diff --git a/CHANGES.md b/CHANGES.md
index 5c079cfca58..00b259198f0 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -5,6 +5,7 @@ CKEditor 4 Changelog
 
 Fixed Issues:
 
+* [#11677](http://dev.ckeditor.com/ticket/11677): Fixed: undo/redo keystrokes are blocked in source mode.
 * [#11717](http://dev.ckeditor.com/ticket/11717): [Docprops](http://ckeditor.com/addon/docprops) plugin requires the [Colordialog](http://ckeditor.com/addon/colordialog) plugin to work.
 
 ## CKEditor 4.3.4
diff --git a/core/config.js b/core/config.js
index dbdcaf1df08..221a1eeeb14 100644
--- a/core/config.js
+++ b/core/config.js
@@ -333,10 +333,7 @@ CKEDITOR.config = {
 	 *		config.blockedKeystrokes = [
 	 *			CKEDITOR.CTRL + 66, // CTRL+B
 	 *			CKEDITOR.CTRL + 73, // CTRL+I
-	 *			CKEDITOR.CTRL + 85, // CTRL+U
-	 *			CKEDITOR.CTRL + 89, // CTRL+Y
-	 *			CKEDITOR.CTRL + 90, // CTRL+Z
-	 *			CKEDITOR.CTRL + CKEDITOR.SHIFT + 90  // CTRL+SHIFT+Z
+	 *			CKEDITOR.CTRL + 85 // CTRL+U
 	 *		];
 	 *
 	 * @cfg {Array} [blockedKeystrokes=see example]
@@ -344,11 +341,7 @@ CKEDITOR.config = {
 	blockedKeystrokes: [
 		CKEDITOR.CTRL + 66, // CTRL+B
 		CKEDITOR.CTRL + 73, // CTRL+I
-		CKEDITOR.CTRL + 85, // CTRL+U
-
-		CKEDITOR.CTRL + 89, // CTRL+Y
-		CKEDITOR.CTRL + 90, // CTRL+Z
-		CKEDITOR.CTRL + CKEDITOR.SHIFT + 90  // CTRL+SHIFT+Z
+		CKEDITOR.CTRL + 85 // CTRL+U
 	]
 };
 
diff --git a/plugins/undo/plugin.js b/plugins/undo/plugin.js
index baa6f9cf35a..828b4acd11a 100644
--- a/plugins/undo/plugin.js
+++ b/plugins/undo/plugin.js
@@ -38,12 +38,23 @@
 				canUndo: false
 			} );
 
+			var keystrokes = [ CKEDITOR.CTRL + 90 /*Z*/, CKEDITOR.CTRL + 89 /*Y*/, CKEDITOR.CTRL + CKEDITOR.SHIFT + 90 /*Z*/ ];
+
 			editor.setKeystroke( [
-				[ CKEDITOR.CTRL + 90 /*Z*/, 'undo' ],
-				[ CKEDITOR.CTRL + 89 /*Y*/, 'redo' ],
-				[ CKEDITOR.CTRL + CKEDITOR.SHIFT + 90 /*Z*/, 'redo' ]
+				[ keystrokes[ 0 ], 'undo' ],
+				[ keystrokes[ 1 ], 'redo' ],
+				[ keystrokes[ 2 ], 'redo' ]
 			] );
 
+			// Block undo/redo keystrokes when at the bottom/top of the undo stack (#11126 and #11677).
+			editor.on( 'contentDom', function() {
+				var editable = editor.editable();
+				editable.attachListener( editable, 'keydown', function( evt ) {
+					if ( CKEDITOR.tools.indexOf( keystrokes, evt.data.getKeystroke() ) > -1 )
+						evt.data.preventDefault();
+				} );
+			} );
+
 			undoManager.onChange = function() {
 				undoCommand.setState( undoManager.undoable() ? CKEDITOR.TRISTATE_OFF : CKEDITOR.TRISTATE_DISABLED );
 				redoCommand.setState( undoManager.redoable() ? CKEDITOR.TRISTATE_OFF : CKEDITOR.TRISTATE_DISABLED );