diff --git a/includes/common.inc b/includes/common.inc
index 30853a37fa..045dd9b303 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -2257,6 +2257,7 @@ function drupal_add_js($data = NULL, $type = 'module', $scope = 'header', $defer
         'core' => array(
           'misc/jquery.js' => array('cache' => TRUE, 'defer' => FALSE, 'preprocess' => TRUE),
           'misc/jquery-extend-3.4.0.js' => array('cache' => TRUE, 'defer' => FALSE, 'preprocess' => TRUE),
+          'misc/jquery-html-prefilter-3.5.0-backport.js' => array('cache' => TRUE, 'defer' => FALSE, 'preprocess' => TRUE),
           'misc/drupal.js' => array('cache' => TRUE, 'defer' => FALSE, 'preprocess' => TRUE),
         ),
         'module' => array(),
diff --git a/misc/jquery-html-prefilter-3.5.0-backport.js b/misc/jquery-html-prefilter-3.5.0-backport.js
new file mode 100644
index 0000000000..9377150221
--- /dev/null
+++ b/misc/jquery-html-prefilter-3.5.0-backport.js
@@ -0,0 +1,251 @@
+/**
+ * For jQuery versions less than 3.5.0, this replaces the jQuery.htmlPrefilter()
+ * function with one that fixes these security vulnerabilities while also
+ * retaining the pre-3.5.0 behavior where it's safe to do so.
+ * - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-11022
+ * - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-11023
+ *
+ * Additionally, for jQuery versions that do not have a jQuery.htmlPrefilter()
+ * function (1.x prior to 1.12 and 2.x prior to 2.2), this adds it, and
+ * extends the functions that need to call it to do so.
+ *
+ * Drupal core's jQuery version is 1.4.4, but jQuery Update can provide a
+ * different version, so this covers all versions between 1.4.4 and 3.4.1.
+ * The GitHub links in the code comments below link to jQuery 1.5 code, because
+ * 1.4.4 isn't on GitHub, but the referenced code didn't change from 1.4.4 to
+ * 1.5.
+ */
+
+(function (jQuery) {
+
+  // Parts of this backport differ by jQuery version.
+  var versionParts = jQuery.fn.jquery.split('.');
+  var majorVersion = parseInt(versionParts[0]);
+  var minorVersion = parseInt(versionParts[1]);
+
+  // No backport is needed if we're already on jQuery 3.5 or higher.
+  if ( (majorVersion > 3) || (majorVersion === 3 && minorVersion >= 5) ) {
+    return;
+  }
+
+  // Prior to jQuery 3.5, jQuery converted XHTML-style self-closing tags to
+  // their XML equivalent: e.g., "<div />" to "<div></div>". This is
+  // problematic for several reasons, including that it's vulnerable to XSS
+  // attacks. However, since this was jQuery's behavior for many years, many
+  // Drupal modules and jQuery plugins may be relying on it. Therefore, we
+  // preserve that behavior, but for a limited set of tags only, that we believe
+  // to not be vulnerable. This is the set of HTML tags that satisfy all of the
+  // following conditions:
+  // - In DOMPurify's list of HTML tags. If an HTML tag isn't safe enough to
+  //   appear in that list, then we don't want to mess with it here either.
+  //   @see https://github.com/cure53/DOMPurify/blob/2.0.11/dist/purify.js#L128
+  // - A normal element (not a void, template, text, or foreign element).
+  //   @see https://html.spec.whatwg.org/multipage/syntax.html#elements-2
+  // - An element that is still defined by the current HTML specification
+  //   (not a deprecated element), because we do not want to rely on how
+  //   browsers parse deprecated elements.
+  //   @see https://developer.mozilla.org/en-US/docs/Web/HTML/Element
+  // - Not 'html', 'head', or 'body', because this pseudo-XHTML expansion is
+  //   designed for fragments, not entire documents.
+  // - Not 'colgroup', because due to an idiosyncrasy of jQuery's original
+  //   regular expression, it didn't match on colgroup, and we don't want to
+  //   introduce a behavior change for that.
+  var selfClosingTagsToReplace = [
+    'a', 'abbr', 'address', 'article', 'aside', 'audio', 'b', 'bdi', 'bdo',
+    'blockquote', 'button', 'canvas', 'caption', 'cite', 'code', 'data',
+    'datalist', 'dd', 'del', 'details', 'dfn', 'div', 'dl', 'dt', 'em',
+    'fieldset', 'figcaption', 'figure', 'footer', 'form', 'h1', 'h2', 'h3',
+    'h4', 'h5', 'h6', 'header', 'hgroup', 'i', 'ins', 'kbd', 'label', 'legend',
+    'li', 'main', 'map', 'mark', 'menu', 'meter', 'nav', 'ol', 'optgroup',
+    'option', 'output', 'p', 'picture', 'pre', 'progress', 'q', 'rp', 'rt',
+    'ruby', 's', 'samp', 'section', 'select', 'small', 'source', 'span',
+    'strong', 'sub', 'summary', 'sup', 'table', 'tbody', 'td', 'tfoot', 'th',
+    'thead', 'time', 'tr', 'u', 'ul', 'var', 'video'
+  ];
+
+  // Define regular expressions for <TAG/> and <TAG ATTRIBUTES/>. Doing this as
+  // two expressions makes it easier to target <a/> without also targeting
+  // every tag that starts with "a".
+  var xhtmlRegExpGroup = '(' + selfClosingTagsToReplace.join('|') + ')';
+  var whitespace = '[\\x20\\t\\r\\n\\f]';
+  var rxhtmlTagWithoutSpaceOrAttributes = new RegExp('<' + xhtmlRegExpGroup + '\\/>', 'gi');
+  var rxhtmlTagWithSpaceAndMaybeAttributes = new RegExp('<' + xhtmlRegExpGroup + '(' + whitespace + '[^>]*)\\/>', 'gi');
+
+  // jQuery 3.5 also fixed a vulnerability for when </select> appears within
+  // an <option> or <optgroup>, but it did that in local code that we can't
+  // backport directly. Instead, we filter such cases out. To do so, we need to
+  // determine when jQuery would otherwise invoke the vulnerable code, which it
+  // uses this regular expression to determine. The regular expression changed
+  // for version 3.0.0 and changed again for 3.4.0.
+  // @see https://github.com/jquery/jquery/blob/1.5/jquery.js#L4958
+  // @see https://github.com/jquery/jquery/blob/3.0.0/dist/jquery.js#L4584
+  // @see https://github.com/jquery/jquery/blob/3.4.0/dist/jquery.js#L4712
+  var rtagName;
+  if (majorVersion < 3) {
+    rtagName = /<([\w:]+)/;
+  }
+  else if (minorVersion < 4) {
+    rtagName = /<([a-z][^\/\0>\x20\t\r\n\f]+)/i;
+  }
+  else {
+    rtagName = /<([a-z][^\/\0>\x20\t\r\n\f]*)/i;
+  }
+
+  // The regular expression that jQuery uses to determine which self-closing
+  // tags to expand to open and close tags. This is vulnerable, because it
+  // matches all tag names except the few excluded ones. We only use this
+  // expression for determining vulnerability. The expression changed for
+  // version 3, but we only need to check for vulnerability in versions 1 and 2,
+  // so we use the expression from those versions.
+  // @see https://github.com/jquery/jquery/blob/1.5/jquery.js#L4957
+  var rxhtmlTag = /<(?!area|br|col|embed|hr|img|input|link|meta|param)(([\w:]+)[^>]*)\/>/gi;
+
+  jQuery.extend({
+    htmlPrefilter: function (html) {
+      // This is how jQuery determines the first tag in the HTML.
+      // @see https://github.com/jquery/jquery/blob/1.5/jquery.js#L5521
+      var tag = ( rtagName.exec( html ) || [ "", "" ] )[ 1 ].toLowerCase();
+
+      // It is not valid HTML for <option> or <optgroup> to have <select> as
+      // either a descendant or sibling, and attempts to inject one can cause
+      // XSS on jQuery versions before 3.5. Since this is invalid HTML and a
+      // possible XSS attack, reject the entire string.
+      // @see https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-11023
+      if ((tag === 'option' || tag === 'optgroup') && html.match(/<\/?select/i)) {
+        html = '';
+      }
+
+      // Retain jQuery's prior to 3.5 conversion of pseudo-XHTML, but for only
+      // the tags in the `selfClosingTagsToReplace` list defined above.
+      // @see https://github.com/jquery/jquery/blob/1.5/jquery.js#L5518
+      // @see https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-11022
+      html = html.replace(rxhtmlTagWithoutSpaceOrAttributes, "<$1></$1>");
+      html = html.replace(rxhtmlTagWithSpaceAndMaybeAttributes, "<$1$2></$1>");
+
+      // Prior to jQuery 1.12 and 2.2, this function gets called (via code later
+      // in this file) in addition to, rather than instead of, the unsafe
+      // expansion of self-closing tags (including ones not in the list above).
+      // We can't prevent that unsafe expansion from running, so instead we
+      // check to make sure that it doesn't affect the DOM returned by the
+      // browser's parsing logic. If it does affect it, then it's vulnerable to
+      // XSS, so we reject the entire string.
+      if ( (majorVersion === 1 && minorVersion < 12) || (majorVersion === 2 && minorVersion < 2) ) {
+        var htmlRisky = html.replace(rxhtmlTag, "<$1></$2>");
+        if (htmlRisky !== html) {
+          // Even though htmlRisky and html are different strings, they might
+          // represent the same HTML structure once parsed, in which case,
+          // htmlRisky is actually safe. We can ask the browser to parse both
+          // to find out, but the browser can't parse table fragments (e.g., a
+          // root-level "<td>"), so we need to wrap them. We just need this
+          // technique to work on all supported browsers; we don't need to
+          // copy from the specific jQuery version we're using.
+          // @see https://github.com/jquery/jquery/blob/3.5.1/dist/jquery.js#L4939
+          var wrapMap = {
+            thead: [ 1, "<table>", "</table>" ],
+            col: [ 2, "<table><colgroup>", "</colgroup></table>" ],
+            tr: [ 2, "<table><tbody>", "</tbody></table>" ],
+            td: [ 3, "<table><tbody><tr>", "</tr></tbody></table>" ],
+          };
+          wrapMap.tbody = wrapMap.tfoot = wrapMap.colgroup = wrapMap.caption = wrapMap.thead;
+          wrapMap.th = wrapMap.td;
+
+          // Function to wrap HTML into something that a browser can parse.
+          // @see https://github.com/jquery/jquery/blob/3.5.1/dist/jquery.js#L5032
+          var getWrappedHtml = function (html) {
+            var wrap = wrapMap[tag];
+            if (wrap) {
+              html = wrap[1] + html + wrap[2];
+            }
+            return html;
+          };
+
+          // Function to return canonical HTML after parsing it. This parses
+          // only; it doesn't execute scripts.
+          // @see https://github.com/jquery/jquery-migrate/blob/3.3.0/src/jquery/manipulation.js#L5
+          var getParsedHtml = function (html) {
+            var doc = window.document.implementation.createHTMLDocument( "" );
+            doc.body.innerHTML = html;
+            return doc.body ? doc.body.innerHTML : '';
+          };
+
+          // If the browser couldn't parse either one successfully, or if
+          // htmlRisky parses differently than html, then html is vulnerable,
+          // so reject it.
+          var htmlParsed = getParsedHtml(getWrappedHtml(html));
+          var htmlRiskyParsed = getParsedHtml(getWrappedHtml(htmlRisky));
+          if (htmlRiskyParsed === '' || htmlParsed === '' || (htmlRiskyParsed !== htmlParsed)) {
+            html = '';
+          }
+        }
+      }
+
+      return html;
+    }
+  });
+
+  // Prior to jQuery 1.12 and 2.2, jQuery.clean(), jQuery.buildFragment(), and
+  // jQuery.fn.html() did not call jQuery.htmlPrefilter(), so we add that.
+  if ( (majorVersion === 1 && minorVersion < 12) || (majorVersion === 2 && minorVersion < 2) ) {
+    // Filter the HTML coming into jQuery.fn.html().
+    var fnOriginalHtml = jQuery.fn.html;
+    jQuery.fn.extend({
+      // @see https://github.com/jquery/jquery/blob/1.5/jquery.js#L5147
+      html: function (value) {
+        if (typeof value === "string") {
+          value = jQuery.htmlPrefilter(value);
+        }
+        // .html() can be called as a setter (with an argument) or as a getter
+        // (without an argument), so invoke fnOriginalHtml() the same way that
+        // we were invoked.
+        return fnOriginalHtml.apply(this, arguments.length ? [value] : []);
+      }
+    });
+
+    // The regular expression that jQuery uses to determine if a string is HTML.
+    // Used by both clean() and buildFragment().
+    // @see https://github.com/jquery/jquery/blob/1.5/jquery.js#L4960
+    var rhtml = /<|&#?\w+;/;
+
+    // Filter HTML coming into:
+    // - jQuery.clean() for versions prior to 1.9.
+    // - jQuery.buildFragment() for 1.9 and above.
+    //
+    // The looping constructs in the two functions might be essentially
+    // identical, but they're each expressed here in the way that most closely
+    // matches their original expression in jQuery, so that we filter all of
+    // the items and only the items that jQuery will treat as HTML strings.
+    if (majorVersion === 1 && minorVersion < 9) {
+      var originalClean = jQuery.clean;
+      jQuery.extend({
+        // @see https://github.com/jquery/jquery/blob/1.5/jquery.js#L5493
+        'clean': function (elems, context, fragment, scripts) {
+          for ( var i = 0, elem; (elem = elems[i]) != null; i++ ) {
+            if ( typeof elem === "string" && rhtml.test( elem ) ) {
+              elems[i] = elem = jQuery.htmlPrefilter(elem);
+            }
+          }
+          return originalClean.call(this, elems, context, fragment, scripts);
+        }
+      });
+    }
+    else {
+      var originalBuildFragment = jQuery.buildFragment;
+      jQuery.extend({
+        // @see https://github.com/jquery/jquery/blob/1.9.0/jquery.js#L6419
+        'buildFragment': function (elems, context, scripts, selection) {
+          var l = elems.length;
+          for ( var i = 0; i < l; i++ ) {
+            var elem = elems[i];
+            if (elem || elem === 0) {
+              if ( jQuery.type( elem ) !== "object" && rhtml.test( elem ) ) {
+                elems[i] = elem = jQuery.htmlPrefilter(elem);
+              }
+            }
+          }
+          return originalBuildFragment.call(this, elems, context, scripts, selection);
+        }
+      });
+    }
+  }
+
+})(jQuery);