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
As a contributor, put, the following shortcode in a page/post
[lightbox2 url='" onmouseover=javascript:alert(/XSS/) "']
The XSS will be triggered when viewing/previewing the page/post and moving the mouse over the "Click Here" text in the post