The SSH client must be configured to not use CBC-based ciphers.

From VMware ESXi Server 5.0 Security Technical Implementation Guide

Part of SRG-OS-000157

Associated with: CCI-001127

SV-51260r2_rule The SSH client must be configured to not use CBC-based ciphers.

Vulnerability discussion

The Cipher-Block Chaining (CBC) mode of encryption as implemented in the SSHv2 protocol is vulnerable to chosen plain text attacks and must not be used.

Check content

Disable lock down mode. Enable the ESXi Shell. Check the SSH client configuration for allowed ciphers. # grep -i ciphers /etc/ssh/ssh_config | grep -v '^#' Re-enable lock down mode. If the returned ciphers list contains any cipher ending with cbc, this is a finding. If the /etc/ssh/ssh_config file does not exist or the Ciphers option is not set, this is not a finding.

Fix text

Disable lock down mode. Enable the ESXi Shell. Edit the SSH client configuration and add/modify the "Ciphers" configuration (examples of disallowed ciphers: aes128-cbc, aes192-cbc, aes256-cbc, arcfour256blowfish-cbc, cast128-cbc, 3des-cbc). # vi /etc/ssh/ssh_config Re-enable lock down mode.

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