diff --git a/core/modules/ckeditor/js/plugins/drupalimage/plugin.js b/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
index 724fa50..047ae24 100644
--- a/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
+++ b/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
@@ -44,13 +44,14 @@
         var allowedContentDefinition = {
           element: 'img',
           attributes: {
-            '!data-entity-type': '',
-            '!data-entity-uuid': ''
+            'data-entity-type': '',
+            'data-entity-uuid': ''
           }
         };
         var imgAttributes = widgetDefinition.allowedContent.img.attributes.split(/\s*,\s*/);
         for (var i = 0; i < imgAttributes.length; i++) {
-          allowedContentDefinition.attributes[imgAttributes[i]] = '';
+          // Exclamation marks are only used in the string notation.
+          allowedContentDefinition.attributes[imgAttributes[i].replace(/^!/, '')] = '';
         }
         if (widgetDefinition.allowedContent.img.classes) {
           allowedContentDefinition.attributes['class'] = widgetDefinition.allowedContent.img.classes.split(/\s*,\s*/).join(' ');
diff --git a/core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js b/core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js
index 0963c04..8cad394 100644
--- a/core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js
+++ b/core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js
@@ -56,8 +56,8 @@
         requiredContent.attributes['data-caption'] = '';
         widgetDefinition.requiredContent = new CKEDITOR.style(requiredContent);
         var allowedContent = widgetDefinition.allowedContent.getDefinition();
-        allowedContent.attributes['!data-align'] = '';
-        allowedContent.attributes['!data-caption'] = '';
+        allowedContent.attributes['data-align'] = '';
+        allowedContent.attributes['data-caption'] = '';
         widgetDefinition.allowedContent = new CKEDITOR.style(allowedContent);
 
         // Override allowedContent setting for the 'caption' nested editable.
diff --git a/core/modules/ckeditor/js/plugins/drupallink/plugin.js b/core/modules/ckeditor/js/plugins/drupallink/plugin.js
index e595815..0db1cde 100644
--- a/core/modules/ckeditor/js/plugins/drupallink/plugin.js
+++ b/core/modules/ckeditor/js/plugins/drupallink/plugin.js
@@ -16,7 +16,7 @@
         allowedContent: new CKEDITOR.style({
           element: 'a',
           attributes: {
-            '!href': '',
+            'href': '',
             'target': ''
           }
         }),
@@ -125,7 +125,7 @@
         allowedContent: new CKEDITOR.style({
           element: 'a',
           attributes: {
-            '!href': '',
+            'href': '',
             'target': ''
           }
         }),