Mercurial version 4.5 and earlier contains a Incorrect Access Control (CWE-285) vulnerability in Protocol server that can result in Unauthorized data access. This attack appear to be exploitable via network connectivity. This vulnerability appears to have been fixed in 4.5.1.
https://www.mercurial-scm.org/wiki/WhatsNew#Mercurial_4.5.1_.2F_4.5.2_.282018-03-06.29
https://lists.debian.org/debian-lts-announce/2020/07/msg00032.html
https://lists.debian.org/debian-lts-announce/2018/07/msg00005.html
https://lists.debian.org/debian-lts-announce/2018/03/msg00034.html