Meks Flexible Shortcodes < 1.3.5 - Contributor+ Stored XSS

The plugin does not validate and escape some of its shortcode attributes before outputting them back in the page, which could allow users with a role as low as contributor to perform Stored Cross-Site Scripting attacks which could be used against high privilege users such as admins.

PoC

Exploit: [mks_button txt_color=‘" onmouseover=“alert(1)”’]