A vulnerability in xdg-dbus-proxy prior to version 0.1.2-3.oe2203sp4 allows attackers to bypass message filtering on the D-Bus session bus by setting a reply serial number on non-reply messages. This flaw can enable a malicious or compromised Flatpak application to execute arbitrary code outside its sandbox. xdg-dbus-proxy is used as a sandbox boundary component for Flatpak and other app frameworks such as Firejail. The issue is fixed in version 0.1.2-3.oe2203sp4.