A form validation method in GitHub Plugin did not check the permission of the user accessing it, allowing anyone with Overall/Read access to Jenkins to cause Jenkins to send a POST request to a specified URL. If that request’s HTTP response code indicates success, the form validation is returning a generic success message, otherwise the HTTP status code is returned. Additionally, this form validation method did not require POST requests, resulting in a CSRF vulnerability. External References: https://jenkins.io/security/advisory/2018-06-04/