This patch is for ALL versions of Zen Cart up to 1.5.6B
It affects product notifications which could give access to the database and open your database up to an SQL injection by a hacker. It is VERY IMPORTANT that you either get this patch installed, or upgrade to the latest version of Zen Cart that has the patch in it already.
It contains one file that does not overwrite any core files.  You can read more about it and / or download the file to upload yourself if you have the time and capability at this Zen Cart post.
If you need me to install it for you , you can find it here.