Vulnerability Discussion

The most important characteristic of a random number generator is its randomness, namely its ability to deliver random numbers that are impossible to predict. Entropy in computer security is associated with the unpredictability of a source of randomness. The random source with high entropy tends to achieve a uniform distribution of random values. Random number generators are one of the most important building blocks of cryptosystems.

The SSH implementation in RHEL 8 uses the OPENSSL library, which does not use high-entropy sources by default. By using the SSH_USE_STRONG_RNG environment variable the OPENSSL random generator is reseeded from /dev/random. This setting is not recommended on computers without the hardware random generator because insufficient entropy causes the connection to be blocked until enough entropy is available.

Check

Note: If the operating system is RHEL versions 8.0 or 8.1, this requirement is not applicable.

Verify the operating system SSH server uses strong entropy with the following command:

$ sudo grep -i ssh_use_strong_rng /etc/sysconfig/sshd

SSH_USE_STRONG_RNG=32

If the "SSH_USE_STRONG_RNG" line does not equal "32", is commented out, or is missing, this is a finding.

Fix

Configure the operating system SSH server to use strong entropy.

Add or modify the following line in the "/etc/sysconfig/sshd" file.

SSH_USE_STRONG_RNG=32

The SSH service must be restarted for changes to take effect.

Restart the SSH Daemon with the following command:

$ sudo systemctl restart sshd.service