Mercurial prior to 4.3 did not adequately sanitize hostnames passed to ssh, leading to possible shell-injection attacks.
https://www.mercurial-scm.org/wiki/WhatsNew#Mercurial_4.3_.2F_4.3.1_.282017-08-10.29
https://security.gentoo.org/glsa/201709-18
https://access.redhat.com/errata/RHSA-2017:2489