diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index e1c15fea0e7bdf9942e8fcb0d4587daa87496869..effc5838746dea6071ca6746eb06cfcb9d3d772a 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -1622,7 +1622,7 @@ function format_string($string, array $args = array()) {
  * @ingroup sanitization
  */
 function check_plain($text) {
-  return htmlspecialchars($text, ENT_QUOTES, 'UTF-8');
+  return htmlspecialchars($text, ENT_QUOTES, 'UTF-8', FALSE);
 }
 
 /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/XssUnitTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/XssUnitTest.php
index dff85bd957fea677c4217eb42ae71ffb0bdd0697..1643641f65ca63260050c5e2812d4881427ddf7e 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/XssUnitTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/XssUnitTest.php
@@ -60,6 +60,9 @@ function testEscaping() {
      $this->assertEqual($text, '<script>', 'check_plain() escapes <script>');
      $text = check_plain('<>&"\'');
      $this->assertEqual($text, '&lt;&gt;&amp;&quot;&#039;', 'check_plain() escapes reserved HTML characters.');
+     // Test that check_plain() is not double-escaping entities in strings.
+     $text = check_plain('&lt;<script>&gt;');
+     $this->assertEqual($text, '&lt;&lt;script&gt;&gt;', 'check_plain() does not double-escape entities in strings.');
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/System/ShutdownFunctionsTest.php b/core/modules/system/lib/Drupal/system/Tests/System/ShutdownFunctionsTest.php
index d3a160e5ad70f38a157b594f081d1f67ad985519..bec5fa87a14bc2a7e939e4313713b7f18ff88821 100644
--- a/core/modules/system/lib/Drupal/system/Tests/System/ShutdownFunctionsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/System/ShutdownFunctionsTest.php
@@ -41,6 +41,6 @@ function testShutdownFunctions() {
 
     // Make sure exceptions displayed through _drupal_render_exception_safe()
     // are correctly escaped.
-    $this->assertRaw('Drupal is &amp;lt;blink&amp;gt;awesome&amp;lt;/blink&amp;gt;.');
+    $this->assertRaw('Drupal is &lt;blink&gt;awesome&lt;/blink&gt;.');
   }
 }