The SSH client must be configured to not use Cipher-Block Chaining (CBC) based ciphers.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-22462	GEN005511	SV-35193r1_rule	ECSC-1	Medium

Description
The CBC mode of encryption as implemented in the SSHv2 protocol is vulnerable to chosen plaintext attacks and must not be used.

STIG	Date
HP-UX 11.23 Security Technical Implementation Guide	2012-05-25

Details

Check Text ( C-36637r1_chk )
Check the SSH client configuration for allowed ciphers. Note that keywords are case-insensitive and arguments (args) are case-sensitive. keyword=Ciphers arg(s)= Default values include: "aes128-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr,3des-cbc,arcfour,arcfour128,arcfour256blowfish-cbc,cast128-cbc". For this check, the only allowed keyword values are those from the above list with the "aes" prefix and the "-ctr" suffix. Note: When the default "arg" value exactly matches the required "arg" value (see above), the entry is not required to exist (commented or uncommented) in the ssh (client) or sshd (server) configuration file. While not required, it is recommended that the configuration file(s) be populated with all keywords and assigned arg values as a means to explicitly document the ssh(d) binary's expected behavior. Examine the file. # cat /opt/ssh/etc/ssh_config \| tr '\011' ' ' \| tr -s ' ' \| sed -e 's/^[ \t]*//' \| grep -v '^#' \| grep -i "ciphers" If the returned ciphers list contains any cipher other than those with the "aes" prefix and the "-ctr" suffix, this is a finding.

Check Text ( C-36637r1_chk )

Check the SSH client configuration for allowed ciphers. Note that keywords are case-insensitive and arguments (args) are case-sensitive.

keyword=Ciphers
arg(s)=

Default values include: "aes128-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr,3des-cbc,arcfour,arcfour128,arcfour256blowfish-cbc,cast128-cbc".

For this check, the only allowed keyword values are those from the above list with the "aes" prefix and the "-ctr" suffix.

Note: When the default "arg" value exactly matches the required "arg" value (see above), the entry is not required to exist (commented or uncommented) in the ssh (client) or sshd (server) configuration file. While not required, it is recommended that the configuration file(s) be populated with all keywords and assigned arg values as a means to explicitly document the ssh(d) binary's expected behavior.

Examine the file.
# cat /opt/ssh/etc/ssh_config | tr '\011' ' ' | tr -s ' ' | sed -e 's/^[ \t]*//' | grep -v '^#' | grep -i "ciphers"

If the returned ciphers list contains any cipher other than those with the "aes" prefix and the "-ctr" suffix, this is a finding.

Fix Text (F-32008r1_fix)
Edit the configuration file and remove any ciphers other than those with the "aes" prefix and the "-ctr" suffix.