In dbus-proxy/flatpak-proxy.c in Flatpak before 0.8.9, and 0.9.x and 0.10.x before 0.10.3, crafted D-Bus messages to the host can be used to break out of the sandbox, because whitespace handling in the proxy is not identical to whitespace handling in the daemon.
Red Hat Security Advisory 2018-2766-01 - Flatpak is a system for building, distributing, and running sandboxed desktop applications on Linux. Issues addressed include a sandbox escape.