CVE-2026-94422: xdg-dbus-proxy: message filtering bypass via reply serial allows sandbox escape

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.

Impact: 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.

Fixed in 0.1.9 by commits: e5702fca4dba9600721921fbca2dbc39dc5ca400 "proxy: Don't assume that only returns and errors have a reply-serial" fc027f759316fb2a6c45648200b6f100202eb84e "proxy: Make it clearer which direction messages are going in" e4465a0dfe96da3b39929a30a1ac3a22b16223e3 "proxy: Only allow replies to go to the correct destination"

Reported by @refi64.

Reference: https://github.com/flatpak/xdg-dbus-proxy/security/advisories/GHSA-2cgv-pwcq-wvpq