Flatpak Permissions on Upgrade, Unravelled

March 2, 2024

Flatpak has been around for a while, but I’ve not cared about it. I got interested in Fedora Silverblue as a ChromeOS replacement so have been trying Flatpak out on Arch.

It has a sandbox, which is nice, but what is the security model of the sandbox? I can see the permissions before installing a package, but what if those permissions change? Is the sandbox there to restrict the impact of a remote-code-execution of a hacked app, or is it there to protect me against the software packager? Glancing through pages wasn’t clear.

The main difference between the options is, “what happens when an app adds new permissions to its manifest?” If Flatpak happily upgrades it, then the sandbox trusts the software packager. But I didn’t find any documentation about how added permissions are handled.

I followed the org.flatpak.Hello example, building the app and adding it to a repository, then installing the app. I then rebuilt the app to see what the ordinary flow looked like:

$ flatpak update
Looking for updates…


        ID                  Branch   Op   Remote          Download
 1.     org.flatpak.Hello   master   u    tutorial-repo   < 601 bytes

Proceed with these changes to the user installation? [Y/n]:

I then appended to org.flatpak.Hello.yml:

finish-args:
  - --share=network

I rebuilt the app and then updated:

$ flatpak update
Looking for updates…

New org.flatpak.Hello permissions:
    network



        ID                  Branch   Op   Remote          Download
 1.     org.flatpak.Hello   master   u    tutorial-repo   < 647 bytes

Proceed with these changes to the user installation? [Y/n]:

Well, that’s good that it tells me, but it also isn’t the least bit scary and is easy to miss, especially if there are many updates. I’m also left wondering what happens with automatic updates. flatpak update --assumeyes is probably a bad idea (which isn’t mentioned in --help), but --noninteractive looks promising:

$ flatpak info --show-permissions org.flatpak.Hello
$ flatpak update --noninteractive
Updating app/org.flatpak.Hello/x86_64/master
$ flatpak info --show-permissions org.flatpak.Hello
[Context]
shared=network;

It happily accepts the new permissions without any notification. The output is identical to when permissions stay the same. I had expected it to either error or ignore the update. I’d gladly accept it making an override that denies the new permission, but it doesn’t do that either. I wasn’t really expecting that, because there wasn’t an option to do that interactively.

Now, you could argue “CLI is for advanced usage; use Gnome Software.” Unfortunately, this hello world example doesn’t show up at all in Gnome Software, so I couldn’t test. But if I search specifically for gnome software upgrade change permission then I can find Gnome Software issue 943: UX for Flatpak permission changes on update. It shows Gnome Software is in a good place, but it doesn’t fill me with confidence about Flatpak overall. If I had Discover installed and logged into KDE, would I still be okay?

Installs default to system-wide

Using flatpak list I can see apps installed in the “system” Installation. This is not clear in Gnome Software. Upgrading these apps changes the permissions for all users, yet I don’t get a “you are about to change your system” dialog before using elevated privileges. Gnome Software does sorta tell me if I go to Software Repositories and see “System Installation • 3 apps installed”.

Apparently it is system-wide because Flathub is configured in /etc/flatpak/remotes.d/. If the repo is system-wide, then the install is system-wide. I can accept that logic, but this was done by default by the Arch flatpak package. The Fedora package doesn’t include Flathub. Flatpak is aware of this in their setup docs: Arch vs Fedora. They encourage the system installation in their docs, and the flatpak CLI defaults to system installation.

Easy mistakes with flatpak CLI

I also discovered that flatpak permission-show and flatpak permissions don’t do what they would appear. They show only some of the permissions; the extra permissions granted, above those in the app’s manifest. They also report no warnings/errors if the app name is mispelled. Very easy to be misled.

flatpak override is poor as well, as there does not seem to be a way to list all overrides; you need to dig into ~/.local/share/flatpak/overrides, which at least is documented in man flatpak-override.

$ flatpak override --user --unshare=network org.flatpak.Hello
$ flatpak override
error: Permission denied
$ flatpak override --show
$ flatpak override --user
$ flatpak override --user --show
$ flatpak override --show org.flatpak.Hello
$ flatpak override --user --show org.flatpak.Hello
[Context]
shared=!network;

There’s no UIs for those things in Gnome Software. Given how much I’ve heard about the sandbox in Flatpak, I’m disappointed in the cavalier security usability stance.

How can a user accept permissions?

While doing the above I wondered about Flatseal, because it is particularly powerful in a “can attack you” sense. Flatseal lists “No Network Access” in its permissions in Gnome Software, but it also has an entry for “Arbitrary Permissions” with the description “Can acquire arbitrary permissions”. It seems in that case the various green “No $X access” should be removed.

How bad are other cases? If an app has access to “Pictures folder,” it seems that is probably mostly predictable. But isn’t it trivial to escape the sandbox with “Session Services” (--socket=session-bus) using systemd-run? How would a user know that “No Network Access” is meaningless when the service “Can access D-Bus services on the session bus”?

But also an app might have access to the session bus because of abstract Unix sockets. From the Flatpak Sandbox Permissions Reference:

Giving network access also grants access to all host services listening on abstract Unix sockets (due to how network namespaces work), and these have no permission checks. This unfortunately affects e.g. the X server and the session bus which listens to abstract sockets by default. A secure distribution should disable these and just use regular sockets.

On Arch, lsof -U | grep @ shows dbus-broker and gnome-session available as abstract sockets.

If an app has write access to the XDG config directory (--filesystem=xdg-config) it can write configuration for systemd to start a service on next login. Write access to the home directory can write .bashrc. Write access to all settings can change which program is executed with my custom keyboard shortcuts.

Having the sandbox in many of these cases seems useless, because it takes 10 minutes for an attacker to work around it. This isn’t even to the level that “defense in depth” becomes helpful. I think the only benefit of the sandbox for these is when developing an application and incrementally making your application sandbox-ready. But the user should be told there isn’t a sandbox. Even if it is nominally present, it isn’t providing its function.

Gnome Software does give such permissions red and yellow security levels, but the explanations of the permissions undersells their power and then acts like other permissions are still restricted.

Relatively small beans

When opening a file with a Flatpak app in Nautilus and xdg-open, my entire environment is copied onto the command line and visible through ps. I understand some OSes allow users to view each other’s environment and that secrets need to be shared through files or pipes. But this is still surprising.

By poking around with the CLI I noticed that every time I open a file in a portal or with File Roller it adds a new (permanent?) permission for the opening app. But Gnome Software doesn’t have a UI to show me the added permissions. And the permissions are kept even if I uninstall and reinstall the app.

I can clear out all permissions for a specific app using flatpak permission-reset APP_ID or all apps with flatpak permission-reset --all. Although the permission rows remains, listing each opened file, just with the app removed. To delete the row I need flatpak permission-remove TABLE ID, but each row has to be deleted individually.

Takeaways

I started with a very precise concern, didn’t get a clear answer, and now I have additional concerns. Before this investigation I had thought “if an app doesn’t have both network and data access, then that is progress.”

But I don’t think Flatpak is actually communicating the security impact of permissions. And while I now know the full severity of a longer list of risky permissions, what attacks am I not noticing? And do I actually trust Flatpak that a fully-locked-down “all green” app can’t easily escape given so much wasted effort on permissions elsewhere? (I accept that the sandbox can be escaped, but I want to avoid someone knowledgable only needing 10 minutes to break out.)

Fedora (and Silverblue) doesn’t use Flathub by default. That seems like a wise decision. On that system I could use Flatpak without much worry as it is just a different package format for the distro.

Looking through the ~20 apps I’m interested in, all of them show red in Gnome Software except for two yellows: “Calculator” and “Connections,” both of which have network access (Calculator looks up currency exchange rates). Flathub is looking like just another distro but with each user needing to vet most packagers.

Eric Anderson

Archive