Jekyll through 3.6.2, 3.7.x through 3.7.3, and 3.8.x through 3.8.3 allows attackers to access arbitrary files by specifying a symlink in the "include" key in the "_config.yml" file.
https://jekyllrb.com/news/2018/09/19/security-fixes-for-3-6-3-7-3-8/