Title

RHEL 8 must ensure the SSH server uses strong entropy. (Cat III impact)

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 Content

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 Text

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

Additional Identifiers

Rule ID: SV-230253r1044799_rule

Vulnerability ID: V-230253

Group Title: SRG-OS-000480-GPOS-00227

Expert Comments

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