Windows GUI Client config is user-writable

Summary

The Windows Firezone Client read its configuration from two locations that the logged-in user could write to:

• Advanced settings — the file advanced_settings.json (authentication URL, control-plane URL, log filter) stored under %LOCALAPPDATA%\dev.firezone.client\config.
• MDM policy — read from the per-user HKCU\Software\Policies\Firezone registry hive.

Both locations are writable by the logged-in user, and the values flow through to the privileged tunnel service, so a same-user process could modify the configuration the Client relied on.

Impact

An unprivileged process running as the same user could edit the advanced settings file, or write policy values into the per-user HKCU hive, to change how the Client connects — for example, redirecting the authentication and control-plane URLs to attacker-controlled infrastructure — without any elevation or user prompt. The attacker only needs to run code in the user's session.

Who is affected

Windows GUI Clients prior to 1.5.13. The Windows headless Client is not affected — it does not use the GUI's advanced settings file or read MDM policy from the registry.

Remediation

Upgrade to Windows GUI Client 1.5.13 or later.

1.5.13 moves ownership of the advanced settings into the tunnel service's configuration directory under C:\ProgramData\dev.firezone.client\config, protected by a DACL that grants access only to SYSTEM and Administrators, and reads MDM policy from the machine-scoped HKLM\Software\Policies\Firezone hive instead of HKCU. Existing advanced settings and per-user MDM values are migrated automatically on first upgrade.

Administrators who manage the Client through MDM must import the updated, machine-scoped policy template; see the Deploy the Clients guide.

Workarounds

There is no configuration-level workaround short of upgrading. Until you can upgrade, reduce exposure by keeping untrusted software off the machine.

Credits

Found and reported internally by the Firezone team.