Improper authorization in Nextcloud server 17.0.0 causes leaking of previews and files when a file-drop share link is opened via the gallery app.
https://nextcloud.com/security/advisory/?id=NC-SA-2019-012
https://hackerone.com/reports/719426
http://lists.opensuse.org/opensuse-security-announce/2020-02/msg00022.html
http://lists.opensuse.org/opensuse-security-announce/2020-02/msg00019.html