diff --git a/CHANGES.md b/CHANGES.md
index 1501a4548b4..5296fef8da3 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -12,6 +12,7 @@ CKEditor 4 Changelog
 * [#6504](http://dev.ckeditor.com/ticket/6504): Resolved race condition while loading several customConfig files.
 * [#10298](http://dev.ckeditor.com/ticket/10298): Data processor breaks attributes containing protected parts.
 * [#9945](http://dev.ckeditor.com/ticket/9945): [iOS] Scrolling not possible on iPad.
+* [#10165](http://dev.ckeditor.com/ticket/10165): [IE] Access denied error if document.domain has been altered.
 
 ## CKEditor 4.1.1
 
diff --git a/core/dom/document.js b/core/dom/document.js
index ec7e6d5c1f4..9ba43d927f4 100644
--- a/core/dom/document.js
+++ b/core/dom/document.js
@@ -261,7 +261,12 @@ CKEDITOR.tools.extend( CKEDITOR.dom.document.prototype, {
 		this.$.open( 'text/html', 'replace' );
 
 		// Support for custom document.domain in IE.
-		CKEDITOR.env.isCustomDomain() && ( this.$.domain = document.domain );
+		//
+		// The script must be appended because if placed before the
+		// doctype, IE will go into quirks mode and mess with
+		// the editable, e.g. by changing its default height.
+		if ( CKEDITOR.env.ie )
+			html = html.replace( /(?:^\s*<!DOCTYPE[^>]*?>)|^/i, '$&\n<script data-cke-temp="1">(' + CKEDITOR.tools.fixDomain + ')();</script>' );
 
 		this.$.write( html );
 		this.$.close();
diff --git a/core/env.js b/core/env.js
index b04fcc70280..88de856decd 100644
--- a/core/env.js
+++ b/core/env.js
@@ -109,6 +109,7 @@ if ( !CKEDITOR.env ) {
 			 *			alert( 'I\'m in a custom domain!' );
 			 *
 			 * @returns {Boolean} `true` if a custom domain is enabled.
+			 * @deprecated
 			 */
 			isCustomDomain: function() {
 				if ( !this.ie )
diff --git a/core/tools.js b/core/tools.js
index 7c69e73903f..5e9e0890ef5 100644
--- a/core/tools.js
+++ b/core/tools.js
@@ -971,6 +971,45 @@
 				obj[ arr[ i ] ] = fillWith;
 
 			return obj;
+		},
+
+		/**
+		 * Tries to fix the document.domain of the current document to match the
+		 * parent window domain, avoiding "Same Origin" policy issues.
+		 * This is a IE only requirement.
+		 *
+		 * @since 4.1.2
+		 * @returns {boolean} True if the current domain is already good or if
+		 * 		it has been fixed successfully.
+		 */
+		fixDomain: function() {
+			var domain;
+
+			while( 1 ) {
+				try {
+					// Try to access the parent document. It throws
+					// "access denied" if restricted by the "Same Origin" policy.
+					domain = parent.document.domain;
+					return true;
+				} catch ( e ) {
+					// Calculate the value to set to document.domain.
+					domain = domain ?
+
+						// If it is not the first pass, strip one part of the
+						// name. E.g.  "test.example.com"  => "example.com"
+						domain.replace( /.+?(?:\.|$)/, '' ) :
+
+						// In the first pass, we'll handle the
+						// "document.domain = document.domain" case.
+						document.domain;
+
+					// Stop here if there is no more domain parts available.
+					if ( !domain )
+						return false;
+
+					document.domain = domain;
+				}
+			}
 		}
 	};
 })();
diff --git a/plugins/clipboard/dialogs/paste.js b/plugins/clipboard/dialogs/paste.js
index 793f759a962..aeb7b00f208 100644
--- a/plugins/clipboard/dialogs/paste.js
+++ b/plugins/clipboard/dialogs/paste.js
@@ -5,7 +5,6 @@
 
 CKEDITOR.dialog.add( 'paste', function( editor ) {
 	var lang = editor.lang.clipboard;
-	var isCustomDomain = CKEDITOR.env.isCustomDomain();
 
 	function onPasteFrameLoad( win ) {
 		var doc = new CKEDITOR.dom.document( win.document ),
@@ -130,12 +129,12 @@ CKEDITOR.dialog.add( 'paste', function( editor ) {
 					var src =
 							CKEDITOR.env.air ?
 								'javascript:void(0)' :
-							isCustomDomain ?
-								'javascript:void((function(){' +
+							CKEDITOR.env.ie ?
+								'javascript:void((function(){' + encodeURIComponent(
 									'document.open();' +
-									'document.domain=\'' + document.domain + '\';' +
-									'document.close();' +
-								'})())"'
+									'(' + CKEDITOR.tools.fixDomain + ')();' +
+									'document.close();'
+								) + '})())"'
 							: '';
 
 					var iframe = CKEDITOR.dom.element.createFromHtml( '<iframe' +
@@ -172,7 +171,11 @@ CKEDITOR.dialog.add( 'paste', function( editor ) {
 					if ( CKEDITOR.env.ie ) {
 						var focusGrabber = CKEDITOR.dom.element.createFromHtml( '<span tabindex="-1" style="position:absolute" role="presentation"></span>' );
 						focusGrabber.on( 'focus', function() {
-							iframe.$.contentWindow.focus();
+							// Since fixDomain is called in src attribute,
+							// IE needs some slight delay to correctly move focus.
+							setTimeout( function() {
+								iframe.$.contentWindow.focus();
+							});
 						});
 						container.append( focusGrabber );
 
diff --git a/plugins/dialog/plugin.js b/plugins/dialog/plugin.js
index 9a8e2dd74e1..f08c93c5e53 100644
--- a/plugins/dialog/plugin.js
+++ b/plugins/dialog/plugin.js
@@ -161,8 +161,7 @@ CKEDITOR.DIALOG_RESIZE_BOTH = 3;
 
 		// IFrame shim for dialog that masks activeX in IE. (#7619)
 		if ( CKEDITOR.env.ie && !CKEDITOR.env.ie6Compat ) {
-			var isCustomDomain = CKEDITOR.env.isCustomDomain(),
-				src = 'javascript:void(function(){' + encodeURIComponent( 'document.open();' + ( isCustomDomain ? ( 'document.domain="' + document.domain + '";' ) : '' ) + 'document.close();' ) + '}())',
+			var src = 'javascript:void(function(){' + encodeURIComponent( 'document.open();(' + CKEDITOR.tools.fixDomain + ')();document.close();' ) + '}())',
 				iframe = CKEDITOR.dom.element.createFromHtml( '<iframe' +
 					' frameBorder="0"' +
 					' class="cke_iframe_shim"' +
@@ -1976,8 +1975,7 @@ CKEDITOR.DIALOG_RESIZE_BOTH = 3;
 
 			if ( CKEDITOR.env.ie6Compat ) {
 				// Support for custom document.domain in IE.
-				var isCustomDomain = CKEDITOR.env.isCustomDomain(),
-					iframeHtml = '<html><body style=\\\'background-color:' + backgroundColorStyle + ';\\\'></body></html>';
+				var iframeHtml = '<html><body style=\\\'background-color:' + backgroundColorStyle + ';\\\'></body></html>';
 
 				html.push( '<iframe' +
 					' hidefocus="true"' +
@@ -1985,12 +1983,13 @@ CKEDITOR.DIALOG_RESIZE_BOTH = 3;
 					' id="cke_dialog_background_iframe"' +
 					' src="javascript:' );
 
-				html.push( 'void((function(){' +
+				html.push( 'void((function(){' + encodeURIComponent(
 					'document.open();' +
-					( isCustomDomain ? 'document.domain=\'' + document.domain + '\';' : '' ) +
+					// Support for custom document.domain in IE.
+					'(' + CKEDITOR.tools.fixDomain + ')();' +
 					'document.write( \'' + iframeHtml + '\' );' +
-					'document.close();' +
-					'})())' );
+					'document.close();'
+				) + '})())' );
 
 				html.push( '"' +
 					' style="' +
diff --git a/plugins/dialogui/plugin.js b/plugins/dialogui/plugin.js
index 4bb9b729c09..0278397cc28 100644
--- a/plugins/dialogui/plugin.js
+++ b/plugins/dialogui/plugin.js
@@ -592,9 +592,6 @@ CKEDITOR.plugins.add( 'dialogui', {
 				var innerHTML = function() {
 						_.frameId = CKEDITOR.tools.getNextId() + '_fileInput';
 
-						// Support for custom document.domain in IE.
-						var isCustomDomain = CKEDITOR.env.isCustomDomain();
-
 						var html = [
 							'<iframe' +
 								' frameborder="0"' +
@@ -605,11 +602,13 @@ CKEDITOR.plugins.add( 'dialogui', {
 								' title="', elementDefinition.label, '"' +
 								' src="javascript:void(' ];
 
-						html.push( isCustomDomain ? '(function(){' +
-							'document.open();' +
-							'document.domain=\'' + document.domain + '\';' +
-							'document.close();' +
-							'})()'
+						// Support for custom document.domain on IE. (#10165)
+						html.push( CKEDITOR.env.ie ?
+							'(function(){' + encodeURIComponent(
+								'document.open();' +
+								'(' + CKEDITOR.tools.fixDomain + ')();' +
+								'document.close();'
+							) + '})()'
 							:
 							'0' );
 
@@ -1256,33 +1255,36 @@ CKEDITOR.plugins.add( 'dialogui', {
 				function generateFormField() {
 					frameDocument.$.open();
 
-					// Support for custom document.domain in IE.
-					if ( CKEDITOR.env.isCustomDomain() )
-						frameDocument.$.domain = document.domain;
-
 					var size = '';
 					if ( elementDefinition.size )
 						size = elementDefinition.size - ( CKEDITOR.env.ie ? 7 : 0 ); // "Browse" button is bigger in IE.
 
-				var inputId = _.frameId + '_input';
-
-					frameDocument.$.write( [ '<html dir="' + langDir + '" lang="' + langCode + '"><head><title></title></head><body style="margin: 0; overflow: hidden; background: transparent;">',
-														'<form enctype="multipart/form-data" method="POST" dir="' + langDir + '" lang="' + langCode + '" action="',
-														CKEDITOR.tools.htmlEncode( elementDefinition.action ),
-														'">',
-													// Replicate the field label inside of iframe.
-																	'<label id="', _.labelId, '" for="', inputId, '" style="display:none">',
-													CKEDITOR.tools.htmlEncode( elementDefinition.label ),
-													'</label>',
-													'<input id="', inputId, '" aria-labelledby="', _.labelId, '" type="file" name="',
-														CKEDITOR.tools.htmlEncode( elementDefinition.id || 'cke_upload' ),
-														'" size="',
-														CKEDITOR.tools.htmlEncode( size > 0 ? size : "" ),
-														'" />',
-														'</form>',
-														'</body></html>',
-														'<script>window.parent.CKEDITOR.tools.callFunction(' + callNumber + ');',
-														'window.onbeforeunload = function() {window.parent.CKEDITOR.tools.callFunction(' + unloadNumber + ')}</script>' ].join( '' ) );
+					var inputId = _.frameId + '_input';
+
+					frameDocument.$.write( [
+						'<html dir="' + langDir + '" lang="' + langCode + '"><head><title></title></head><body style="margin: 0; overflow: hidden; background: transparent;">',
+							'<form enctype="multipart/form-data" method="POST" dir="' + langDir + '" lang="' + langCode + '" action="',
+								CKEDITOR.tools.htmlEncode( elementDefinition.action ),
+							'">',
+								// Replicate the field label inside of iframe.
+								'<label id="', _.labelId, '" for="', inputId, '" style="display:none">',
+									CKEDITOR.tools.htmlEncode( elementDefinition.label ),
+								'</label>',
+								'<input id="', inputId, '" aria-labelledby="', _.labelId, '" type="file" name="',
+									CKEDITOR.tools.htmlEncode( elementDefinition.id || 'cke_upload' ),
+									'" size="',
+									CKEDITOR.tools.htmlEncode( size > 0 ? size : "" ),
+								'" />',
+							'</form>',
+						'</body></html>',
+						'<script>',
+							// Support for custom document.domain in IE.
+							CKEDITOR.env.ie ? '(' + CKEDITOR.tools.fixDomain + ')();' : '',
+
+							'window.parent.CKEDITOR.tools.callFunction(' + callNumber + ');',
+							'window.onbeforeunload = function() {window.parent.CKEDITOR.tools.callFunction(' + unloadNumber + ')}',
+						'</script>'
+					].join( '' ) );
 
 					frameDocument.$.close();
 
diff --git a/plugins/docprops/dialogs/docprops.js b/plugins/docprops/dialogs/docprops.js
index d6704bacfd1..7828089f8da 100644
--- a/plugins/docprops/dialogs/docprops.js
+++ b/plugins/docprops/dialogs/docprops.js
@@ -146,13 +146,12 @@ CKEDITOR.dialog.add( 'docProps', function( editor ) {
 			};
 		};
 	var previewSrc = 'javascript:' +
-		'void((function(){' +
-			encodeURIComponent( 'document.open();' +
-		( CKEDITOR.env.isCustomDomain() ? 'document.domain=\'' + document.domain + '\';' : '' ) +
-		'document.write( \'<html style="background-color: #ffffff; height: 100%"><head></head><body style="width: 100%; height: 100%; margin: 0px">' + lang.previewHtml + '</body></html>\' );' +
-		'document.close();'
-		) +
-		'})())';
+		'void((function(){' + encodeURIComponent(
+			'document.open();' +
+			( CKEDITOR.env.ie ? '(' + CKEDITOR.tools.fixDomain + ')();' : '' ) +
+			'document.write( \'<html style="background-color: #ffffff; height: 100%"><head></head><body style="width: 100%; height: 100%; margin: 0px">' + lang.previewHtml + '</body></html>\' );' +
+			'document.close();'
+		) + '})())';
 
 	return {
 		title: lang.title,
diff --git a/plugins/panel/plugin.js b/plugins/panel/plugin.js
index dbce1fa92f5..61067b69978 100644
--- a/plugins/panel/plugin.js
+++ b/plugins/panel/plugin.js
@@ -163,10 +163,22 @@
 			};
 
 			if ( this.isFramed ) {
+				// With IE, the custom domain has to be taken care at first,
+				// for other browers, the 'src' attribute should be left empty to
+				// trigger iframe's 'load' event.
+				var src =
+					CKEDITOR.env.air ? 'javascript:void(0)' :
+					CKEDITOR.env.ie ? 'javascript:void(function(){' + encodeURIComponent(
+						'document.open();' +
+						// In IE, the document domain must be set any time we call document.open().
+						'(' + CKEDITOR.tools.fixDomain + ')();' +
+						'document.close();'
+					) + '}())' :
+					'';
+
 				data.frame = frameTpl.output({
 					id: this.id + '_frame',
-					src: 'javascript:void(document.open(),' + ( CKEDITOR.env.isCustomDomain() ? 'document.domain=\'' + document.domain + '\',' : '' )
-						+ 'document.close())">'
+					src: src
 				});
 			}
 
diff --git a/plugins/preview/plugin.js b/plugins/preview/plugin.js
index 9af5c6d3dbb..db577664a99 100644
--- a/plugins/preview/plugin.js
+++ b/plugins/preview/plugin.js
@@ -17,7 +17,6 @@
 			var sHTML,
 				config = editor.config,
 				baseTag = config.baseHref ? '<base href="' + config.baseHref + '"/>' : '',
-				isCustomDomain = CKEDITOR.env.isCustomDomain(),
 				eventData;
 
 			if ( config.fullPage ) {
@@ -63,15 +62,17 @@
 				return false;
 
 			var sOpenUrl = '';
-			if ( isCustomDomain ) {
+			if ( CKEDITOR.env.ie ) {
 				window._cke_htmlToLoad = eventData.dataValue;
 				sOpenUrl = 'javascript:void( (function(){' +
 					'document.open();' +
-					'document.domain="' + document.domain + '";' +
+					// Support for custom document.domain.
+					// Strip comments and replace parent with window.opener in the function body.
+					( '(' + CKEDITOR.tools.fixDomain + ')();' ).replace( /\/\/.*?\n/g, '' ).replace( /parent\./g, 'window.opener.' ) +
 					'document.write( window.opener._cke_htmlToLoad );' +
 					'document.close();' +
 					'window.opener._cke_htmlToLoad = null;' +
-					'})() )';
+				'})() )';
 			}
 
 			// With Firefox only, we need to open a special preview page, so
@@ -84,7 +85,7 @@
 			var oWindow = window.open( sOpenUrl, null, 'toolbar=yes,location=no,status=yes,menubar=yes,scrollbars=yes,resizable=yes,width=' +
 				iWidth + ',height=' + iHeight + ',left=' + iLeft );
 
-			if ( !isCustomDomain && !CKEDITOR.env.gecko ) {
+			if ( !CKEDITOR.env.ie && !CKEDITOR.env.gecko ) {
 				var doc = oWindow.document;
 				doc.open();
 				doc.write( eventData.dataValue );
diff --git a/plugins/wysiwygarea/plugin.js b/plugins/wysiwygarea/plugin.js
index 6232ef95f51..c6672cfa1ac 100644
--- a/plugins/wysiwygarea/plugin.js
+++ b/plugins/wysiwygarea/plugin.js
@@ -19,17 +19,9 @@
 			}
 
 			editor.addMode( 'wysiwyg', function( callback ) {
-				var iframe = CKEDITOR.document.createElement( 'iframe' );
-				iframe.setStyles({ width: '100%', height: '100%' } );
-				iframe.addClass( 'cke_wysiwyg_frame cke_reset' );
-
-				var contentSpace = editor.ui.space( 'contents' );
-				contentSpace.append( iframe );
-
 				var src = 'document.open();' +
-					// The document domain must be set any time we
-				// call document.open().
-				( isCustomDomain ? ( 'document.domain="' + document.domain + '";' ) : '' ) +
+					// In IE, the document domain must be set any time we call document.open().
+					( CKEDITOR.env.ie ? '(' + CKEDITOR.tools.fixDomain + ')();' : '' ) +
 					'document.close();';
 
 				// With IE, the custom domain has to be taken care at first,
@@ -39,6 +31,14 @@
 					:
 					'';
 
+				var iframe = CKEDITOR.dom.element.createFromHtml( '<iframe src="' + src + '" frameBorder="0"></iframe>' );
+				iframe.setStyles({ width: '100%', height: '100%' } );
+				iframe.addClass( 'cke_wysiwyg_frame cke_reset' );
+
+				var contentSpace = editor.ui.space( 'contents' );
+				contentSpace.append( iframe );
+
+
 				// Asynchronous iframe loading is only required in IE>8 and Gecko (other reasons probably).
 				// Do not use it on WebKit as it'll break the browser-back navigation.
 				var useOnloadEvent = CKEDITOR.env.ie || CKEDITOR.env.gecko;
@@ -63,10 +63,8 @@
 				});
 
 				iframe.setAttributes({
-					frameBorder: 0,
 					'aria-describedby' : labelId,
 					title: frameLabel,
-					src: src,
 					tabIndex: editor.tabIndex,
 					allowTransparency: 'true'
 				});
@@ -102,9 +100,6 @@
 		}
 	});
 
-	// Support for custom document.domain in IE.
-	var isCustomDomain = CKEDITOR.env.isCustomDomain();
-
 	function onDomReady( win ) {
 		var editor = this.editor,
 			doc = win.document,
@@ -412,7 +407,6 @@
 					// is fully editable even before the editing iframe is fully loaded (#4455).
 					var bootstrapCode =
 						'<script id="cke_actscrpt" type="text/javascript"' + ( CKEDITOR.env.ie ? ' defer="defer" ' : '' ) + '>' +
-							( isCustomDomain ? ( 'document.domain="' + document.domain + '";' ) : '' ) +
 							'var wasLoaded=0;' +	// It must be always set to 0 as it remains as a window property.
 							'function onload(){' +
 								'if(!wasLoaded)' +	// FF3.6 calls onload twice when editor.setData. Stop that.