WP AD Gallery <= 1.3 - Authenticated (Contributor+) Stored Cross-Site Scripting

The WP AD Gallery plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'startindex' parameter of the ad-gallery shortcode in all versions up to, and including, 1.3. This is due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.