Description
Git Plugin connects to a user-specified Git repository as part of form validation. An attacker with no direct access to Jenkins but able to guess at a username/password credentials ID could trick a developer with job configuration permissions into following a link with a maliciously crafted Jenkins URL which would result in the Jenkins Git client sending the username and password to an attacker-controlled server.
Remediation
References
http://www.securityfocus.com/bid/100435
https://jenkins.io/security/advisory/2017-07-10/