diff --git a/core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php
index b5fddb1..b44c561 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php
@@ -72,14 +72,33 @@ public function onConfigImporterValidate(ConfigImporterEvent $event) {
       }
     }
     $config_importer = $event->getConfigImporter();
-    if ($config_importer->getStorageComparer()->getSourceStorage()->exists('core.extension')) {
+    if ($this->validateCoreExtension($config_importer)) {
       $this->validateModules($config_importer);
       $this->validateThemes($config_importer);
       $this->validateDependencies($config_importer);
     }
-    else {
+  }
+
+  /**
+   * Validates the core.extension configuration file.
+   *
+   * @param \Drupal\Core\Config\ConfigImporter $config_importer
+   *   The configuration importer.
+   *
+   * @return bool
+   *   TRUE if the core.extension is valid, FALSE if not.
+   */
+  protected function validateCoreExtension(ConfigImporter $config_importer) {
+    $data = $config_importer->getStorageComparer()->getSourceStorage()->read('core.extension');
+    if ($data === FALSE) {
       $config_importer->logError($this->t('The core.extension configuration does not exist.'));
+      return FALSE;
+    }
+    elseif (empty($data['versions'])) {
+      $config_importer->logError($this->t('The core.extension file is out-of-date. Update the codebase of the source site, run update.php and export the configuration again.'));
+      return FALSE;
     }
+    return TRUE;
   }
 
   /**
@@ -97,6 +116,23 @@ protected function validateModules(ConfigImporter $config_importer) {
       $config_importer->logError($this->t('Unable to install the %module module since it does not exist.', array('%module' => $module)));
     }
 
+    $existing_modules = array_keys(array_intersect_key($core_extension['module'], $config_importer->getStorageComparer()->getTargetStorage()->read('core.extension')['module']));
+    foreach ($existing_modules as $module) {
+      if ($core_extension['versions'][$module]['current'] !== $module_data[$module]->info['version']) {
+        $config_importer->logError($this->t(
+          'Unable to import configuration because the version of %module module on the source site (@source_version) does not match this site (@target_version).',
+          ['%module' => $module, '@source_version' => $core_extension['versions'][$module]['current'], '@target_version' => $module_data[$module]->info['version']]
+        ));
+      }
+      elseif ($core_extension['versions'][$module]['schema'] !== (string) drupal_get_installed_schema_version($module)) {
+        $config_importer->logError($this->t(
+          'Unable to import configuration because the schema version of %module module on the source site (@source_schema) does not match this site (@target_schema).',
+          ['%module' => $module, '@source_schema' => $core_extension['versions'][$module]['schema'], '@target_schema' => drupal_get_installed_schema_version($module)]
+        ));
+
+      }
+    }
+
     // Ensure that all modules being installed have their dependencies met.
     $installs = $config_importer->getExtensionChangelist('module', 'install');
     foreach ($installs as $module) {
@@ -166,6 +202,16 @@ protected function validateThemes(ConfigImporter $config_importer) {
       }
     }
 
+    $existing_themes = array_keys(array_intersect_key($core_extension['theme'], $config_importer->getStorageComparer()->getTargetStorage()->read('core.extension')['theme']));
+    foreach ($existing_themes as $theme) {
+      if ($core_extension['versions'][$theme]['current'] !== $theme_data[$theme]->info['version']) {
+        $config_importer->logError($this->t(
+          'Unable to import configuration because the version of %theme theme on the source site (@source_version) does not match this site (@target_version).',
+          ['%theme' => $theme, '@source_version' => $core_extension['versions'][$theme]['current'], '@target_version' => $theme_data[$theme]->info['version']]
+        ));
+      }
+    }
+
     // Ensure that all themes being installed have their dependencies met.
     foreach ($installs as $theme) {
       foreach (array_keys($theme_data[$theme]->requires) as $required_theme) {
diff --git a/core/modules/config/src/Tests/ConfigImportUITest.php b/core/modules/config/src/Tests/ConfigImportUITest.php
index c98915c..85529c1 100644
--- a/core/modules/config/src/Tests/ConfigImportUITest.php
+++ b/core/modules/config/src/Tests/ConfigImportUITest.php
@@ -523,6 +523,9 @@ public function testExtensionValidation() {
     // This theme does exist but the version is wrong.
     $core['theme']['seven'] = 0;
     $core['versions']['seven'] = ['current' => '8.0.0-rc3', 'install' => '8.0.0-rc3'];
+    $core['versions']['bartik']['current'] = '8.0.0-rc3';
+    $core['versions']['text']['current'] = '8.0.0-rc3';
+    $core['versions']['config_test']['schema'] = '8999';
 
     $sync->write('core.extension', $core);
 
@@ -534,6 +537,8 @@ public function testExtensionValidation() {
     $this->assertText('Unable to install the does_not_exist theme since it does not exist.');
     $this->assertText('Unable to install the ban module since the installed version (8.0.0-rc3) does not match the code base (' . \Drupal::VERSION . ').');
     $this->assertText('Unable to install the seven theme since the installed version (8.0.0-rc3) does not match the code base (' . \Drupal::VERSION . ').');
+    $this->assertText('Unable to import configuration because the schema version of config_test module on the source site (8999) does not match this site (' . drupal_get_installed_schema_version('config_test') . ').');
+    $this->assertText('Unable to import configuration because the version of bartik theme on the source site (8.0.0-rc3) does not match this site (' . \Drupal::VERSION . ').');
   }
 
 }
diff --git a/core/modules/config/src/Tests/ConfigImporterTest.php b/core/modules/config/src/Tests/ConfigImporterTest.php
index e62a777..1f5724d 100644
--- a/core/modules/config/src/Tests/ConfigImporterTest.php
+++ b/core/modules/config/src/Tests/ConfigImporterTest.php
@@ -651,6 +651,58 @@ public function testMissingCoreExtension() {
   }
 
   /**
+   * Tests missing core.extension:versions during configuration import.
+   *
+   * Configuration exported before the versions information was added should
+   * fail.
+   *
+   * @see \Drupal\Core\EventSubscriber\ConfigImportSubscriber
+   */
+  public function testMissingVersionInformation() {
+    $sync = $this->container->get('config.storage.sync');
+    $data = $sync->read('core.extension');
+    unset($data['versions']);
+    $sync->write('core.extension', $data);
+    try {
+      $this->configImporter->reset()->import();
+      $this->fail('ConfigImporterException not thrown, invalid import was not stopped due to missing version information.');
+    }
+    catch (ConfigImporterException $e) {
+      $this->assertEqual($e->getMessage(), 'There were errors validating the config synchronization.');
+      $error_log = $this->configImporter->getErrors();
+      $this->assertEqual(['The core.extension file is out-of-date. Update the codebase of the source site, run update.php and export the configuration again.'], $error_log);
+    }
+  }
+
+  /**
+   * Tests missing core.extension:versions during configuration import.
+   *
+   * Configuration exported before the versions information was added should
+   * fail.
+   *
+   * @see \Drupal\Core\EventSubscriber\ConfigImportSubscriber
+   */
+  public function testVersionMismatch() {
+    $sync = $this->container->get('config.storage.sync');
+    $data = $sync->read('core.extension');
+    $data['versions']['system']['current'] = '8.0.0-beta1';
+    $data['versions']['config_test']['schema'] = '8999';
+    $sync->write('core.extension', $data);
+    try {
+      $this->configImporter->reset()->import();
+      $this->fail('ConfigImporterException not thrown, invalid import was not stopped due to missing version information.');
+    }
+    catch (ConfigImporterException $e) {
+      $this->assertEqual($e->getMessage(), 'There were errors validating the config synchronization.');
+      $error_log = $this->configImporter->getErrors();
+      $this->assertEqual([
+        'Unable to import configuration because the schema version of <em class="placeholder">config_test</em> module on the source site (8999) does not match this site (-1).',
+        'Unable to import configuration because the version of <em class="placeholder">system</em> module on the source site (8.0.0-beta1) does not match this site (8.0.0-dev).'
+      ], $error_log);
+    }
+  }
+
+  /**
    * Tests install profile validation during configuration import.
    *
    * @see \Drupal\Core\EventSubscriber\ConfigImportSubscriber
diff --git a/core/modules/simpletest/src/KernelTestBase.php b/core/modules/simpletest/src/KernelTestBase.php
index a3d17c6..3f686e7 100644
--- a/core/modules/simpletest/src/KernelTestBase.php
+++ b/core/modules/simpletest/src/KernelTestBase.php
@@ -522,7 +522,7 @@ protected function enableModules(array $modules) {
         'current' => $version,
         'install' => $version,
         // @todo How to get schema - or should installSchema do this?
-        'schema' => NULL,
+        'schema' => SCHEMA_UNINSTALLED,
       ];
     }
     $active_storage->write('core.extension', $extensions);
diff --git a/core/tests/Drupal/KernelTests/KernelTestBase.php b/core/tests/Drupal/KernelTests/KernelTestBase.php
index fa05e4d..0149613 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBase.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBase.php
@@ -835,7 +835,7 @@ protected function enableModules(array $modules) {
         'current' => $version,
         'install' => $version,
         // @todo How to get schema - or should installSchema do this?
-        'schema' => NULL,
+        'schema' => SCHEMA_UNINSTALLED,
       ];
     }
     $active_storage->write('core.extension', $extension_config);