diff --git a/core/lib/Drupal/Core/Render/ReadOnlyRenderArray.php b/core/lib/Drupal/Core/Render/ProtectedRenderArray.php
similarity index 85%
rename from core/lib/Drupal/Core/Render/ReadOnlyRenderArray.php
rename to core/lib/Drupal/Core/Render/ProtectedRenderArray.php
index b17a680..7e675c4 100644
--- a/core/lib/Drupal/Core/Render/ReadOnlyRenderArray.php
+++ b/core/lib/Drupal/Core/Render/ProtectedRenderArray.php
@@ -7,10 +7,10 @@
  *
  * @ingroup utility
  */
-class ReadOnlyRenderArray extends \ArrayObject implements RenderableInterface {
+class ProtectedRenderArray extends \ArrayObject implements RenderableInterface {
 
   /**
-   * ReadOnlyRenderArray constructor.
+   * ProtectedRenderArray constructor.
    *
    * @param array $input
    *   The render array to wrap into a read-only object.
@@ -32,7 +32,7 @@ public function toRenderable() {
    * @param mixed $index
    *   The index with the value.
    *
-   * @return \Drupal\Core\Render\ReadOnlyRenderArray|mixed
+   * @return \Drupal\Core\Render\ProtectedRenderArray|mixed
    */
   public function __get($index) {
     return $this->offsetGet($index);
@@ -44,7 +44,7 @@ public function __get($index) {
   public function offsetGet($index) {
     $value = parent::offsetGet($index);
     if (is_array($value) && !empty($value)) {
-      $value = new ReadOnlyRenderArray($value);
+      $value = new ProtectedRenderArray($value);
     }
     return $value;
   }
diff --git a/core/lib/Drupal/Core/Render/RenderArrayIterator.php b/core/lib/Drupal/Core/Render/RenderArrayIterator.php
index 55a4302..e7ffded 100644
--- a/core/lib/Drupal/Core/Render/RenderArrayIterator.php
+++ b/core/lib/Drupal/Core/Render/RenderArrayIterator.php
@@ -15,7 +15,7 @@ class RenderArrayIterator extends \ArrayIterator {
   public function offsetGet($index) {
     $value = parent::offsetGet($index);
     if (is_array($value) && !empty($value)) {
-      $value = new ReadOnlyRenderArray($value);
+      $value = new ProtectedRenderArray($value);
     }
     return $value;
   }
@@ -26,7 +26,7 @@ public function offsetGet($index) {
   public function current() {
     $value = parent::current();
     if (is_array($value) && !empty($value)) {
-      $value = new ReadOnlyRenderArray($value);
+      $value = new ProtectedRenderArray($value);
     }
     return $value;
   }
diff --git a/core/lib/Drupal/Core/Template/TwigEnvironment.php b/core/lib/Drupal/Core/Template/TwigEnvironment.php
index 584c600..91ddcc1 100644
--- a/core/lib/Drupal/Core/Template/TwigEnvironment.php
+++ b/core/lib/Drupal/Core/Template/TwigEnvironment.php
@@ -4,7 +4,7 @@
 
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Render\Markup;
-use Drupal\Core\Render\ReadOnlyRenderArray;
+use Drupal\Core\Render\ProtectedRenderArray;
 use Drupal\Core\State\StateInterface;
 
 /**
@@ -155,7 +155,7 @@ public function wrapContext(&$context) {
     if (is_array($context)) {
       foreach ($context as $key => $value) {
         if (is_array($context[$key]) && !empty($context[$key])) {
-          $context[$key] = new ReadOnlyRenderArray($context[$key]);
+          $context[$key] = new ProtectedRenderArray($context[$key]);
         }
       }
     }
@@ -168,12 +168,12 @@ public function wrapContext(&$context) {
    *   An array of parameters.
    */
   public function unWrapContext(&$context) {
-    if (is_object($context) && $context instanceof ReadOnlyRenderArray) {
+    if (is_object($context) && $context instanceof ProtectedRenderArray) {
       $context = $context->getArrayCopy();
     }
     if (is_array($context)) {
       foreach ($context as $key => $value) {
-        if (is_object($context[$key]) && $context[$key] instanceof ReadOnlyRenderArray) {
+        if (is_object($context[$key]) && $context[$key] instanceof ProtectedRenderArray) {
           $context[$key] = $context[$key]->getArrayCopy();
         }
       }
diff --git a/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
index d0d8779..014c2e3 100644
--- a/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -9,7 +9,7 @@
 use Drupal\Core\Render\AttachmentsInterface;
 use Drupal\Core\Render\BubbleableMetadata;
 use Drupal\Core\Render\Markup;
-use Drupal\Core\Render\ReadOnlyRenderArray;
+use Drupal\Core\Render\ProtectedRenderArray;
 use Drupal\Core\Render\RenderableInterface;
 use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\Routing\UrlGeneratorInterface;
@@ -248,7 +248,7 @@ public function getPath($name, $parameters = [], $options = []) {
    *   (optional) An associative array of additional options. The 'absolute'
    *   option is forced to be TRUE.
    *
-   * @return \Drupal\Core\Render\ReadOnlyRenderArray
+   * @return \Drupal\Core\Render\ProtectedRenderArray
    *   The generated absolute URL for the given route.
    *
    * @todo Add an option for scheme-relative URLs.
@@ -261,7 +261,7 @@ public function getUrl($name, $parameters = [], $options = []) {
     // Return as render array, so we can bubble the bubbleable metadata.
     $build = ['#markup' => $generated_url->getGeneratedUrl()];
     $generated_url->applyTo($build);
-    return new ReadOnlyRenderArray($build);
+    return new ProtectedRenderArray($build);
   }
 
   /**
@@ -274,7 +274,7 @@ public function getUrl($name, $parameters = [], $options = []) {
    * @param array|\Drupal\Core\Template\Attribute $attributes
    *   An optional array or Attribute object of link attributes.
    *
-   * @return \Drupal\Core\Render\ReadOnlyRenderArray
+   * @return \Drupal\Core\Render\ProtectedRenderArray
    *   A render array representing a link to the given URL.
    */
   public function getLink($text, $url, $attributes = []) {
@@ -301,7 +301,7 @@ public function getLink($text, $url, $attributes = []) {
       '#title' => $text,
       '#url' => $url,
     ];
-    return new ReadOnlyRenderArray($build);
+    return new ProtectedRenderArray($build);
   }
 
   /**
diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index e0a173a..60391ff 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -7,9 +7,10 @@
 
 use Drupal\comment\CommentInterface;
 use Drupal\comment\Plugin\Field\FieldType\CommentItemInterface;
+use Drupal\Component\Utility\Xss;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityTypeInterface;
-use Drupal\Core\Render\ReadOnlyRenderArray;
+use Drupal\Core\Render\Markup;
 use Drupal\Core\Url;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
@@ -546,7 +547,9 @@ function template_preprocess_forum_list(&$variables) {
   $row = 0;
   // Sanitize each forum so that the template can safely print the data.
   foreach ($variables['forums'] as $id => $forum) {
-    $variables['forums'][$id]->description = new ReadOnlyRenderArray(['#markup' => $forum->description->value]);
+    // Work-around the fact that this is an object instead of a render array
+    // by setting the property to a filtered Markup object so it's not escaped.
+    $variables['forums'][$id]->description = Markup::create(Xss::filter($forum->description->value));
     $variables['forums'][$id]->link = forum_uri($forum);
     $variables['forums'][$id]->name = $forum->label();
     $variables['forums'][$id]->is_container = !empty($forum->forum_container->value);
diff --git a/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
index 49169c4..4024a8a 100644
--- a/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
@@ -3,7 +3,7 @@
 namespace Drupal\Tests\Core\Template;
 
 use Drupal\Core\GeneratedLink;
-use Drupal\Core\Render\ReadOnlyRenderArray;
+use Drupal\Core\Render\ProtectedRenderArray;
 use Drupal\Core\Render\RenderableInterface;
 use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Drupal\Core\Template\Loader\StringLoader;
@@ -219,7 +219,7 @@ public function testSafeJoin() {
     $markup->__toString()->willReturn('<em>will be markup</em>');
     $markup = $markup->reveal();
 
-    $items = new ReadOnlyRenderArray([
+    $items = new ProtectedRenderArray([
       '<em>will be escaped</em>',
       $markup,
       ['#markup' => '<strong>will be rendered</strong>'],
@@ -228,7 +228,7 @@ public function testSafeJoin() {
     $this->assertEquals('&lt;em&gt;will be escaped&lt;/em&gt;<br/><em>will be markup</em><br/><strong>will be rendered</strong>', $result);
 
     // Ensure safe_join Twig filter supports Traversable variables.
-    $items = new ReadOnlyRenderArray([
+    $items = new ProtectedRenderArray([
       '<em>will be escaped</em>',
       $markup,
       ['#markup' => '<strong>will be rendered</strong>'],