diff --git a/AjaxControlToolkit.Tests/HtmlEditorExtenderTests.cs b/AjaxControlToolkit.Tests/HtmlEditorExtenderTests.cs
index d99ca8ddd..c83db08d8 100644
--- a/AjaxControlToolkit.Tests/HtmlEditorExtenderTests.cs
+++ b/AjaxControlToolkit.Tests/HtmlEditorExtenderTests.cs
@@ -61,7 +61,7 @@ public void StripScriptTagWithoutAttributes() {
             var text = @" <script>";
             var actual = extender.Decode(text);
 
-            Assert.AreEqual("", actual);
+            Assert.AreEqual(" ", actual);
         }
 
         [Test]
@@ -78,5 +78,49 @@ public void DoNotStripDataEncodedImage() {
                 Assert.False(decodedText.Contains("data:text/javascript"));
             }
         }
+
+        [Test]
+        public void RemoveInsecureHtml() {
+            using(var html = new HtmlEditorExtender()) {
+                html.EnableSanitization = false;
+
+                // was "Z"
+                Assert.AreEqual("A", html.Decode("A<script>Z"));
+
+                // was "<script>Z"
+                Assert.AreEqual("", html.Decode("<script>Z"));
+
+                Assert.AreEqual("AZ", html.Decode("A<script attr>...</script attr>Z"));
+
+                // was "Hello world!alert()" - see https://github.com/DevExpress/AjaxControlToolkit/issues/525
+                Assert.AreEqual(
+                    "Hello world! ",
+                    html.Decode("Hello world! <script>alert();</script>")
+                );
+
+                // was "<style</style>" - see https://github.com/DevExpress/AjaxControlToolkit/issues/513
+                var issue513html = "<style><!-- body { font-family: Script; } --></style>";
+                Assert.AreEqual(issue513html, html.Decode(issue513html));
+
+                // was "<pClick me</a></p>"
+                Assert.AreEqual(
+                    "<p><a>Click me</a></p>",
+                    html.Decode("<p><a href='javascript:alert()'>Click me</a></p>")
+                );
+
+                var comment = "<!-- behavior of expression filter data:text/plain -->";
+                Assert.AreEqual(comment, html.Decode(comment));
+
+                Assert.AreEqual(
+                    "<p><p><p><p>",
+                    html.Decode(
+                        "<p style='width: expression(1)'>" +
+                        "<p style='position: absolute'>" +
+                        "<p style='filter: inherit'>" +
+                        "<p style='behavior: url(something.htc)>"
+                    )
+                );
+            }
+        }
     }
 }
diff --git a/AjaxControlToolkit/HtmlEditorExtender/HtmlEditorExtender.cs b/AjaxControlToolkit/HtmlEditorExtender/HtmlEditorExtender.cs
index ec3e2ff63..a9cb55470 100644
--- a/AjaxControlToolkit/HtmlEditorExtender/HtmlEditorExtender.cs
+++ b/AjaxControlToolkit/HtmlEditorExtender/HtmlEditorExtender.cs
@@ -173,23 +173,6 @@ public string Decode(string value) {
 
             result = Regex.Replace(result, "&amp;", "&", RegexOptions.IgnoreCase);
             result = Regex.Replace(result, "&nbsp;", "\xA0", RegexOptions.IgnoreCase);
-            result = Regex.Replace(result, "[^<]<[^>]*expression[^>]*>", "", RegexOptions.IgnoreCase | RegexOptions.ECMAScript);
-
-            result = Regex.Replace(result, "[^<]<([^>]*)(data\\:[^>]*)>", m => {
-                var tagGroup = m.Groups[1].Value.ToLower();
-                var urlGroup = m.Groups[2].Value.ToLower();
-
-                if(tagGroup.StartsWith("img") && urlGroup.StartsWith("data:image/"))
-                    return m.Value;
-
-                return "";
-            }, RegexOptions.IgnoreCase | RegexOptions.ECMAScript);
-
-            result = Regex.Replace(result, "[^<]<[^>]*script(?!\\w)[^>]*>", "", RegexOptions.IgnoreCase | RegexOptions.ECMAScript);
-            result = Regex.Replace(result, "[^<]<[^>]*filter[^>]*>", "", RegexOptions.IgnoreCase | RegexOptions.ECMAScript);
-            result = Regex.Replace(result, "[^<]<[^>]*behavior[^>]*>", "", RegexOptions.IgnoreCase | RegexOptions.ECMAScript);
-            result = Regex.Replace(result, "[^<]<[^>]*javascript\\:[^>]*>", "", RegexOptions.IgnoreCase | RegexOptions.ECMAScript);
-            result = Regex.Replace(result, "[^<]<[^>]*position\\:[^>]*>", "", RegexOptions.IgnoreCase | RegexOptions.ECMAScript);
 
             // Check Whether EnableSanitization is disabled or not.
             if(EnableSanitization && Sanitizer != null) {
@@ -205,6 +188,8 @@ public string Decode(string value) {
                     elementWhiteList.Add("img", new[] { "src" });
 
                 result = Sanitizer.GetSafeHtmlFragment(result, elementWhiteList);
+            } else {
+                result = RemoveInsecureHtml(result);
             }
 
             // HtmlAgilityPack vanishes self-closing <hr /> tag, so replace it after sanitization
@@ -213,6 +198,39 @@ public string Decode(string value) {
             return result;
         }
 
+        static string RemoveInsecureHtml(string html) {
+            var reFlags = RegexOptions.IgnoreCase | RegexOptions.Singleline;
+
+            html = Regex.Replace(html, @"<script[^>]*>.*?(</script[^>]*>|$)", "", reFlags);
+
+            html = Regex.Replace(html, @"<(/?[^\s>]+)([^>]*)>", m => {
+                var tag = m.Groups[1].Value;
+
+                if(tag.StartsWith("!--"))
+                    return m.Value;
+
+                var attrs = m.Groups[2].Value;
+                var dropAttrs = false;
+
+                // Non-image data URLs
+                dropAttrs = dropAttrs || Regex.IsMatch(attrs, @"\bdata:(?!image/)", reFlags);
+
+                // Insecure/harmful CSS props
+                dropAttrs = dropAttrs || Regex.IsMatch(attrs, @"\b(filter|behavior|position)\s*:", reFlags);
+                dropAttrs = dropAttrs || Regex.IsMatch(attrs, @":\s*expression\b", reFlags);
+
+                // JavaScript URLs
+                dropAttrs = dropAttrs || Regex.IsMatch(attrs, @"\bjavascript:", reFlags);
+
+                if(dropAttrs)
+                    return "<" + tag + ">";
+
+                return m.Value;
+            }, reFlags);
+
+            return html;
+        }
+
         // On Init add popup div and ajaxfileupload control to support Add image
         protected override void OnInit(EventArgs e) {
             base.OnInit(e);