Title

The SSH daemon must be configured to not use Cipher-Block Chaining (CBC) ciphers. (Cat II impact)

Discussion

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

Check Content

Check the SSH daemon configuration for allowed ciphers. Note that keywords are case-insensitive and arguments (args) are case-sensitive. keyword=Ciphers arg(s)=<comma separated cipher(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 <keyword=arg> 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/sshd_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

Edit the configuration file and remove any ciphers other than those with the "aes" prefix and the "-ctr" suffix.

Additional Identifiers

Rule ID: SV-35218r1_rule

Vulnerability ID: V-22459

Group Title: GEN005506

Expert Comments

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