[Bug]: #6876

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug that matches the one I want to file, without success.
• If this is an issue with a particular app, I have tried filing it in the appropriate issue tracker for the app (e.g. under https://github.com/flathub/) and determined that it is an issue with Flatpak itself.
• This issue is not a report of a security vulnerability (see here if you need to report a security issue).

Flatpak version

1.16.6

What Linux distribution are you using?

Other (specify below)

Linux distribution version

Kubuntu 26.04 LTS

What architecture are you using?

x86_64

How to reproduce

Platform: Ubuntu 26.04 (resolute), kernel 7.0
Flatpak 1.16.6-1, bubblewrap 0.11.1-1ubuntu0.3
sysctl: unprivileged_userns_clone=1, apparmor_restrict_unprivileged_userns=0
user.max_user_namespaces=20522

Reproduce: flatpak run org.mozilla.firefox
Trace:
F: Running 'bwrap --args 73 -- xdg-dbus-proxy --args=72'
F: Running 'bwrap --args 71 -- firefox'
[2] Sandbox: CanCreateUserNamespace() clone() failure

Expected Behavior

Username to Sandbox

Actual Behavior

Fails to create username

Additional Information

No response