WordPress through 4.8.2, when domain-based flashmediaelement.swf sandboxing is not used, allows remote attackers to conduct cross-domain Flash injection (XSF) attacks by leveraging code contained within the wp-includes/js/mediaelement/flashmediaelement.swf file.
https://opnsec.com/2017/10/cve-2016-9263-unpatched-xsf-vulnerability-in-wordpress/