Brief.
We have issued a XSS fix for the common_functions.php script for releases 3.*.
 
Solution.
Please download an updated version of the file for your version from here:
http://www.viart.com/downloads/common_functions-3.0.zip
http://www.viart.com/downloads/common_functions-3.1.zip
http://www.viart.com/downloads/common_functions-3.2.zip
http://www.viart.com/downloads/common_functions-3.3.zip
Then extract the common_functions.php file into the 'includes' folder of your shop replacing existing ones. Don't forget to make a backup copy of the current files in case something goes wrong.