CVE-2024-42472

Flatpak is a Linux application sandboxing and distribution framework. Prior
to versions 1.14.0 and 1.15.10, a malicious or compromised Flatpak app
using persistent directories could access and write files outside of what
it would otherwise have access to, which is an attack on integrity and
confidentiality.
When persistent=subdir is used in the application permissions
(represented as --persist=subdir in the command-line interface), that
means that an application which otherwise doesn’t have access to the real
user home directory will see an empty home directory with a writeable
subdirectory subdir. Behind the scenes, this directory is actually a bind
mount and the data is stored in the per-application directory as
~/.var/app/$APPID/subdir. This allows existing apps that are not aware of
the per-application directory to still work as intended without general
home directory access.
However, the application does have write access to the application
directory ~/.var/app/$APPID where this directory is stored. If the source
directory for the persistent/--persist option is replaced by a symlink,
then the next time the application is started, the bind mount will follow
the symlink and mount whatever it points to into the sandbox.
Partial protection against this vulnerability can be provided by patching
Flatpak using the patches in commits ceec2ffc and 98f79773. However, this
leaves a race condition that could be exploited by two instances of a
malicious app running in parallel. Closing the race condition requires
updating or patching the version of bubblewrap that is used by Flatpak to
add the new --bind-fd option using the patch and then patching Flatpak to
use it. If Flatpak has been configured at build-time with
-Dsystem_bubblewrap=bwrap (1.15.x) or --with-system-bubblewrap=bwrap
(1.14.x or older), or a similar option, then the version of bubblewrap that
needs to be patched is a system copy that is distributed separately,
typically /usr/bin/bwrap. This configuration is the one that is typically
used in Linux distributions. If Flatpak has been configured at build-time
with -Dsystem_bubblewrap= (1.15.x) or with --without-system-bubblewrap
(1.14.x or older), then it is the bundled version of bubblewrap that is
included with Flatpak that must be patched. This is typically installed as
/usr/libexec/flatpak-bwrap. This configuration is the default when
building from source code.
For the 1.14.x stable branch, these changes are included in Flatpak
1.14.10. The bundled version of bubblewrap included in this release has
been updated to 0.6.3. For the 1.15.x development branch, these changes are
included in Flatpak 1.15.10. The bundled version of bubblewrap in this
release is a Meson “wrap” subproject, which has been updated to 0.10.0. The
1.12.x and 1.10.x branches will not be updated for this vulnerability.
Long-term support OS distributions should backport the individual changes
into their versions of Flatpak and bubblewrap, or update to newer versions
if their stability policy allows it. As a workaround, avoid using
applications using the persistent (--persist) permission.