CVE-2019-3902

Starting with version 1.5.3, Mercurial allows environment variable expansion on path names for sub repositories when creating it or cloning a parent repository, but it doesn’t validate whether the final path name outside the repository root directory. An attacker can leverage this weakness using a combination of symbolic links and environment variables to craft a tampered repository, leading Mercurial to write files outside the repository as long the destination location is empty.