In Mercurial before 4.4.1, it is possible that a specially malformed repository can cause Git subrepositories to run arbitrary code in the form of a .git/hooks/post-update script checked into the repository. Typical use of Mercurial prevents construction of such repositories, but they can be created programmatically.

The validaterepo function in sshpeer in Mercurial before 3.2.4 allows remote attackers to execute arbitrary commands via a crafted repository name in a clone command.

In Mercurial before 4.1.3, "hg serve --stdio" allows remote authenticated users to launch the Python debugger, and consequently execute arbitrary code, by using --debugger as a repository name.

Mercurial before 1.6.4 fails to verify the Common Name field of SSL certificates which allows remote attackers who acquire a certificate signed by a Certificate Authority to perform a man-in-the-middle attack.

A flaw was found in Mercurial before 4.9. It was possible to use symlinks and subrepositories to defeat Mercurial's path-checking logic and write files outside a repository.