diff --git a/CHANGELOG b/CHANGELOG
index 92ee97ca6..0f40a12b0 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -82,7 +82,8 @@ CHANGELOG Roundcube Webmail
 - Fix so links over images are not removed in plain text signatures converted from HTML (#4473)
 - Fix various issues when downloading files with names containing non-ascii chars, use RFC 2231 (#5772)
 - Fix PHP Warning: Use of undefined constant IDNA_DEFAULT on systems without php-intl (#6244)
-- Fix bug where some parts of quota information could be ignored (#6280)
+- Fix bug where some parts of quota information could have been ignored (#6280)
+- Fix bug where some escape sequences in html styles could bypass security checks
 
 RELEASE 1.3.6
 -------------
diff --git a/program/lib/Roundcube/rcube_utils.php b/program/lib/Roundcube/rcube_utils.php
index 5fcb4476e..f010ff117 100644
--- a/program/lib/Roundcube/rcube_utils.php
+++ b/program/lib/Roundcube/rcube_utils.php
@@ -518,7 +518,8 @@ class rcube_utils
 
         $out = html_entity_decode(html_entity_decode($content));
         $out = trim(preg_replace('/(^<!--|-->$)/', '', trim($out)));
-        $out = preg_replace_callback('/\\\([0-9a-f]{2,4})\s*/i', $callback, $out);
+        $out = preg_replace_callback('/\\\([0-9a-f]{2,6})\s*/i', $callback, $out);
+        $out = preg_replace('/\\\([^0-9a-f])/i', '\\1', $out);
         $out = preg_replace('#/\*.*\*/#Ums', '', $out);
         $out = strip_tags($out);
 
diff --git a/program/lib/Roundcube/rcube_washtml.php b/program/lib/Roundcube/rcube_washtml.php
index d8b12f53e..fff1f44e8 100644
--- a/program/lib/Roundcube/rcube_washtml.php
+++ b/program/lib/Roundcube/rcube_washtml.php
@@ -246,8 +246,11 @@ class rcube_washtml
         // Remove unwanted white-space characters so regular expressions below work better
         $style = preg_replace('/[\n\r\s\t]+/', ' ', $style);
 
+        // Decode insecure character sequences
+        $style = rcube_utils::xss_entity_decode($style);
+
         foreach (explode(';', $style) as $declaration) {
-            if (preg_match('/^\s*([a-z\-]+)\s*:\s*(.*)\s*$/i', $declaration, $match)) {
+            if (preg_match('/^\s*([a-z\\\-]+)\s*:\s*(.*)\s*$/i', $declaration, $match)) {
                 $cssid = $match[1];
                 $str   = $match[2];
                 $value = '';
diff --git a/tests/Framework/Utils.php b/tests/Framework/Utils.php
index 205874caa..9c3857e1f 100644
--- a/tests/Framework/Utils.php
+++ b/tests/Framework/Utils.php
@@ -203,12 +203,15 @@ class Framework_Utils extends PHPUnit_Framework_TestCase
         $mod = rcube_utils::mod_css_styles("left:exp/*  */ression( alert(&#039;xss3&#039;) )", 'rcmbody');
         $this->assertEquals("/* evil! */", $mod, "Don't allow encoding quirks");
 
-        $mod = rcube_utils::mod_css_styles("background:\\0075\\0072\\006c( javascript:alert(&#039;xss&#039;) )", 'rcmbody');
+        $mod = rcube_utils::mod_css_styles("background:\\0075\\0072\\00006c( javascript:alert(&#039;xss&#039;) )", 'rcmbody');
         $this->assertEquals("/* evil! */", $mod, "Don't allow encoding quirks (2)");
 
         $mod = rcube_utils::mod_css_styles("background: \\75 \\72 \\6C ('/images/img.png')", 'rcmbody');
         $this->assertEquals("/* evil! */", $mod, "Don't allow encoding quirks (3)");
 
+        $mod = rcube_utils::mod_css_styles("background: u\\r\\l('/images/img.png')", 'rcmbody');
+        $this->assertEquals("/* evil! */", $mod, "Don't allow encoding quirks (4)");
+
         // position: fixed (#5264)
         $mod = rcube_utils::mod_css_styles(".test { position: fixed; }", 'rcmbody');
         $this->assertEquals("#rcmbody .test { position: absolute; }", $mod, "Replace position:fixed with position:absolute (0)");
@@ -265,6 +268,9 @@ class Framework_Utils extends PHPUnit_Framework_TestCase
         $mod = rcube_utils::xss_entity_decode('#foo:after{content:"\003Cimg/src=x onerror=alert(2)>";}');
         $this->assertNotContains('<img', $mod, "Strip (encoded) tags from content property");
 
+        $mod = rcube_utils::xss_entity_decode("background: u\\r\\00006c('/images/img.png')");
+        $this->assertContains("url(", $mod, "Escape sequences resolving");
+
         // #5747
         $mod = rcube_utils::xss_entity_decode('<!-- #foo { content:css; } -->');
         $this->assertContains('#foo', $mod, "Strip HTML comments from content, but not the content");
diff --git a/tests/Framework/Washtml.php b/tests/Framework/Washtml.php
index ed523d23a..397eb718b 100644
--- a/tests/Framework/Washtml.php
+++ b/tests/Framework/Washtml.php
@@ -377,6 +377,7 @@ class Framework_Washtml extends PHPUnit_Framework_TestCase
             array("<link href=\"src:abc\">", false),
             array("<img src=\"http://TRACKING_URL/\">", true),
             array("<img src=\"data:image\">", false),
+            array('<p style="backgr\\ound-image: \\ur\\l(\'http://TRACKING_URL\')"></p>', true),
         );
 
         foreach ($html as $item) {
diff --git a/tests/MailFunc.php b/tests/MailFunc.php
index 8f951863a..2689001e2 100644
--- a/tests/MailFunc.php
+++ b/tests/MailFunc.php
@@ -216,7 +216,7 @@ class MailFunc extends PHPUnit_Framework_TestCase
       $body = rcmail_print_body($html, $this->get_html_part(), array('safe' => false, 'plain' => false));
 
       $this->assertNotContains('onerror=alert(1)//">test', $body);
-      $this->assertContains('<a style="x: &gt;&lt;img src=x onerror=alert(1)//"', $body);
+      $this->assertContains('<a style="x: &gt;"', $body);
     }
 
     /**