ajax.functions.php in the MailUp plugin before 1.3.3 for WordPress does not properly restrict access to unspecified Ajax functions, which allows remote attackers to modify plugin settings and conduct cross-site scripting (XSS) attacks by setting the wordpress_logged_in cookie. NOTE: this is due to an incomplete fix for a similar issue that was fixed in 1.3.2.
https://exchange.xforce.ibmcloud.com/vulnerabilities/82847
http://www.securityfocus.com/bid/58467
http://wordpress.org/extend/plugins/wp-mailup/changelog/