diff --git a/core/modules/system/src/Tests/Update/PathAliasUpdateTest.php b/core/modules/system/src/Tests/Update/PathAliasUpdateTest.php
new file mode 100644
index 0000000..87156bc
--- /dev/null
+++ b/core/modules/system/src/Tests/Update/PathAliasUpdateTest.php
@@ -0,0 +1,56 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Update\PathAliasUpdateTest.
+ */
+
+namespace Drupal\system\Tests\Update;
+use Drupal\Component\Utility\Unicode;
+use Drupal\Core\Language\LanguageInterface;
+
+/**
+ * Tests system_update_8012().
+ *
+ * @group Update
+ */
+class PathAliasUpdateTest extends UpdatePathTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setDatabaseDumpFiles() {
+    $this->databaseDumpFiles = [
+      __DIR__ . '/../../../tests/fixtures/update/drupal-8.bare.standard.php.gz',
+    ];
+  }
+
+  /**
+   * Ensures that the system_update_8012() runs as expected.
+   */
+  public function testUpdate() {
+    $connection = \Drupal::database();
+    $schema = $connection->schema();
+
+    if (!$schema->tableExists('url_alias')) {
+      return;
+    }
+    $fields = array(
+      'source' => '/user',
+      'alias' => '/User/PAge',
+      'langcode' => LanguageInterface::LANGCODE_NOT_SPECIFIED,
+    );
+    $query = $connection->insert('url_alias')
+      ->fields($fields);
+    $pid = $query->execute();
+    $this->runUpdates();
+
+    // Check that the mixed case path alias was converted to all lower case.
+    /** @var \Drupal\Core\Path\AliasStorageInterface $alias_storage */
+    $alias_storage = \Drupal::service('path.alias_storage');
+
+    $alias = $alias_storage->load(['pid' => $pid]);
+    $this->assertIdentical(Unicode::strtolower($fields['alias']), $alias['alias']);
+  }
+
+}
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 47f6dca..da34040 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -1808,5 +1808,37 @@ function system_update_8011() {
 }
 
 /**
+ * Load and save all path aliases to make sure they are lower case.
+ */
+function system_update_8012(&$sandbox = NULL) {
+  // The path alias storage interface has no way to count or iterate over all
+  // aliases, so we have to fall back to SQL for this function.
+  /** @var \Drupal\Core\Path\AliasStorageInterface $alias_storage */
+  $alias_storage = \Drupal::service('path.alias_storage');
+  $connection = \Drupal::database();
+  $schema = $connection->schema();
+  if ($schema->tableExists('url_alias')) {
+    if (!isset($sandbox['current'])) {
+      $sandbox['current'] = 0;
+      $sandbox['max'] = $connection->query('SELECT COUNT(pid) FROM {url_alias}')
+        ->fetchField();
+    }
+    $aliases = $connection->queryRange('SELECT pid, source, alias, langcode FROM {url_alias} ORDER BY pid ASC', $sandbox['current'], $sandbox['current'] + 50)
+      ->fetchAllAssoc('pid', PDO::FETCH_ASSOC);
+
+    foreach ($aliases as $alias) {
+      $alias_storage->save($alias['source'], $alias['alias'], $alias['langcode'], $alias['pid']);
+      $sandbox['current']++;
+    }
+
+    $sandbox['#finished'] = empty($sandbox['max']) ? 1 : ($sandbox['current'] / $sandbox['max']);
+    return t('Path aliases converted');
+  }
+  else {
+    return t('Path alias conversion skipped, because the {url_alias} table did not exist.');
+  }
+}
+
+/**
  * @} End of "addtogroup updates-8.0.0-beta".
  */