A flaw was found in Jenkins weekly up to and including 2.106 and Jenkins LTS up to and including 2.89.3. The form validation for the proxy configuration form did not check the permission of the user accessing it, allowing anyone with Overall/Read access to Jenkins to cause Jenkins to send a GET request to a specified URL, optionally with a specified proxy configuration.

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. It was not possible to reuse an existing proxy configuration to send those requests; that configuration had to be provided by the attacker.


References:
https://jenkins.io/security/advisory/2018-02-14/