diff --git a/app/assets/javascripts/discourse/lib/markdown.js b/app/assets/javascripts/discourse/lib/markdown.js
index cee428cad..a28496cdb 100644
--- a/app/assets/javascripts/discourse/lib/markdown.js
+++ b/app/assets/javascripts/discourse/lib/markdown.js
@@ -164,6 +164,9 @@ Discourse.Markdown = {
   urlAllowed: function (uri, effect, ltype, hints) {
     var url = typeof(uri) === "string" ? uri : uri.toString();
 
+    // escape single quotes
+    url = url.replace(/'/g, "'");
+
     // whitelist some iframe only
     if (hints && hints.XML_TAG === "iframe" && hints.XML_ATTR === "src") {
       for (var i = 0, length = _validIframes.length; i < length; i++) {
diff --git a/test/javascripts/lib/markdown_test.js b/test/javascripts/lib/markdown_test.js
index c557b7f23..b40e2a779 100644
--- a/test/javascripts/lib/markdown_test.js
+++ b/test/javascripts/lib/markdown_test.js
@@ -401,14 +401,20 @@ test("URLs in BBCode tags", function() {
 });
 
 test("urlAllowed", function() {
+  var urlAllowed = Discourse.Markdown.urlAllowed;
+
   var allowed = function(url, msg) {
-    equal(Discourse.Markdown.urlAllowed(url), url, msg);
+    equal(urlAllowed(url), url, msg);
   };
 
   allowed("/foo/bar.html", "allows relative urls");
   allowed("http://eviltrout.com/evil/trout", "allows full urls");
   allowed("https://eviltrout.com/evil/trout", "allows https urls");
   allowed("//eviltrout.com/evil/trout", "allows protocol relative urls");
+
+  equal(urlAllowed("http://google.com/test'onmouseover=alert('XSS!');//.swf"),
+        "http://google.com/test&#39;onmouseover=alert(&#39;XSS!&#39;);//.swf",
+        "escape single quotes");
 });
 
 test("images", function() {