An incorrect implementation of message filtering in xdg-dbus-proxy versions before 0.1.9 allows an attacker to bypass the intended message filtering on the D-Bus session bus by setting a reply serial number on non-reply messages.

xdg-dbus-proxy was designed to be part of the sandbox boundary for Flatpak, but it is released as a separate project and is sometimes used by other app frameworks such as Firejail.

A malicious or compromised Flatpak app could achieve arbitrary code execution outside its sandbox.

If other app frameworks rely on xdg-dbus-proxy in the same way that Flatpak does, then they will have an equivalent vulnerability until xdg-dbus-proxy is updated.