LowSSHBLWK-SSH-004

SSH X11 forwarding is enabled

What this means

X11Forwarding is set to “‹x11 forwarding›” in sshd_config. X11 forwarding exposes any local X server to whatever runs on the remote end of the session, and has a history of X11-protocol-level compromise (keystroke injection, screen capture).

How to fix it

Set X11Forwarding no in /etc/ssh/sshd_config and run systemctl restart sshd, unless remote GUI apps are a deliberate requirement.

Check it yourself

Run a scan and look for BLWK-SSH-004 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:

x11_forwarding == "yes"

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

References

• CIS Benchmark 5.2.4

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 TCP forwarding/tunneling is enabled · BLWK-SSH-005
• SSH permits user-set environment variables · BLWK-SSH-006
• SSH tun-device (layer 2/3) tunneling is enabled · BLWK-SSH-007