Flatpak before 1.0.7, and 1.1.x and 1.2.x before 1.2.3, exposes /proc in the apply_extra script sandbox, which allows attackers to modify a host-side executable file.
https://github.com/flatpak/flatpak/releases/tag/1.2.3
https://github.com/flatpak/flatpak/releases/tag/1.0.7
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=922059
https://access.redhat.com/errata/RHSA-2019:0375
http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00088.html