source: https://www.securityfocus.com/bid/18666/info

OpenGuestbook is prone to multiple input-validation vulnerabilities, including cross-site scripting and SQL-injection issues, because the application fails to properly sanitize user-supplied input.

A successful exploit of these vulnerabilities could allow an attacker to compromise the application, access or modify data, steal cookie-based authentication credentials, or even exploit vulnerabilities in the underlying database implementation. Other attacks are also possible.

http://www.example.com/openguestbook/view.php?offset=[SQL]