1. Problem with /ajax.php in v1.5.4 only - Severity: High
In Zen Cart v1.5.4 the /ajax.php file has a vulnerability which can be used to cause a server exploit under very specific conditions.
Requires replacing the ajax.php file with new one

Below are some additional lower-severity patches affecting prior versions, which should be reviewed carefully for your site, to merge with existing customizations you may have made:

2. XSS problem for unsanitized comment field - Severity: Medium
In Zen Cart versions up to and including v1.5.4 an XSS problem exists with the order-comments field.
XSS problems are where someone can drop in executable/javascript code that can cause problems later when that content is output back to the screen.
Requires patching /includes/modules/pages/checkout_confirmation/header_php.php,

3. Failed customer login puts password back in input box - Severity: Low
When attempting a login with an invalid password, the resulting response contains that invalid password.
Requires patching /includes/functions/html_output.php file