MediumSSHBLWK-SSH-006

SSH permits user-set environment variables

What this means

PermitUserEnvironment is set to “‹permit user environment›” in sshd_config, letting an SSH client set arbitrary environment variables for its session (via ~/.ssh/environment or SendEnv) — including ones like LD_PRELOAD that can hijack the behavior of programs the session later runs.

How to fix it

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

Check it yourself

Run a scan and look for BLWK-SSH-006 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_user_environment == "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 5.2.6

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 tun-device (layer 2/3) tunneling is enabled · BLWK-SSH-007