MediumSSHBLWK-SSH-007

SSH tun-device (layer 2/3) tunneling is enabled

What this means

PermitTunnel is set to “‹permit tunnel›” in sshd_config. Unlike ordinary TCP port forwarding, tun-device tunneling gives an SSH client a full virtual network interface into this host's network — a much broader pivot/VPN-like capability that's rarely needed and should be off by default.

How to fix it

Set PermitTunnel no in /etc/ssh/sshd_config and run systemctl restart sshd.

Check it yourself

Run a scan and look for BLWK-SSH-007 in the results:

bulwark scan

Installed the standalone CLI package instead of the app? The command is bulwarkctl.

How Bulwark decides

Bulwark reads sshd_config and reports this check when:

permit_tunnel == "yes"

If a value can't be read, Bulwark reports the check as undetermined rather than passing it. View the rule source.

References

• MITRE ATT&CK T1572

Related checks

• SSH password authentication is enabled · BLWK-SSH-001
• SSH root login is permitted · BLWK-SSH-002
• SSH allows empty passwords · BLWK-SSH-003
• SSH X11 forwarding is enabled · BLWK-SSH-004
• SSH TCP forwarding/tunneling is enabled · BLWK-SSH-005
• SSH permits user-set environment variables · BLWK-SSH-006