xdg-dbus-proxy versions before 0.1.2-3.oe2403sp4 contain a vulnerability that allows bypassing message filtering on the D-Bus session bus by setting a reply serial number on non-reply messages. This flaw can be exploited by a malicious or compromised Flatpak app to execute arbitrary code outside its sandbox. The issue arises from incorrect message filtering implementation. A fix is available in version 0.1.2-3.oe2403sp4.