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.
https://github.com/flatpak/flatpak/releases/tag/0.8.9
https://github.com/flatpak/flatpak/releases/tag/0.10.3
https://github.com/flatpak/flatpak/commit/52346bf187b5a7f1c0fe9075b328b7ad6abe78f6