Cross-site scripting (XSS) vulnerability in wp-includes/pluggable.php in WordPress before 3.9.2, when Multisite is enabled, allows remote authenticated administrators to inject arbitrary web script or HTML, and obtain Super Admin privileges, via a crafted avatar URL.
http://openwall.com/lists/oss-security/2014/08/13/3
https://core.trac.wordpress.org/changeset/29398