualPermissionAdminController.php
@@ -0,0 +1,14 @@
+<?php
+
+namespace SilverStripe\Admin\Tests\LeftAndMainTest;
+
+use SilverStripe\Admin\AdminController;
+use SilverStripe\Dev\TestOnly;
+
+class DualPermissionAdminController extends AdminController implements TestOnly
+{
+    private static string|array $required_permission_codes = [
+        'PERMISSION1',
+        'PERMISSION2',
+    ];
+}
diff --git a/tests/php/AdminControllerTest/TestAdminController.php b/tests/php/AdminControllerTest/TestAdminController.php
new file mode 100644
index 000000000..17265081f
--- /dev/null
+++ b/tests/php/AdminControllerTest/TestAdminController.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace SilverStripe\Admin\Tests\LeftAndMainTest;
+
+use SilverStripe\Admin\AdminController;
+use SilverStripe\Dev\TestOnly;
+
+class TestAdminController extends AdminController implements TestOnly
+{
+    // no-op - we just want a non-abstract controller.
+}
diff --git a/tests/php/LeftAndMainTest.php b/tests/php/LeftAndMainTest.php
index ec76a65ec..29b1c335a 100644
--- a/tests/php/LeftAndMainTest.php
+++ b/tests/php/LeftAndMainTest.php
@@ -11,21 +11,15 @@
 use SilverStripe\Core\Config\Config;
 use SilverStripe\Core\Manifest\ModuleLoader;
 use SilverStripe\Dev\FunctionalTest;
-use SilverStripe\Security\Member;
 use SilverStripe\View\Requirements;
 use SilverStripe\Admin\Tests\LeftAndMainTest\MyTree;
 use SilverStripe\Admin\Tests\LeftAndMainTest\MyTreeController;
-use SilverStripe\Control\HTTPResponse_Exception;
 use stdClass;
-use ReflectionObject;
-use InvalidArgumentException;
 use PHPUnit\Framework\Attributes\DataProvider;
 use SilverStripe\Core\Manifest\VersionProvider;
 
 class LeftAndMainTest extends FunctionalTest
 {
-    protected static $fixture_file = 'LeftAndMainTest.yml';
-
     protected static $extra_dataobjects = [
         MyTree::class
     ];
@@ -80,8 +74,7 @@ protected function tearDown(): void
 
     public function testExtraCssAndJavascript()
     {
-        $admin = $this->objFromFixture(Member::class, 'admin');
-        $this->logInAs($admin);
+        $this->logInWithPermission('ADMIN');
         $response = $this->get('admin/security');
 
         // Check css
@@ -135,65 +128,6 @@ public function testLeftAndMainSubclasses()
         $this->assertMatchesRegularExpression('/<body[^>]*>/i', $response->getBody(), "$link should contain <body> tag");
     }
 
-    public function testCanView()
-    {
-        $adminuser = $this->objFromFixture(Member::class, 'admin');
-        $securityonlyuser = $this->objFromFixture(Member::class, 'securityonlyuser');
-        $allcmssectionsuser = $this->objFromFixture(Member::class, 'allcmssectionsuser');
-
-        // anonymous user
-        $this->logOut();
-        $this->resetMenu();
-        $menuItems = LeftAndMain::singleton()->MainMenu(false);
-        $this->assertEquals(
-            $menuItems->column('Code'),
-            [],
-            'Without valid login, members cant access any menu entries'
-        );
-
-        // restricted cms user
-        $this->logInAs($securityonlyuser);
-        $this->resetMenu();
-        $menuItems = LeftAndMain::singleton()->MainMenu(false);
-        $menuItems = $menuItems->column('Code');
-        sort($menuItems);
-
-        $this->assertEquals(
-            [
-                'SilverStripe-Admin-CMSProfileController',
-                'SilverStripe-Admin-SecurityAdmin'
-            ],
-            $menuItems,
-            'Groups with limited access can only access the interfaces they have permissions for'
-        );
-
-        // all cms sections user
-        $this->logInAs($allcmssectionsuser);
-        $this->resetMenu();
-        $menuItems = LeftAndMain::singleton()->MainMenu(false);
-        $this->assertContains(
-            'SilverStripe-Admin-CMSProfileController',
-            $menuItems->column('Code'),
-            'Group with CMS_ACCESS_SilverStripe\\Admin\\LeftAndMain permission can edit own profile'
-        );
-        $this->assertContains(
-            'SilverStripe-Admin-SecurityAdmin',
-            $menuItems->column('Code'),
-            'Group with CMS_ACCESS_SilverStripe\\Admin\\LeftAndMain permission can access all sections'
-        );
-
-        // admin
-        $this->logInAs($adminuser);
-        $this->resetMenu();
-        $menuItems = LeftAndMain::singleton()->MainMenu(false);
-        $this->assertContains(
-            'SilverStripe-Admin-SecurityAdmin',
-            $menuItems->column('Code'),
-            'Administrators can access Security Admin'
-        );
-
-        $this->logOut();
-    }
 
     /**
      * Test that getHelpLinks transforms $help_links into the correct format
@@ -255,8 +189,7 @@ public static function provideTestCMSVersionNumber()
 
     public function testValidationResult()
     {
-        $this->logInAs('admin');
-
+        $this->logInWithPermission('ADMIN');
         $obj = MyTree::create();
         $obj->write();
 
@@ -289,143 +222,4 @@ public function testValidationResult()
         $this->assertSame($result->messages[0]->fieldName, 'Content');
         $this->assertSame($result->messages[0]->message, MyTree::INVALID_CONTENT_MESSAGE);
     }
-
-    public static function provideJsonSuccess(): array
-    {
-        return [
-            [
-                'statusCode' => 201,
-                'data' => null,
-                'expectedBody' => '',
-                'expectedException' => '',
-            ],
-            [
-                'statusCode' => 200,
-                'data' => [],
-                'expectedBody' => '[]',
-                'expectedException' => '',
-            ],
-            [
-                'statusCode' => 200,
-                'data' => [1, "two", 3.3],
-                'expectedBody' => '[1,"two",3.3]',
-                'expectedException' => '',
-            ],
-            [
-                'statusCode' => 200,
-                'data' => ['foo' => 'bar', 'quotes' => '"something"', 'array' => [1, 2, 3]],
-                'expectedBody' => '{"foo":"bar","quotes":"\"something\"","array":[1,2,3]}',
-                'expectedException' => '',
-            ],
-            [
-                'statusCode' => 200,
-                'data' => ['unicode' => ['one' => 'ōōō', 'two' => '℅℅℅', 'three' => '👍👍👍']],
-                'expectedBody' => '{"unicode":{"one":"ōōō","two":"℅℅℅","three":"👍👍👍"}}',
-                'expectedException' => '',
-            ],
-            [
-                'statusCode' => 199,
-                'data' => [],
-                'expectedBody' => '',
-                'expectedException' => InvalidArgumentException::class,
-            ],
-            [
-                'statusCode' => 302,
-                'data' => [],
-                'expectedBody' => '',
-                'expectedException' => InvalidArgumentException::class,
-            ],
-        ];
-    }
-
-    #[DataProvider('provideJsonSuccess')]
-    public function testJsonSuccess(
-        int $statusCode,
-        ?array $data,
-        string $expectedBody,
-        string $expectedException
-    ): void {
-        $leftAndMain = new LeftAndMain();
-        $refelectionObject = new ReflectionObject($leftAndMain);
-        $method = $refelectionObject->getMethod('jsonSuccess');
-        $method->setAccessible(true);
-        if ($expectedException) {
-            $this->expectException($expectedException);
-        }
-        $response = $method->invoke($leftAndMain, $statusCode, $data);
-        $this->assertSame('application/json', $response->getHeader('Content-type'));
-        $this->assertSame($statusCode, $response->getStatusCode());
-        $this->assertSame($expectedBody, $response->getBody());
-    }
-
-    public static function provideJsonError(): array
-    {
-        return [
-            [
-                'statusCode' => 400,
-                'errorMessage' => null,
-                'expectedValue' => 'Sorry, it seems there was something wrong with the request.',
-            ],
-            [
-                'statusCode' => 401,
-                'errorMessage' => null,
-                'expectedValue' => 'Sorry, it seems you are not authorised to access this section or object.',
-            ],
-            [
-                'statusCode' => 403,
-                'errorMessage' => null,
-                'expectedValue' => 'Sorry, it seems the action you were trying to perform is forbidden.',
-            ],
-            [
-                'statusCode' => 404,
-                'errorMessage' => null,
-                'expectedValue' => 'Sorry, it seems you were trying to access a section or object that doesn\'t exist.',
-            ],
-            [
-                'statusCode' => 500,
-                'errorMessage' => null,
-                'expectedValue' => 'Sorry, it seems there was an internal server error.',
-            ],
-            [
-                'statusCode' => 503,
-                'errorMessage' => null,
-                'expectedValue' => 'Sorry, it seems the service is temporarily unavailable.',
-            ],
-            [
-                'statusCode' => 418,
-                'errorMessage' => null,
-                'expectedValue' => 'Error',
-            ],
-            [
-                'statusCode' => 400,
-                'errorMessage' => 'Test custom error message',
-                'expectedValue' => 'Test custom error message',
-            ],
-        ];
-    }
-
-    #[DataProvider('provideJsonError')]
-    public function testJsonError(
-        int $statusCode,
-        ?string $errorMessage,
-        ?string $expectedValue,
-    ): void {
-        $leftAndMain = new LeftAndMain();
-        $refelectionObject = new ReflectionObject($leftAndMain);
-        $method = $refelectionObject->getMethod('jsonError');
-        $method->setAccessible(true);
-        $this->expectException(HTTPResponse_Exception::class);
-        $expectedMessage = json_encode((object) [
-            'status' => 'error',
-            'errors' => [
-                (object) [
-                    'type' => 'error',
-                    'code' => $statusCode,
-                    'value' => $expectedValue,
-                ],
-            ],
-        ]);
-        $this->expectExceptionMessage($expectedMessage);
-        $method->invoke($leftAndMain, $statusCode, $errorMessage);
-    }
 }
diff --git a/tests/php/SudoModeControllerTest.php b/tests/php/SudoModeControllerTest.php
index bd35f8162..320c505d6 100644
--- a/tests/php/SudoModeControllerTest.php
+++ b/tests/php/SudoModeControllerTest.php
@@ -88,7 +88,7 @@ public function testActivateFailsWithIncorrectPassword()
             'Password' => 'wrongpassword!',
         ]);
 
-        $this->assertSame(200, $response->getStatusCode());
+        $this->assertSame(400, $response->getStatusCode());
         $result = json_decode((string) $response->getBody(), true);
         $this->assertFalse($result['result'], 'Should have failed with incorrect password');
         $this->assertEquals('Incorrect password', $result['message']);