core/lib/Drupal/Core/Access/AccessResult.php          | 17 +++++++++++++++--
 core/lib/Drupal/Core/Access/AccessResultForbidden.php | 18 ++++++++++++++++++
 core/lib/Drupal/Core/Routing/AccessAwareRouter.php    |  3 ++-
 core/modules/rest/src/Access/CSRFAccessCheck.php      |  2 +-
 4 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/core/lib/Drupal/Core/Access/AccessResult.php b/core/lib/Drupal/Core/Access/AccessResult.php
index 8148d4c..3ece571 100644
--- a/core/lib/Drupal/Core/Access/AccessResult.php
+++ b/core/lib/Drupal/Core/Access/AccessResult.php
@@ -51,11 +51,16 @@ public static function allowed() {
   /**
    * Creates an AccessResultInterface object with isForbidden() === TRUE.
    *
+   * @param string|null $reason
+   *   (optional) The reason why access is forbidden. Intended for developers,
+   *   hence not translatable.
+   *
    * @return \Drupal\Core\Access\AccessResult
    *   isForbidden() will be TRUE.
    */
-  public static function forbidden() {
-    return new AccessResultForbidden();
+  public static function forbidden($reason = NULL) {
+    assert('is_string($reason) || is_null($reason)');
+    return new AccessResultForbidden($reason);
   }
 
   /**
@@ -334,8 +339,16 @@ public function andIf(AccessResultInterface $other) {
     if ($this->isForbidden() || $other->isForbidden()) {
       $result = static::forbidden();
       if (!$this->isForbidden()) {
+        if ($other instanceof AccessResultForbidden) {
+          $result->setReason($other->getReason());
+        }
         $merge_other = TRUE;
       }
+      else {
+        if ($this instanceof AccessResultForbidden) {
+          $result->setReason($this->getReason());
+        }
+      }
     }
     elseif ($this->isAllowed() && $other->isAllowed()) {
       $result = static::allowed();
diff --git a/core/lib/Drupal/Core/Access/AccessResultForbidden.php b/core/lib/Drupal/Core/Access/AccessResultForbidden.php
index 4dc0120..85b5639 100644
--- a/core/lib/Drupal/Core/Access/AccessResultForbidden.php
+++ b/core/lib/Drupal/Core/Access/AccessResultForbidden.php
@@ -8,10 +8,28 @@
 class AccessResultForbidden extends AccessResult {
 
   /**
+   * The reason why access is forbidden. For use in error messages.
+   *
+   * @var string|null
+   */
+  protected $reason;
+
+  /**
    * {@inheritdoc}
    */
   public function isForbidden() {
     return TRUE;
   }
 
+  // @todo Create interface.
+  public function getReason() {
+    return $this->reason;
+  }
+
+  // @todo Create interface.
+  public function setReason($reason) {
+    $this->reason = $reason;
+    return $this;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Routing/AccessAwareRouter.php b/core/lib/Drupal/Core/Routing/AccessAwareRouter.php
index 80e16eb..1f11987 100644
--- a/core/lib/Drupal/Core/Routing/AccessAwareRouter.php
+++ b/core/lib/Drupal/Core/Routing/AccessAwareRouter.php
@@ -3,6 +3,7 @@
 namespace Drupal\Core\Routing;
 
 use Drupal\Core\Access\AccessManagerInterface;
+use Drupal\Core\Access\AccessResultForbidden;
 use Drupal\Core\Session\AccountInterface;
 use Symfony\Cmf\Component\Routing\ChainRouter;
 use Symfony\Component\HttpFoundation\Request;
@@ -105,7 +106,7 @@ protected function checkAccess(Request $request) {
       $request->attributes->set(AccessAwareRouterInterface::ACCESS_RESULT, $access_result);
     }
     if (!$access_result->isAllowed()) {
-      throw new AccessDeniedHttpException();
+      throw new AccessDeniedHttpException($access_result instanceof AccessResultForbidden ? $access_result->getReason() : NULL);
     }
   }
 
diff --git a/core/modules/rest/src/Access/CSRFAccessCheck.php b/core/modules/rest/src/Access/CSRFAccessCheck.php
index c6dc04f..d2f25d6 100644
--- a/core/modules/rest/src/Access/CSRFAccessCheck.php
+++ b/core/modules/rest/src/Access/CSRFAccessCheck.php
@@ -78,7 +78,7 @@ public function access(Request $request, AccountInterface $account) {
     ) {
       $csrf_token = $request->headers->get('X-CSRF-Token');
       if (!\Drupal::csrfToken()->validate($csrf_token, 'rest')) {
-        return AccessResult::forbidden()->setCacheMaxAge(0);
+        return AccessResult::forbidden()->setReason('X-CSRF-Token request header is missing')->setCacheMaxAge(0);
       }
     }
     // Let other access checkers decide if the request is legit.