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://access.redhat.com/errata/RHSA-2019:2276
https://lists.debian.org/debian-lts-announce/2018/03/msg00034.html
https://lists.debian.org/debian-lts-announce/2018/07/msg00005.html
https://lists.debian.org/debian-lts-announce/2020/07/msg00032.html