source: https://www.securityfocus.com/bid/29133/info

BlogPHP is prone to multiple input-validation vulnerabilities, including a cross-site scripting issue, an HTML-injection issue, and a cookie-manipulation issue.

Attackers can exploit these issues to execute arbitrary script code in the context of the webserver, compromise the application, steal cookie-based authentication credentials from legitimate users of the site, modify the way the site is rendered, and gain access as an arbitrary user.

BlogPHP 2.0 is vulnerable; other versions may also be affected. 

http://www.example.com/index.php?act=sendmessage&user=admin[XSS]