The Wordpress plugin "MailPoet Newsletters" (wysija-newsletters) before 2.6.8 is vulnerable to an unauthenticated file upload. The exploit uses the Upload Theme functionality to upload a zip file containing the payload. The plugin used the admin_init hook, which is also executed for unauthenticated users when accessing a specific URL. The developers tried to fix the vulnerability in version 2.6.7 but the fix can be bypassed. In PHPs default configuration, a POST variable overwrites a GET variable in the $_REQUEST array. The plugin uses $_REQUEST to check for access rights. By setting the POST parameter to something not beginning with 'wysija_', the check is bypassed. Wordpress uses the $_GET array to determine the page and is so not affected by this.
ce2cffe8515677c0d219f665bad07fe8ecea2cce4c18e01fcea51556c3c8c876