diff --git a/core/lib/Drupal/Core/Session/SessionConfiguration.php b/core/lib/Drupal/Core/Session/SessionConfiguration.php
index 70aae9c..ec98e0a 100644
--- a/core/lib/Drupal/Core/Session/SessionConfiguration.php
+++ b/core/lib/Drupal/Core/Session/SessionConfiguration.php
@@ -140,7 +140,7 @@ protected function getName(Request $request) {
    *   The session name without the prefix (SESS/SSESS).
    */
   protected function getUnprefixedName(Request $request) {
-    if ($test_prefix = drupal_valid_test_ua()) {
+    if ($test_prefix = $this->drupalValidTestUa()) {
       $session_name = $test_prefix;
     }
     elseif (isset($this->options['cookie_domain'])) {
@@ -179,7 +179,7 @@ protected function getCookieDomain(Request $request) {
       if (strpos($host, 'www.') === 0) {
         $host = substr($host, 4);
       }
-      $cookie_domain = $host;
+      $cookie_domain = '.' . $host;
     }
 
     // Per RFC 2109, cookie domains must contain at least one dot other than the
@@ -190,4 +190,15 @@ protected function getCookieDomain(Request $request) {
     }
   }
 
+  /**
+   * Wraps drupal_valid_test_ua().
+   *
+   * @return string|FALSE
+   *   Either the simpletest prefix (the string "simpletest" followed by any
+   *   number of digits) or FALSE if the user agent does not contain a valid
+   *   HMAC and timestamp.
+   */
+  protected function drupalValidTestUa() {
+    return drupal_valid_test_ua();
+  }
 }
diff --git a/core/tests/Drupal/Tests/Core/Session/SessionConfigurationTest.php b/core/tests/Drupal/Tests/Core/Session/SessionConfigurationTest.php
new file mode 100644
index 0000000..dbd9497
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Session/SessionConfigurationTest.php
@@ -0,0 +1,252 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Session\SessionConfigurationTest.
+ */
+
+namespace Drupal\Tests\Core\Session;
+
+use Drupal\Tests\UnitTestCase;
+use Drupal\Core\Session\SessionConfiguration;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Session\SessionConfiguration
+ * @group Session
+ */
+class SessionConfigurationTest extends UnitTestCase {
+
+  /**
+   * Constructs a partially mocked SUT.
+   *
+   * @returns \Drupal\Core\Session\SessionConfiguration|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected function createSessionConfiguration($options = []) {
+    return $this->getMock('Drupal\Core\Session\SessionConfiguration', ['drupalValidTestUa'], [$options]);
+  }
+
+  /**
+   * Tests whether the session.cookie_domain ini settings is computed correctly.
+   *
+   * @covers ::getOptions()
+   *
+   * @dataProvider providerTestGeneratedCookieDomain
+   */
+  public function testGeneratedCookieDomain($uri, $expected_domain) {
+    $config = $this->createSessionConfiguration();
+
+    $request = Request::create($uri);
+    $options = $config->getOptions($request);
+
+    $this->assertEquals($expected_domain, $options['cookie_domain']);
+  }
+
+  /**
+   * Data provider for the cookie domain test.
+   *
+   * @returns array
+   *   Test data
+   */
+  public function providerTestGeneratedCookieDomain() {
+    return [
+      ['http://example.com/path/index.php', '.example.com'],
+      ['http://www.example.com/path/index.php', '.example.com'],
+      ['http://subdomain.example.com/path/index.php', '.subdomain.example.com'],
+      ['http://example.com:8080/path/index.php', '.example.com'],
+      ['https://example.com/path/index.php', '.example.com'],
+      ['http://localhost/path/index.php', ''],
+      ['http://127.0.0.1/path/index.php', ''],
+      ['http://127.0.0.1:8888/path/index.php', ''],
+      ['http://1.1.1.1/path/index.php', ''],
+      ['http://[::1]/path/index.php', ''],
+      ['http://[::1]:8888/path/index.php', ''],
+    ];
+  }
+
+  /**
+   * Tests the constructor injected session.cookie_domain ini setting.
+   *
+   * @covers ::__construct()
+   * @covers ::getOptions()
+   *
+   * @dataProvider providerTestEnforcedCookieDomain
+   */
+  public function testEnforcedCookieDomain($uri, $expected_domain) {
+    $config = $this->createSessionConfiguration(['cookie_domain' => '.example.com']);
+
+    $request = Request::create($uri);
+    $options = $config->getOptions($request);
+
+    $this->assertEquals($expected_domain, $options['cookie_domain']);
+  }
+
+  /**
+   * Data provider for the cookie domain test.
+   *
+   * @returns array
+   *   Test data
+   */
+  public function providerTestEnforcedCookieDomain() {
+    return [
+      ['http://example.com/path/index.php', '.example.com'],
+      ['http://www.example.com/path/index.php', '.example.com'],
+      ['http://subdomain.example.com/path/index.php', '.example.com'],
+      ['http://example.com:8080/path/index.php', '.example.com'],
+      ['https://example.com/path/index.php', '.example.com'],
+      ['http://localhost/path/index.php', '.example.com'],
+      ['http://127.0.0.1/path/index.php', '.example.com'],
+      ['http://127.0.0.1:8888/path/index.php', '.example.com'],
+      ['http://1.1.1.1/path/index.php', '.example.com'],
+      ['http://[::1]/path/index.php', '.example.com'],
+      ['http://[::1]:8888/path/index.php', '.example.com'],
+    ];
+  }
+
+  /**
+   * Tests whether the session.cookie_secure ini settings is computed correctly.
+   *
+   * @covers ::getOptions()
+   *
+   * @dataProvider providerTestCookieSecure
+   */
+  public function testCookieSecure($uri, $expected_secure) {
+    $config = $this->createSessionConfiguration();
+
+    $request = Request::create($uri);
+    $options = $config->getOptions($request);
+
+    $this->assertEquals($expected_secure, $options['cookie_secure']);
+  }
+
+  /**
+   * Tests that session.cookie_secure ini settings cannot be overridden.
+   *
+   * @covers ::__construct()
+   * @covers ::getOptions()
+   *
+   * @dataProvider providerTestCookieSecure
+   */
+  public function testCookieSecureNotOverridable($uri, $expected_secure) {
+    $config = $this->createSessionConfiguration(['cookie_secure' => FALSE]);
+
+    $request = Request::create($uri);
+    $options = $config->getOptions($request);
+
+    $this->assertEquals($expected_secure, $options['cookie_secure']);
+  }
+
+  /**
+   * Data provider for the cookie secure test.
+   *
+   * @returns array
+   *   Test data
+   */
+  public function providerTestCookieSecure() {
+    return [
+      ['http://example.com/path/index.php', FALSE],
+      ['https://www.example.com/path/index.php', TRUE],
+      ['http://127.0.0.1/path/index.php', FALSE],
+      ['https://127.0.0.1:8888/path/index.php', TRUE],
+      ['http://[::1]/path/index.php', FALSE],
+      ['https://[::1]:8888/path/index.php', TRUE],
+    ];
+  }
+
+  /**
+   * Tests whether the session.name ini settings is computed correctly.
+   *
+   * @covers ::getOptions()
+   *
+   * @dataProvider providerTestGeneratedSessionName
+   */
+  public function testGeneratedSessionName($uri, $expected_name) {
+    $config = $this->createSessionConfiguration();
+
+    $request = Request::create($uri);
+    $options = $config->getOptions($request);
+
+    $this->assertEquals($expected_name, $options['name']);
+  }
+
+  /**
+   * Data provider for the cookie name test.
+   *
+   * @returns array
+   *   Test data
+   */
+  public function providerTestGeneratedSessionName() {
+    $data = [
+      ['http://example.com/path/index.php', 'SESS', 'example.com'],
+      ['http://www.example.com/path/index.php', 'SESS', 'www.example.com'],
+      ['http://subdomain.example.com/path/index.php', 'SESS', 'subdomain.example.com'],
+      ['http://example.com:8080/path/index.php', 'SESS', 'example.com'],
+      ['https://example.com/path/index.php', 'SSESS', 'example.com'],
+      ['http://example.com/path/core/install.php', 'SESS', 'example.com'],
+      ['http://localhost/path/index.php', 'SESS', 'localhost'],
+      ['http://127.0.0.1/path/index.php', 'SESS', '127.0.0.1'],
+      ['http://127.0.0.1:8888/path/index.php', 'SESS', '127.0.0.1'],
+      ['https://127.0.0.1/path/index.php', 'SSESS', '127.0.0.1'],
+      ['https://127.0.0.1:8443/path/index.php', 'SSESS', '127.0.0.1'],
+      ['http://1.1.1.1/path/index.php', 'SESS', '1.1.1.1'],
+      ['https://1.1.1.1/path/index.php', 'SSESS', '1.1.1.1'],
+      ['http://[::1]/path/index.php', 'SESS', '[::1]'],
+      ['http://[::1]:8888/path/index.php', 'SESS', '[::1]'],
+      ['https://[::1]/path/index.php', 'SSESS', '[::1]'],
+      ['https://[::1]:8443/path/index.php', 'SSESS', '[::1]'],
+    ];
+
+    return array_map(function ($record) {
+      return [$record[0], $record[1] . substr(hash('sha256', $record[2]), 0, 32)];
+    }, $data);
+  }
+
+  /**
+   * Tests whether the session.name ini settings is computed correctly.
+   *
+   * @covers ::getOptions()
+   *
+   * @dataProvider providerTestEnforcedSessionName
+   */
+  public function testEnforcedSessionNameViaCookieDomain($uri, $expected_name) {
+    $config = $this->createSessionConfiguration(['cookie_domain' => '.example.com']);
+
+    $request = Request::create($uri);
+    $options = $config->getOptions($request);
+
+    $this->assertEquals($expected_name, $options['name']);
+  }
+
+  /**
+   * Data provider for the cookie name test.
+   *
+   * @returns array
+   *   Test data
+   */
+  public function providerTestEnforcedSessionName() {
+    $data = [
+      ['http://example.com/path/index.php', 'SESS', '.example.com'],
+      ['http://www.example.com/path/index.php', 'SESS', '.example.com'],
+      ['http://subdomain.example.com/path/index.php', 'SESS', '.example.com'],
+      ['http://example.com:8080/path/index.php', 'SESS', '.example.com'],
+      ['https://example.com/path/index.php', 'SSESS', '.example.com'],
+      ['http://example.com/path/core/install.php', 'SESS', '.example.com'],
+      ['http://localhost/path/index.php', 'SESS', '.example.com'],
+      ['http://127.0.0.1/path/index.php', 'SESS', '.example.com'],
+      ['http://127.0.0.1:8888/path/index.php', 'SESS', '.example.com'],
+      ['https://127.0.0.1/path/index.php', 'SSESS', '.example.com'],
+      ['https://127.0.0.1:8443/path/index.php', 'SSESS', '.example.com'],
+      ['http://1.1.1.1/path/index.php', 'SESS', '.example.com'],
+      ['https://1.1.1.1/path/index.php', 'SSESS', '.example.com'],
+      ['http://[::1]/path/index.php', 'SESS', '.example.com'],
+      ['http://[::1]:8888/path/index.php', 'SESS', '.example.com'],
+      ['https://[::1]/path/index.php', 'SSESS', '.example.com'],
+      ['https://[::1]:8443/path/index.php', 'SSESS', '.example.com'],
+    ];
+
+    return array_map(function ($record) {
+      return [$record[0], $record[1] . substr(hash('sha256', $record[2]), 0, 32)];
+    }, $data);
+  }
+
+}