An issue was discovered in the gon gem before gon-6.4.0 for Ruby. MultiJson does not honor the escape_mode parameter to escape fields as an XSS protection mechanism. To mitigate, json_dumper.rb in gon now does escaping for XSS by default without relying on MultiJson.
Ubuntu Security Notice 4560-1 - It was discovered that Gon gem did not properly escape certain input. An attacker could use this vulnerability to execute a cross-site scripting attack.