The remote server is hosting an outdated installation of WordPress that is vulnerable to a cross-site scripting (XSS) attack.
Versions of WordPress prior to 4.3.1 are susceptible to a flaw that allows a cross-site scripting (XSS) attack. This flaw exists because the 'wp-includes/class-wp-theme.php' script does not validate input before returning it to users in error messages. This may allow a remote attacker to create a specially crafted request that would execute arbitrary script code in a user's browser session within the trust relationship between their browser and the server. (CVE-2016-1564)
Upgrade to WordPress 4.4.1, or later.