The MailPoet Newsletters (wysija-newsletters) plugin before 2.6.7 for WordPress allows remote attackers to bypass authentication and execute arbitrary PHP code by uploading a crafted theme using wp-admin/admin-post.php and accessing the theme in wp-content/uploads/wysija/themes/mailp/.
https://wordpress.org/plugins/wysija-newsletters/changelog/
http://www.openwall.com/lists/oss-security/2014/07/08/7
http://blog.sucuri.net/2014/07/remote-file-upload-vulnerability-on-mailpoet-wysija-newsletters.html
http://blog.sucuri.net/2014/07/malware-infection-breaking-wordpress-sites.html