source: http://www.securityfocus.com/bid/10209/info

It has been reported that Advanced Guestbook is prone to a SQL injection vulnerability that could allow an attacker to gain administrative access to the application.

This issue is reported to exist in Advanced Guestbook 2.2, however, it is possible that other versions are affected as well.

JQ <idiosyncrasie@xs4all.nl> explains that it is possible to trigger this issue by leaving the username entry blank and entering the following string in the password field:

') OR ('a' = 'a

Spy Hat <spyhat@spyhat.com> comments that it is also possible to leverage this issue by leaving the password field blank and entering the following string into the username field:

? or 1=1 --