Bg Book Publisher <= 1.25 - Authenticated (Contributor+) Stored Cross-Site Scripting

The Bg Book Publisher plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the `book_author` post meta, rendered through the `[book_author]` shortcode, in all versions up to, and including, 1.25. This is due to the plugin not properly escaping the meta value before output. 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.