diff --git a/core/lib/Drupal/Core/EventSubscriber/PathSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/PathSubscriber.php
index a73cf31..d35457d 100644
--- a/core/lib/Drupal/Core/EventSubscriber/PathSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/PathSubscriber.php
@@ -58,7 +58,7 @@ public function __construct(AliasManagerInterface $alias_manager, CurrentPathSta
   public function onKernelController(FilterControllerEvent $event) {
     // Set the cache key on the alias manager cache decorator.
     if ($event->getRequestType() == HttpKernelInterface::MASTER_REQUEST) {
-      $this->aliasManager->setCacheKey(trim($this->currentPath->getPath($event->getRequest()), '/'));
+      $this->aliasManager->setCacheKey(rtrim($this->currentPath->getPath($event->getRequest()), '/'));
     }
   }
 
diff --git a/core/modules/path/src/Tests/PathAliasTest.php b/core/modules/path/src/Tests/PathAliasTest.php
index 27c38c3..70d4bd0 100644
--- a/core/modules/path/src/Tests/PathAliasTest.php
+++ b/core/modules/path/src/Tests/PathAliasTest.php
@@ -52,12 +52,12 @@ function testPathCache() {
     // created.
     \Drupal::cache('data')->deleteAll();
     // Make sure the path is not converted to the alias.
-    $this->drupalGet($edit['source'], array('alias' => TRUE));
+    $this->drupalGet(trim($edit['source'], '/'), array('alias' => TRUE));
     $this->assertTrue(\Drupal::cache('data')->get('preload-paths:' . $edit['source']), 'Cache entry was created.');
 
     // Visit the alias for the node and confirm a cache entry is created.
     \Drupal::cache('data')->deleteAll();
-    $this->drupalGet($edit['alias']);
+    $this->drupalGet(trim($edit['alias'], '/'));
     $this->assertTrue(\Drupal::cache('data')->get('preload-paths:' .  $edit['source']), 'Cache entry was created.');
   }
 
diff --git a/core/modules/system/src/Tests/Routing/RouteProviderTest.php b/core/modules/system/src/Tests/Routing/RouteProviderTest.php
index 06a40fb..44e5102 100644
--- a/core/modules/system/src/Tests/Routing/RouteProviderTest.php
+++ b/core/modules/system/src/Tests/Routing/RouteProviderTest.php
@@ -430,7 +430,7 @@ public function testRouteCaching() {
     // A path with a path alias.
     /** @var \Drupal\Core\Path\AliasStorageInterface $path_storage */
     $path_storage = \Drupal::service('path.alias_storage');
-    $path_storage->save('path/add/one', 'path/add-one');
+    $path_storage->save('/path/add/one', '/path/add-one');
     /** @var \Drupal\Core\Path\AliasManagerInterface $alias_manager */
     $alias_manager = \Drupal::service('path.alias_manager');
     $alias_manager->cacheClear();
diff --git a/core/modules/system/src/Tests/Update/UpdatePathSystemSiteConfigSlashTest.php b/core/modules/system/src/Tests/Update/UpdatePathSystemSiteConfigSlashTest.php
index 9c4569f..96b4fa7 100644
--- a/core/modules/system/src/Tests/Update/UpdatePathSystemSiteConfigSlashTest.php
+++ b/core/modules/system/src/Tests/Update/UpdatePathSystemSiteConfigSlashTest.php
@@ -7,6 +7,6 @@
 
 namespace Drupal\system\Tests\Update;
 
-class UpdatePathSystemSiteConfigSlashTest {
+class UpdatePathSystemSiteConfigSlashTest extends UpdatePathTestBase {
 
 }
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index a0849b1..b33cf96 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -1085,3 +1085,45 @@ function system_schema() {
 
   return $schema;
 }
+
+/**
+ * Ensures that system.site:site_front, site_403 and site_404 start with a slash.
+ */
+function system_update_8000() {
+  /** @var \Drupal\Core\Config\ConfigFactoryInterface $config_factory */
+  $config_factory = \Drupal::service('config.factory');
+  $config = $config_factory->get('system.site');
+
+  if (($page_front = $config->get('page.front')) && $page_front[0] !== '/') {
+    $page_front = '/' . $page_front;
+    $config->set('page.front', $page_front);
+  }
+
+  if (($page_403 = $config->get('page.403')) && $page_403[0] !== '/') {
+    $page_403 = '/' . $page_403;
+    $config->set('page.403', $page_403);
+  }
+
+  if (($page_404 = $config->get('page.404')) && $page_404[0] !== '/') {
+    $page_404 = '/' . $page_404;
+    $config->set('page.404', $page_404);
+  }
+}
+
+/**
+ * Updates {url_alias} to append '/'.
+ */
+function system_update_8001() {
+  $query = \Drupal::database()->update('url_alias');
+
+  if (\Drupal::database()->databaseType() !== 'sqlite') {
+    $query->expression('source', "concat('/', source)");
+    $query->expression('alias', "concat('/', alias)");
+  }
+  else {
+    $query->expression('source', "source || '/'");
+    $query->expression('alias', "alias || '/'");
+  }
+
+  $query->execute();
+}