diff --git a/core/lib/Drupal/Component/Utility/UrlHelper.php b/core/lib/Drupal/Component/Utility/UrlHelper.php
index 3293d5cb1d2273664ed1ba923d580f5d0d9ae2a6..1564f8243dd2293bd69f9195c6f3d036ba7b2ffe 100644
--- a/core/lib/Drupal/Component/Utility/UrlHelper.php
+++ b/core/lib/Drupal/Component/Utility/UrlHelper.php
@@ -214,10 +214,15 @@ public static function encodePath($path) {
    */
   public static function isExternal($path) {
     $colonpos = strpos($path, ':');
-    // Avoid calling stripDangerousProtocols() if there is any
-    // slash (/), hash (#) or question_mark (?) before the colon (:)
-    // occurrence - if any - as this would clearly mean it is not a URL.
-    return $colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) && static::stripDangerousProtocols($path) == $path;
+    // Avoid calling drupal_strip_dangerous_protocols() if there is any slash
+    // (/), hash (#) or question_mark (?) before the colon (:) occurrence - if
+    // any - as this would clearly mean it is not a URL. If the path starts with
+    // 2 slashes then it is always considered an external URL without an
+    // explicit protocol part.
+    return (strpos($path, '//') === 0)
+      || ($colonpos !== FALSE
+        && !preg_match('![/?#]!', substr($path, 0, $colonpos))
+        && static::stripDangerousProtocols($path) == $path);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/EventSubscriber/RedirectResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/RedirectResponseSubscriber.php
index 1a468f1812e1f2048627a7b5b497ba6d24c4badd..0573eea259a6354c5c23297287c85d0495e5a9e1 100644
--- a/core/lib/Drupal/Core/EventSubscriber/RedirectResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/RedirectResponseSubscriber.php
@@ -10,6 +10,7 @@
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Routing\RequestContext;
 use Drupal\Core\Routing\UrlGeneratorInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
 use Symfony\Component\HttpKernel\KernelEvents;
 use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
 use Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent;
@@ -92,6 +93,36 @@ public function checkRedirectUrl(FilterResponseEvent $event) {
     }
   }
 
+  /**
+   * Sanitize the destination parameter to prevent open redirect attacks.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
+   *   The Event to process.
+   */
+  public function sanitizeDestination(GetResponseEvent $event) {
+    $request = $event->getRequest();
+    // Sanitize the destination parameter (which is often used for redirects) to
+    // prevent open redirect attacks leading to other domains. Sanitize both
+    // $_GET['destination'] and $_REQUEST['destination'] to protect code that
+    // relies on either, but do not sanitize $_POST to avoid interfering with
+    // unrelated form submissions. The sanitization happens here because
+    // url_is_external() requires the variable system to be available.
+    $query_info = $request->query;
+    $request_info = $request->request;
+    if ($query_info->has('destination') || $request_info->has('destination')) {
+      // If the destination is an external URL, remove it.
+      if ($query_info->has('destination') && UrlHelper::isExternal($query_info->get('destination'))) {
+        $query_info->remove('destination');
+        $request_info->remove('destination');
+      }
+      // If there's still something in $_REQUEST['destination'] that didn't come
+      // from $_GET, check it too.
+      if ($request_info->has('destination') && (!$query_info->has('destination') || $request_info->get('destination') != $query_info->get('destination')) && UrlHelper::isExternal($request_info->get('destination'))) {
+        $request_info->remove('destination');
+      }
+    }
+  }
+
   /**
    * Registers the methods in this class that should be listeners.
    *
@@ -100,6 +131,7 @@ public function checkRedirectUrl(FilterResponseEvent $event) {
    */
   static function getSubscribedEvents() {
     $events[KernelEvents::RESPONSE][] = array('checkRedirectUrl');
+    $events[KernelEvents::REQUEST][] = array('sanitizeDestination', 100);
     return $events;
   }
 }
diff --git a/core/lib/Drupal/Core/Routing/RedirectDestination.php b/core/lib/Drupal/Core/Routing/RedirectDestination.php
index 58d7184527d33b0685a46fa85d4cb995ec031f53..7c80b116c71f1163b30c5c3ff2dcebbe8b2f010b 100644
--- a/core/lib/Drupal/Core/Routing/RedirectDestination.php
+++ b/core/lib/Drupal/Core/Routing/RedirectDestination.php
@@ -62,7 +62,10 @@ public function getAsArray() {
   public function get() {
     if (!isset($this->destination)) {
       $query = $this->requestStack->getCurrentRequest()->query;
-      if ($query->has('destination')) {
+      if (UrlHelper::isExternal($query->get('destination'))) {
+        $this->destination = '/';
+      }
+      elseif ($query->has('destination')) {
         $this->destination = $query->get('destination');
       }
       else {
diff --git a/core/lib/Drupal/Core/Routing/UrlGenerator.php b/core/lib/Drupal/Core/Routing/UrlGenerator.php
index 1926d44a5fda40a261a32a5b61ae9fe152b1731e..ba8427507827b267984e9bc38aa80f46fa63f6f2 100644
--- a/core/lib/Drupal/Core/Routing/UrlGenerator.php
+++ b/core/lib/Drupal/Core/Routing/UrlGenerator.php
@@ -245,15 +245,20 @@ public function generateFromPath($path = NULL, $options = array()) {
       'prefix' => '',
     );
 
+    // A duplicate of the code from
+    // \Drupal\Component\Utility\UrlHelper::isExternal() to avoid needing
+    // another function call, since performance inside url() is critical.
     if (!isset($options['external'])) {
-      // Return an external link if $path contains an allowed absolute URL. Only
-      // call the slow
-      // \Drupal\Component\Utility\UrlHelper::stripDangerousProtocols() if $path
-      // contains a ':' before any / ? or #. Note: we could use
-      // \Drupal\Component\Utility\UrlHelper::isExternal($path) here, but that
-      // would require another function call, and performance here is critical.
       $colonpos = strpos($path, ':');
-      $options['external'] = ($colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) && UrlHelper::stripDangerousProtocols($path) == $path);
+      // Avoid calling drupal_strip_dangerous_protocols() if there is any slash
+      // (/), hash (#) or question_mark (?) before the colon (:) occurrence -
+      // if any - as this would clearly mean it is not a URL. If the path starts
+      // with 2 slashes then it is always considered an external URL without an
+      // explicit protocol part.
+      $options['external'] = (strpos($path, '//') === 0)
+      || ($colonpos !== FALSE
+        && !preg_match('![/?#]!', substr($path, 0, $colonpos))
+        && UrlHelper::stripDangerousProtocols($path) == $path);
     }
 
     if (isset($options['fragment']) && $options['fragment'] !== '') {
diff --git a/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php b/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
index bc21ea817c19c5e27d06b6601e9885b197fa16c1..2aed98efa89d50b0307167f0fdd2195d6a1cb2ba 100644
--- a/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
+++ b/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
@@ -113,6 +113,11 @@ protected function buildLocalUrl($uri, array $options = []) {
     //   https://www.drupal.org/node/2417459
     $uri = substr($uri, 5);
 
+    // Strip leading slashes from internal paths to prevent them becoming
+    // external URLs without protocol. /example.com should not be turned into
+    // //example.com.
+    $uri = ltrim($uri, '/');
+
     // Allow (outbound) path processing, if needed. A valid use case is the path
     // alias overview form:
     // @see \Drupal\path\Controller\PathController::adminOverview().
diff --git a/core/modules/system/src/Tests/Form/ConfirmFormTest.php b/core/modules/system/src/Tests/Form/ConfirmFormTest.php
index ffcc44a27fd0bb03d207c9ba95629b7359b99011..9521176797f70f82b47a29dbb2ffbc064b2f25f8 100644
--- a/core/modules/system/src/Tests/Form/ConfirmFormTest.php
+++ b/core/modules/system/src/Tests/Form/ConfirmFormTest.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\system\Tests\Form;
 
+use Drupal\Component\Utility\String;
+use Drupal\Core\Url;
 use Drupal\simpletest\WebTestBase;
 
 /**
@@ -50,4 +52,35 @@ function testConfirmForm() {
     $this->assertUrl('form-test/confirm-form', array('query' => array('destination' => 'admin/config')), "The form's complex cancel link was followed.");
   }
 
+  /**
+   * Tests that the confirm form does not use external destinations.
+   */
+  public function testConfirmFormWithExternalDestination() {
+    $this->drupalGet('form-test/confirm-form');
+    $this->assertCancelLinkUrl(Url::fromRoute('form_test.route8'));
+    $this->drupalGet('form-test/confirm-form', array('query' => array('destination' => 'node')));
+    $this->assertCancelLinkUrl(Url::fromUri('internal:/node'));
+    $this->drupalGet('form-test/confirm-form', array('query' => array('destination' => 'http://example.com')));
+    $this->assertCancelLinkUrl(Url::fromRoute('form_test.route8'));
+  }
+
+  /**
+   * Asserts that a cancel link is present pointing to the provided URL.
+   *
+   * @param \Drupal\Core\Url $url
+   *   The url to check for.
+   * @param string $message
+   *   The assert message.
+   * @param string $group
+   *   The assertion group.
+   *
+   * @return bool
+   *   Result of the assertion.
+   */
+  public function assertCancelLinkUrl(Url $url, $message = '', $group = 'Other') {
+    $links = $this->xpath('//a[@href=:url]', [':url' => $url->toString()]);
+    $message = ($message ? $message : String::format('Cancel link with url %url found.', ['%url' => $url->toString()]));
+    return $this->assertTrue(isset($links[0]), $message, $group);
+  }
+
 }
diff --git a/core/modules/system/src/Tests/Routing/DestinationTest.php b/core/modules/system/src/Tests/Routing/DestinationTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..a590d62396977d2fe1096df7fabeaa57f2d048a5
--- /dev/null
+++ b/core/modules/system/src/Tests/Routing/DestinationTest.php
@@ -0,0 +1,83 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Routing\DestinationTest.
+ */
+
+namespace Drupal\system\Tests\Routing;
+
+use Drupal\Core\Url;
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests for $_GET['destination'] and $_REQUEST['destination'] validation.
+ *
+ * Note: This tests basically the same as
+ * \Drupal\Tests\Core\EventSubscriber\RedirectResponseSubscriberTest::testSanitizeDestinationForGet
+ * \Drupal\Tests\Core\EventSubscriber\RedirectResponseSubscriberTest::testSanitizeDestinationForPost
+ * but we want to be absolutely sure it works.
+ *
+ * @group Routing
+ */
+class DestinationTest extends WebTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['system_test'];
+
+  /**
+   * Tests that $_GET/$_REQUEST['destination'] only contain internal URLs.
+   */
+  public function testDestination() {
+    $test_cases = [
+      [
+        'input' => 'node',
+        'output' => 'node',
+        'message' => "Standard internal example node path is present in the 'destination' parameter.",
+      ],
+      [
+        'input' => '/example.com',
+        'output' => '/example.com',
+        'message' => 'Internal path with one leading slash is allowed.',
+      ],
+      [
+        'input' => '//example.com/test',
+        'output' => '',
+        'message' => 'External URL without scheme is not allowed.',
+      ],
+      [
+        'input' => 'example:test',
+        'output' => 'example:test',
+        'message' => 'Internal URL using a colon is allowed.',
+      ],
+      [
+        'input' => 'http://example.com',
+        'output' => '',
+        'message' => 'External URL is not allowed.',
+      ],
+      [
+        'input' => 'javascript:alert(0)',
+        'output' => 'javascript:alert(0)',
+        'message' => 'Javascript URL is allowed because it is treated as an internal URL.',
+      ],
+    ];
+    foreach ($test_cases as $test_case) {
+      // Test $_GET['destination'].
+      $this->drupalGet('system-test/get-destination', ['query' => ['destination' => $test_case['input']]]);
+      $this->assertIdentical($test_case['output'], $this->getRawContent(), $test_case['message']);
+      // Test $_REQUEST['destination'].
+      $post_output = $this->drupalPost('system-test/request-destination', '*', ['destination' => $test_case['input']]);
+      $this->assertIdentical($test_case['output'], $post_output, $test_case['message']);
+    }
+
+    // Make sure that 404 pages do not populate $_GET['destination'] with
+    // external URLs.
+    \Drupal::configFactory()->getEditable('system.site')->set('page.404', 'system-test/get-destination')->save();
+    $this->drupalGet('http://example.com', ['external' => FALSE]);
+    $this->assertResponse(404);
+    $this->assertIdentical(Url::fromRoute('<front>')->toString(), $this->getRawContent(), 'External URL is not allowed on 404 pages.');
+  }
+
+}
diff --git a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
index 5b4bf737a4e1881dad975ac9b9ded9ab1c238fd9..6b6b06333f6b2e93671d20172d95996ee68c878a 100644
--- a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
+++ b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
@@ -87,6 +87,34 @@ public function drupalSetMessageTest() {
     return [];
   }
 
+  /**
+   * Controller to return $_GET['destination'] for testing.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request.
+   *
+   * @return \Symfony\Component\HttpFoundation\Response
+   *   The response.
+   */
+  public function getDestination(Request $request) {
+    $response = new Response($request->query->get('destination'));
+    return $response;
+  }
+
+  /**
+   * Controller to return $_REQUEST['destination'] for testing.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request.
+   *
+   * @return \Symfony\Component\HttpFoundation\Response
+   *   The response.
+   */
+  public function requestDestination(Request $request) {
+    $response = new Response($request->request->get('destination'));
+    return $response;
+  }
+
   /**
    * Try to acquire a named lock and report the outcome.
    */
diff --git a/core/modules/system/tests/modules/system_test/system_test.routing.yml b/core/modules/system/tests/modules/system_test/system_test.routing.yml
index 4251cedc7b564eb67941a2cb41ffb2ead293be79..86d0a31ec92a12f1f8193b6c99ba9589f5005598 100644
--- a/core/modules/system/tests/modules/system_test/system_test.routing.yml
+++ b/core/modules/system/tests/modules/system_test/system_test.routing.yml
@@ -87,3 +87,19 @@ system_test.configure:
     _title_callback: '\Drupal\system_test\Controller\SystemTestController::configureTitle'
   requirements:
     _access: 'TRUE'
+
+system_test.request_destination:
+  path: '/system-test/request-destination'
+  defaults:
+    _controller: '\Drupal\system_test\Controller\SystemTestController::requestDestination'
+  requirements:
+    _access: 'TRUE'
+
+
+system_test.get_destination:
+  path: '/system-test/get-destination'
+  defaults:
+    _controller: '\Drupal\system_test\Controller\SystemTestController::getDestination'
+  requirements:
+    _access: 'TRUE'
+
diff --git a/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
index 0477b7aa48f8e1be18890566d354df6203d2cc73..9480a7e2fd33edfa40916322632d11a033f18761 100644
--- a/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
@@ -358,6 +358,10 @@ public static function providerTestIsExternal() {
       array('/internal/path', FALSE),
       array('https://example.com/external/path', TRUE),
       array('javascript://fake-external-path', FALSE),
+      // External URL without an explicit protocol.
+      array('//drupal.org/foo/bar?foo=bar&bar=baz&baz#foo', TRUE),
+      // Internal URL starting with a slash.
+      array('/drupal.org', FALSE),
     );
   }
 
diff --git a/core/tests/Drupal/Tests/Core/EventSubscriber/RedirectResponseSubscriberTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/RedirectResponseSubscriberTest.php
index 9ff5414a26bce465c1431ccfaa69c02ea4ff229d..51f299169ca137f3bcd8206293820f82ab06abf8 100644
--- a/core/tests/Drupal/Tests/Core/EventSubscriber/RedirectResponseSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/EventSubscriber/RedirectResponseSubscriberTest.php
@@ -14,6 +14,7 @@
 use Symfony\Component\HttpFoundation\RedirectResponse;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 use Symfony\Component\HttpKernel\KernelEvents;
 
@@ -48,7 +49,11 @@ public function testDestinationRedirect(Request $request, $expected) {
         ->expects($this->any())
         ->method('generateFromPath')
           ->willReturnMap([
-            ['test', ['query' => [], 'fragment' => '', 'absolute' => TRUE], 'http://example.com/drupal/test']
+            ['test', ['query' => [], 'fragment' => '', 'absolute' => TRUE], 'http://example.com/drupal/test'],
+            ['example.com', ['query' => [], 'fragment' => '', 'absolute' => TRUE], 'http://example.com/drupal/example.com'],
+            ['example:com', ['query' => [], 'fragment' => '', 'absolute' => TRUE], 'http://example.com/drupal/example:com'],
+            ['javascript:alert(0)', ['query' => [], 'fragment' => '', 'absolute' => TRUE], 'http://example.com/drupal/javascript:alert(0)'],
+            ['/test', ['query' => [], 'fragment' => '', 'absolute' => TRUE], 'http://example.com/test'],
           ]);
     }
 
@@ -58,6 +63,7 @@ public function testDestinationRedirect(Request $request, $expected) {
     $request_context->expects($this->any())
       ->method('getCompleteBaseUrl')
       ->willReturn('http://example.com/drupal');
+    $request->headers->set('HOST', 'example.com');
 
     $listener = new RedirectResponseSubscriber($url_generator, $request_context);
     $dispatcher->addListener(KernelEvents::RESPONSE, array($listener, 'checkRedirectUrl'));
@@ -85,8 +91,118 @@ public static function providerTestDestinationRedirect() {
       array(new Request(array('destination' => 'http://example.com/foobar')), FALSE),
       array(new Request(array('destination' => 'http://example.ca/drupal')), FALSE),
       array(new Request(array('destination' => 'test')), 'http://example.com/drupal/test'),
+      array(new Request(array('destination' => '/test')), 'http://example.com/test'),
+      array(new Request(array('destination' => '/example.com')), 'http://example.com/example.com'),
+      array(new Request(array('destination' => 'example:com')), 'http://example.com/drupal/example:com'),
+      array(new Request(array('destination' => 'javascript:alert(0)')), 'http://example.com/drupal/javascript:alert(0)'),
       array(new Request(array('destination' => 'http://example.com/drupal/')), 'http://example.com/drupal/'),
       array(new Request(array('destination' => 'http://example.com/drupal/test')), 'http://example.com/drupal/test'),
     );
   }
+
+  /**
+   * @expectedException \InvalidArgumentException
+   *
+   * @dataProvider providerTestDestinationRedirectWithInvalidUrl
+   */
+  public function testDestinationRedirectWithInvalidUrl(Request $request) {
+    $dispatcher = new EventDispatcher();
+    $kernel = $this->getMock('Symfony\Component\HttpKernel\HttpKernelInterface');
+    $response = new RedirectResponse('http://example.com/drupal');
+    $url_generator = $this->getMock('Drupal\Core\Routing\UrlGeneratorInterface');
+
+    $request_context = $this->getMockBuilder('Drupal\Core\Routing\RequestContext')
+      ->disableOriginalConstructor()
+      ->getMock();
+
+    $listener = new RedirectResponseSubscriber($url_generator, $request_context);
+    $dispatcher->addListener(KernelEvents::RESPONSE, array($listener, 'checkRedirectUrl'));
+    $event = new FilterResponseEvent($kernel, $request, HttpKernelInterface::SUB_REQUEST, $response);
+    $dispatcher->dispatch(KernelEvents::RESPONSE, $event);
+  }
+
+  /**
+   * Data provider for testDestinationRedirectWithInvalidUrl().
+   */
+  public function providerTestDestinationRedirectWithInvalidUrl() {
+    $data = [];
+    $data[] = [new Request(array('destination' => '//example:com'))];
+    $data[] = [new Request(array('destination' => '//example:com/test'))];
+
+    return $data;
+  }
+
+  /**
+   * Tests that $_GET only contain internal URLs.
+   *
+   * @covers ::sanitizeDestination
+   *
+   * @dataProvider providerTestSanitizeDestination
+   *
+   * @see \Drupal\Component\Utility\UrlHelper::isExternal
+   */
+  public function testSanitizeDestinationForGet($input, $output) {
+    $request = new Request();
+    $request->query->set('destination', $input);
+
+    $url_generator = $this->getMock('Drupal\Core\Routing\UrlGeneratorInterface');
+    $request_context = new RequestContext();
+    $listener = new RedirectResponseSubscriber($url_generator, $request_context);
+    $kernel = $this->getMock('Symfony\Component\HttpKernel\HttpKernelInterface');
+    $event = new GetResponseEvent($kernel, $request, HttpKernelInterface::MASTER_REQUEST);
+
+    $dispatcher = new EventDispatcher();
+    $dispatcher->addListener(KernelEvents::REQUEST, [$listener, 'sanitizeDestination'], 100);
+    $dispatcher->dispatch(KernelEvents::REQUEST, $event);
+
+    $this->assertEquals($output, $request->query->get('destination'));
+  }
+
+  /**
+   * Tests that $_REQUEST['destination'] only contain internal URLs.
+   *
+   * @covers ::sanitizeDestination
+   *
+   * @dataProvider providerTestSanitizeDestination
+   *
+   * @see \Drupal\Component\Utility\UrlHelper::isExternal
+   */
+  public function testSanitizeDestinationForPost($input, $output) {
+    $request = new Request();
+    $request->request->set('destination', $input);
+
+    $url_generator = $this->getMock('Drupal\Core\Routing\UrlGeneratorInterface');
+    $request_context = new RequestContext();
+    $listener = new RedirectResponseSubscriber($url_generator, $request_context);
+    $kernel = $this->getMock('Symfony\Component\HttpKernel\HttpKernelInterface');
+    $event = new GetResponseEvent($kernel, $request, HttpKernelInterface::MASTER_REQUEST);
+
+    $dispatcher = new EventDispatcher();
+    $dispatcher->addListener(KernelEvents::REQUEST, [$listener, 'sanitizeDestination'], 100);
+    $dispatcher->dispatch(KernelEvents::REQUEST, $event);
+
+    $this->assertEquals($output, $request->request->get('destination'));
+  }
+
+  /**
+   * Data provider for testSanitizeDestination().
+   */
+  public function providerTestSanitizeDestination() {
+    $data = [];
+    // Standard internal example node path is present in the 'destination'
+    // parameter.
+    $data[] = ['node', 'node'];
+    // Internal path with one leading slash is allowed.
+    $data[] = ['/example.com', '/example.com'];
+    // External URL without scheme is not allowed.
+    $data[] = ['//example.com/test', ''];
+    // Internal URL using a colon is allowed.
+    $data[] = ['example:test', 'example:test'];
+    // External URL is not allowed.
+    $data[] = ['http://example.com', ''];
+    // Javascript URL is allowed because it is treated as an internal URL.
+    $data[] = ['javascript:alert(0)', 'javascript:alert(0)'];
+
+    return $data;
+  }
 }
diff --git a/core/tests/Drupal/Tests/Core/Routing/RedirectDestinationTest.php b/core/tests/Drupal/Tests/Core/Routing/RedirectDestinationTest.php
index 3f388597783fa0719749564fa13310323d3aa368..e305bd1e2ca0b16667567cccbd3e0d1e4f08cd6e 100644
--- a/core/tests/Drupal/Tests/Core/Routing/RedirectDestinationTest.php
+++ b/core/tests/Drupal/Tests/Core/Routing/RedirectDestinationTest.php
@@ -64,6 +64,13 @@ protected function setupUrlGenerator() {
   }
 
   /**
+   * Tests destination passed via $_GET.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request to test.
+   * @param string $expected_destination
+   *   The expected destination.
+   *
    * @dataProvider providerGet
    *
    * @covers ::get
@@ -108,6 +115,11 @@ public function providerGet() {
     $request->query->set('other', 'value');
     $data[] = [$request, '/current-path?other=value'];
 
+    // A request with a dedicated specified external destination.
+    $request = Request::create('/');
+    $request->query->set('destination', 'https://www.drupal.org');
+    $data[] = [$request, '/'];
+
     return $data;
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Utility/UnroutedUrlAssemblerTest.php b/core/tests/Drupal/Tests/Core/Utility/UnroutedUrlAssemblerTest.php
index 6f5401f3e06ec8722834b42c071170d5c9fe4f8c..6a3642a50d0ba09a42c5ca331ef35eae3ad5ba82 100644
--- a/core/tests/Drupal/Tests/Core/Utility/UnroutedUrlAssemblerTest.php
+++ b/core/tests/Drupal/Tests/Core/Utility/UnroutedUrlAssemblerTest.php
@@ -66,6 +66,14 @@ public function testAssembleWithNeitherExternalNorDomainLocalUri() {
     $this->unroutedUrlAssembler->assemble('wrong-url');
   }
 
+  /**
+   * @covers ::assemble
+   * @expectedException \InvalidArgumentException
+   */
+  public function testAssembleWithLeadingSlash() {
+    $this->unroutedUrlAssembler->assemble('/drupal.org');
+  }
+
   /**
    * @covers ::assemble
    * @covers ::buildExternalUrl
@@ -89,6 +97,7 @@ public function providerTestAssembleWithExternalUrl() {
       ['http://example.com/test', ['https' => TRUE], 'https://example.com/test'],
       ['https://example.com/test', ['https' => FALSE], 'http://example.com/test'],
       ['https://example.com/test?foo=1#bar', [], 'https://example.com/test?foo=1#bar'],
+      ['//drupal.org', [], '//drupal.org'],
     ];
   }
 
@@ -115,6 +124,7 @@ public function providerTestAssembleWithLocalUri() {
       ['base:example', [], TRUE, '/subdir/example'],
       ['base:example', ['query' => ['foo' => 'bar']], TRUE, '/subdir/example?foo=bar'],
       ['base:example', ['fragment' => 'example', ], TRUE, '/subdir/example#example'],
+      ['base:/drupal.org', [], FALSE, '/drupal.org'],
     ];
   }