diff --git a/core/includes/form.inc b/core/includes/form.inc
index 29b6c2d..ef41c4e 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -1211,6 +1211,11 @@ function drupal_validate_form($form_id, &$form, &$form_state) {
     }
   }
 
+  // Ensure the correct protocol when #https is set.
+  if (!empty($form['#https']) && !\Drupal::request()->isSecure() && settings()->get('mixed_mode_sessions', FALSE)) {
+    form_set_error('', t('This form must be submitted over a secure connection.'));
+  }
+
   _form_validate($form, $form_state, $form_id);
   $validated_forms[$form_id] = TRUE;
 
diff --git a/core/lib/Drupal/Core/Access/CsrfTokenGenerator.php b/core/lib/Drupal/Core/Access/CsrfTokenGenerator.php
index 910a77b..93ad32d 100644
--- a/core/lib/Drupal/Core/Access/CsrfTokenGenerator.php
+++ b/core/lib/Drupal/Core/Access/CsrfTokenGenerator.php
@@ -66,7 +66,20 @@ public function setRequest(Request $request) {
    * @see drupal_get_hash_salt()
    */
   public function get($value = '') {
-    return Crypt::hmacBase64($value, session_id() . $this->privateKey->get() . drupal_get_hash_salt());
+    // For mixed HTTP(S) sessions, use a constant identifier so that tokens can
+    // be shared between protocols.
+    $identifier = NULL;
+    if ($this->request->isSecure() && settings()->get('mixed_mode_sessions', FALSE)) {
+      $insecure_session_name = substr(session_name(), 1);
+      if ($this->request->cookies->get($insecure_session_name)) {
+        $identifier = $this->request->cookies->get($insecure_session_name);
+      }
+    }
+    // Otherwise, use the session ID.
+    if (!empty($identifier)) {
+      $identifier = session_id();
+    }
+    return Crypt::hmacBase64($value, $identifier . $this->privateKey->get() . drupal_get_hash_salt());
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/Session/SessionHttpsTest.php b/core/modules/system/lib/Drupal/system/Tests/Session/SessionHttpsTest.php
index 87cd97b..b99c843 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Session/SessionHttpsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Session/SessionHttpsTest.php
@@ -149,13 +149,23 @@ protected function testHttpsSession() {
     $this->drupalGet('user');
     $form = $this->xpath('//form[@id="user-login-form"]');
     $this->assertEqual(substr($form[0]['action'], 0, 6), 'https:', 'Login form action is secure');
-    $form[0]['action'] = $this->httpsUrl('user');
 
+    // Change the form to submit to an insecure URL and make sure the
+    // submission fails.
+    $form[0]['action'] = $this->httpUrl('user');
     $edit = array(
       'name' => $user->getUsername(),
       'pass' => $user->pass_raw,
     );
     $this->drupalPostForm(NULL, $edit, t('Log in'));
+    $this->assertText(t('This form must be submitted over a secure connection.'), 'Form submission failed over HTTP.');
+
+    // Now try with a secure URL and make sure the submission succeeds.
+    $form[0]['action'] = $this->httpsUrl('user');
+    $this->drupalPost(NULL, $edit, t('Log in'));
+    $this->assertNoText(t('This form must be submitted over a secure connection.'), 'Form submission succeeded over HTTPS.');
+    $this->assertLink(t('Log out'), 0, t('User %name successfully logged in.', array('%name' => $user->name)));
+
     // Check secure cookie on secure page.
     $this->assertTrue($this->cookies[$secure_session_name]['secure'], 'The secure cookie has the secure attribute');
     // Check insecure cookie on secure page.
@@ -211,6 +221,20 @@ protected function testHttpsSession() {
     // Test that the user is also authenticated on the insecure site.
     $this->drupalGet("user/" . $user->id() . "/edit");
     $this->assertResponse(200);
+
+    // Test that tokens can be shared in mixed mode.
+    $this->settingsSet('mixed_mode_sessions', TRUE);
+    $this->drupalGet($this->httpUrl('session-test/shared-token'));
+    $matches = array();
+    preg_match('/\s*sharedToken:(.*)\n/', $this->drupalGetContent(), $matches);
+    $this->assertTrue(!empty($matches[1]) , 'Found shared token on the HTTP URL.');
+    $token_plain = $matches[1];
+    $this->drupalGet($this->httpsUrl('session-test/shared-token'));
+    $matches = array();
+    preg_match('/\s*sharedToken:(.*)\n/', $this->drupalGetContent(), $matches);
+    $this->assertTrue(!empty($matches[1]) , 'Found shared token on the HTTPS URL.');
+    $token_secure = $matches[1];
+    $this->assertEqual($token_plain, $token_secure, 'Tokens are shared in mixed HTTPS sessions.');
   }
 
   /**
diff --git a/core/modules/system/tests/modules/session_test/lib/Drupal/session_test/Controller/SessionTestController.php b/core/modules/system/tests/modules/session_test/lib/Drupal/session_test/Controller/SessionTestController.php
index 6b00515..30af2dd 100644
--- a/core/modules/system/tests/modules/session_test/lib/Drupal/session_test/Controller/SessionTestController.php
+++ b/core/modules/system/tests/modules/session_test/lib/Drupal/session_test/Controller/SessionTestController.php
@@ -135,4 +135,14 @@ public function setNotStarted() {
   public function isLoggedIn() {
     return $this->t('User is logged in.');
   }
+
+  /**
+   * Display the result of Drupal::csrfToken()->get().
+   *
+   * @return string
+   *   A notification message with a CSRF token.
+   */
+  public function sharedToken() {
+    return 'sharedToken:' . \Drupal::csrfToken()->get($value);
+  }
 }
diff --git a/core/modules/system/tests/modules/session_test/session_test.routing.yml b/core/modules/system/tests/modules/session_test/session_test.routing.yml
index 1e7b127..4f250ec 100644
--- a/core/modules/system/tests/modules/session_test/session_test.routing.yml
+++ b/core/modules/system/tests/modules/session_test/session_test.routing.yml
@@ -75,3 +75,11 @@ session_test.is_logged_in:
     _controller: '\Drupal\session_test\Controller\SessionTestController::isLoggedIn'
   requirements:
     _user_is_logged_in: 'TRUE'
+
+session_test.shared_token:
+  path: '/session-test/shared-token'
+  defaults:
+    _title: 'Test that tokens can be shared in mixed mode'
+    _controller: '\Drupal\session_test\Controller\SessionTestController::sharedToken'
+  requirements:
+    _permission: 'access content'