A exposure of sensitive information vulnerability exists in Jenkins GitHub Plugin 1.29.0 and older in GitHubServerConfig.java that allows attackers with Overall/Read access to connect to an attacker-specified URL using attacker-specified credentials IDs obtained through another method, capturing credentials stored in Jenkins.
https://github.com/advisories/GHSA-v7g7-cmxx-wxw9
https://jenkins.io/security/advisory/2018-06-04/#SECURITY-804