diff --git a/core/modules/user/lib/Drupal/user/UserAuth.php b/core/modules/user/lib/Drupal/user/UserAuth.php
index a94a47e..01ecf46 100644
--- a/core/modules/user/lib/Drupal/user/UserAuth.php
+++ b/core/modules/user/lib/Drupal/user/UserAuth.php
@@ -48,7 +48,9 @@ public function authenticate($username, $password) {
     $uid = FALSE;
 
     if (!empty($username) && !empty($password)) {
-      if ($account = $this->storage->loadByProperties(array('name' => $username))) {
+      $account_search = $this->storage->loadByProperties(array('name' => $username));
+
+      if ($account = reset($account_search)) {
         if ($this->password->check($password, $account)) {
           // Successful authentication.
           $uid = $account->id();
diff --git a/core/modules/user/tests/Drupal/user/Tests/UserAuthTest.php b/core/modules/user/tests/Drupal/user/Tests/UserAuthTest.php
index d48b4cd..c658e60 100644
--- a/core/modules/user/tests/Drupal/user/Tests/UserAuthTest.php
+++ b/core/modules/user/tests/Drupal/user/Tests/UserAuthTest.php
@@ -7,6 +7,209 @@
 
 namespace Drupal\user\Tests;
 
-class UserAuthTest {
+use Drupal\Tests\UnitTestCase;
+use Drupal\user\UserAuth;
+
+/**
+ * Tests the UserAuth class.
+ *
+ * @group Drupal
+ * @group User
+ *
+ * @coverDefaultClass \Drupal\user\UserAuth
+ */
+class UserAuthTest extends UnitTestCase {
+
+  /**
+   * The mock user storage controller.
+   *
+   * @var \Drupal\Core\Entity\EntityStorageControllerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $userStorage;
+
+  /**
+   * The mocked password service.
+   *
+   * @var \Drupal\Core\Password\PasswordInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $passwordService;
+
+  /**
+   * The mock user.
+   *
+   * @var \Drupal\user\Entity\User|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $testUser;
+
+  /**
+   * The user auth object under test.
+   *
+   * @var \Drupal\user\UserAuth
+   */
+  protected $userAuth;
+
+  /**
+   * The test username.
+   *
+   * @var string
+   */
+  protected $username = 'test_user';
+
+  /**
+   * The test password
+   *
+   * @var string
+   */
+  protected $password = 'password';
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'name' => 'User auth service',
+      'description' => 'Tests the user auth service',
+      'group' => 'User',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    $this->userStorage = $this->getMock('Drupal\Core\Entity\EntityStorageControllerInterface');
+
+    $entity_manager = $this->getMock('Drupal\Core\Entity\EntityManagerInterface');
+    // Getting the user storage controller should only happen once per test.
+    $entity_manager->expects($this->once())
+      ->method('getStorageController')
+      ->with('user')
+      ->will($this->returnValue($this->userStorage));
+
+    $this->passwordService = $this->getMock('Drupal\Core\Password\PasswordInterface');
+
+    $this->testUser = $this->getMockBuilder('Drupal\user\Entity\User')
+      ->disableOriginalConstructor()
+      ->setMethods(array('id', 'setPassword', 'save'))
+      ->getMock();
+
+    $this->userAuth = new UserAuth($entity_manager, $this->passwordService);
+  }
+
+  /**
+   * Tests failing authentication with missing credential parameters.
+   *
+   * @covers ::authenticate()
+   *
+   * @dataProvider providerTestAuthenticateWithMissingCredentials
+   */
+  public function testAuthenticateWithMissingCredentials($username, $password) {
+    $this->userStorage->expects($this->never())
+      ->method('loadByProperties');
+
+    $this->assertFalse($this->userAuth->authenticate($username, $password));
+  }
+
+  /**
+   * Data provider for testAuthenticateWithMissingCredentials().
+   *
+   * @return array
+   */
+  public function providerTestAuthenticateWithMissingCredentials() {
+    return array(
+      array(NULL, NULL),
+      array(NULL, ''),
+      array('', NULL),
+      array('', ''),
+    );
+  }
+
+  /**
+   * Tests the authenticate method with no account returned.
+   *
+   * @covers ::authenticate()
+   */
+  public function testAuthenticateWithNoAccountReturned() {
+    $this->userStorage->expects($this->once())
+      ->method('loadByProperties')
+      ->with(array('name' => $this->username))
+      ->will($this->returnValue(array()));
+
+    $this->assertFalse($this->userAuth->authenticate($this->username, $this->password));
+  }
+
+  /**
+   * Tests the authenticate method with an incorrect password.
+   *
+   * @covers ::authenticate()
+   */
+  public function testAuthenticateWithIncorrectPassword() {
+    $this->userStorage->expects($this->once())
+      ->method('loadByProperties')
+      ->with(array('name' => $this->username))
+      ->will($this->returnValue(array($this->testUser)));
+
+    $this->passwordService->expects($this->once())
+      ->method('check')
+      ->with($this->password, $this->testUser)
+      ->will($this->returnValue(FALSE));
+
+    $this->assertFalse($this->userAuth->authenticate($this->username, $this->password));
+  }
+
+  /**
+   * Tests the authenticate method with a correct password.
+   *
+   * @covers ::authenticate()
+   */
+  public function testAuthenticateWithCorrectPassword() {
+    $this->testUser->expects($this->once())
+      ->method('id')
+      ->will($this->returnValue(1));
+
+    $this->userStorage->expects($this->once())
+      ->method('loadByProperties')
+      ->with(array('name' => $this->username))
+      ->will($this->returnValue(array($this->testUser)));
+
+    $this->passwordService->expects($this->once())
+      ->method('check')
+      ->with($this->password, $this->testUser)
+      ->will($this->returnValue(TRUE));
+
+    $this->assertsame(1, $this->userAuth->authenticate($this->username, $this->password));
+  }
+
+  /**
+   * Tests the authenticate method with a correct password and new password hash.
+   *
+   * @covers ::authenticate()
+   */
+  public function testAuthenticateWithCorrectPasswordAndNewPasswordHash() {
+    $this->testUser->expects($this->once())
+      ->method('id')
+      ->will($this->returnValue(1));
+    $this->testUser->expects($this->once())
+      ->method('setPassword')
+      ->with($this->password);
+    $this->testUser->expects($this->once())
+      ->method('save');
+
+    $this->userStorage->expects($this->once())
+      ->method('loadByProperties')
+      ->with(array('name' => $this->username))
+      ->will($this->returnValue(array($this->testUser)));
+
+    $this->passwordService->expects($this->once())
+      ->method('check')
+      ->with($this->password, $this->testUser)
+      ->will($this->returnValue(TRUE));
+    $this->passwordService->expects($this->once())
+      ->method('userNeedsNewHash')
+      ->with($this->testUser)
+      ->will($this->returnValue(TRUE));
+
+    $this->assertsame(1, $this->userAuth->authenticate($this->username, $this->password));
+  }
 
 }
diff --git a/core/modules/user/user.services.yml b/core/modules/user/user.services.yml
index efebc9c..36d1e79 100644
--- a/core/modules/user/user.services.yml
+++ b/core/modules/user/user.services.yml
@@ -35,4 +35,4 @@ services:
     arguments: ['@private_key', '@cache.cache']
   user.auth:
     class: Drupal\user\UserAuth
-    arguements: ['@entity.manager', '@password']
+    arguments: ['@entity.manager', '@password']