Improper Input Validation in Nextcloud Server 15.0.7 allows group admins to create users with IDs of system folders.
https://nextcloud.com/security/advisory/?id=NC-SA-2019-015
https://hackerone.com/reports/508493
http://lists.opensuse.org/opensuse-security-announce/2020-02/msg00022.html
http://lists.opensuse.org/opensuse-security-announce/2020-02/msg00019.html