Issue #2780285 by alexpott, th_tushar, mpdonadio: XSS in date format configuration

core/modules/system/js/system.date.es6.js

@@ -43,7 +43,7 @@
           (key, value) => (dateFormats[key] ? dateFormats[key] : value),
         );
 
-        $preview.html(dateString);
+        $preview.text(dateString);
         $target.toggleClass('js-hide', !dateString.length);
       }
 

core/modules/system/js/system.date.js

@@ -25,7 +25,7 @@
           return dateFormats[key] ? dateFormats[key] : value;
         });
 
-        $preview.html(dateString);
+        $preview.text(dateString);
         $target.toggleClass('js-hide', !dateString.length);
       }
 

core/modules/system/tests/src/FunctionalJavascript/System/DateFormatTest.php

+<?php
+
+namespace Drupal\Tests\system\FunctionalJavascript\System;
+
+use Drupal\Core\Datetime\Entity\DateFormat;
+use Drupal\FunctionalJavascriptTests\WebDriverTestBase;
+
+/**
+ * Tests that date formats UI with JavaScript enabled.
+ *
+ * @group system
+ */
+class DateFormatTest extends WebDriverTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['block'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    // Create admin user and log in admin user.
+    $this->drupalLogin($this->drupalCreateUser(['administer site configuration']));
+    $this->drupalPlaceBlock('local_actions_block');
+  }
+
+  /**
+   * Tests XSS via date format configuration.
+   */
+  public function testDateFormatXss() {
+    $page = $this->getSession()->getPage();
+    $assert = $this->assertSession();
+
+    $date_format = DateFormat::create([
+      'id' => 'xss_short',
+      'label' => 'XSS format',
+      'pattern' => '\<\s\c\r\i\p\t\>\a\l\e\r\t\(\"\X\S\S\")\;\<\/\s\c\r\i\p\t\>',
+    ]);
+    $date_format->save();
+    $this->drupalGet('admin/config/regional/date-time');
+    $assert->assertEscaped('<script>alert("XSS");</script>', 'The date format was properly escaped');
+    $this->drupalGet('admin/config/regional/date-time/formats/manage/xss_short');
+    $assert->assertEscaped('<script>alert("XSS");</script>', 'The date format was properly escaped');
+
+    // Add a new date format with HTML in it.
+    $this->drupalGet('admin/config/regional/date-time/formats/add');
+    $date_format = '& \<\e\m\>Y\<\/\e\m\>';
+    $page->fillField('date_format_pattern', $date_format);
+    $assert->waitForText('Displayed as');
+    $assert->assertEscaped('<em>' . date("Y") . '</em>');
+    $page->fillField('label', 'date_html_pattern');
+    // Wait for the machine name ID to be completed.
+    $assert->waitForLink('Edit');
+    $page->pressButton('Add format');
+    $assert->pageTextContains('Custom date format added.');
+    $assert->assertEscaped('<em>' . date("Y") . '</em>');
+  }
+
+}