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 RHEL8 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

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

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

$ 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 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.