cis-ubuntu1204-v110-9-3-11

name	cis-ubuntu1204-v110-9-3-11
description	Use Only Approved Cipher in Counter Mode
category	cis-os-hardening
version	1.1.0
author	cyberstrike-official
tags	["cis","ubuntu",12.04,"linux","authentication","ssh","ciphers","encryption"]
cis_id	9.3.11
cis_benchmark	CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

9.3.11 Use Only Approved Cipher in Counter Mode (Scored)

Profile Applicability

• Level 1

Description

This variable limits the types of ciphers that SSH can use during communication.

Rationale

Based on research conducted at various institutions, it was determined that the symmetric portion of the SSH Transport Protocol (as described in RFC 4253) has security weaknesses that allowed recovery of up to 32 bits of plaintext from a block of ciphertext that was encrypted with the Cipher Block Chaining (CBC) method. From that research, new Counter mode algorithms (as described in RFC 4344) were designed that are not vulnerable to these types of attacks and these algorithms are now recommended for standard use.

Audit Procedure

Using Command Line

To verify the correct SSH setting, run the following command and verify that the output is as shown:

grep "Ciphers" /etc/ssh/sshd_config

Expected Result

Ciphers aes128-ctr,aes192-ctr,aes256-ctr

Remediation

Using Command Line

Edit the /etc/ssh/sshd_config file to set the parameter as follows:

Ciphers aes128-ctr,aes192-ctr,aes256-ctr

Default Value

Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc

References

• CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0
• RFC 4253 - The Secure Shell (SSH) Transport Layer Protocol
• RFC 4344 - The Secure Shell (SSH) Transport Layer Encryption Modes

Profile

Level 1 - Scored