GS Books Showcase < 1.3.1 - Contributor+ Stored XSS

The plugin does not validate and escape some of its shortcode attributes before outputting them back in a page/post where the shortcode is embed, which could allow users with the contributor role and above to perform Stored Cross-Site Scripting attacks.

PoC

[gs_book_showcase theme=‘" onmouseover=“alert(1)” style=“background:red;width:100px;height:100px;”’]