Cis Ubuntu1604 V200 5 3 15

---
name: cis-ubuntu1604-v200-5-3-15
description: "Ensure only strong MAC algorithms are used"
category: cis-networking
version: "2.0.0"
author: cyberstrike-official
tags: [cis, ubuntu, linux, ubuntu-16.04, ssh, remote-access]
cis_id: "5.3.15"
cis_benchmark: "CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0"
tech_stack: [ubuntu, linux]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 5.3.15

## Description

This variable Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms must be comma-separated.

**Note:** Some organizations may have stricter requirements for approved MACs. Ensure that MACs used are in compliance with site policy.

## Rationale

MD5 and 96-bit MAC algorithms are considered weak and have been shown to increase exploitability in SSH downgrade attacks. Weak algorithms continue to have a great deal of attention as a weak spot that can be exploited with expanded computing power. An attacker that breaks the algorithm could take advantage of a MiTM position to decrypt the SSH tunnel and capture credentials and information.

## Audit Procedure

### Command Line

Run the following command and verify the output:

```bash
sshd -T -C user=root -C host="$(hostname)" -C addr="$(grep $(hostname) /etc/hosts | awk '{print $1}')" | grep -Ei '^\s*macs\s+([^#]+,)?(hmac-md5|hmac-md5-96|hmac-ripemd160|hmac-sha1|hmac-sha1-96|umac-64@openssh\.com|hmac-md5-etm@openssh\.com|hmac-md5-96-etm@openssh\.com|hmac-ripemd160-etm@openssh\.com|hmac-sha1-etm@openssh\.com|hmac-sha1-96-etm@openssh\.com|umac-64-etm@openssh\.com|umac-128-etm@openssh\.com)\b'
```

Nothing should be returned.

Run the following command and verify the output:

```bash
grep -Ei '^\s*macs\s+([^#]+,)?(hmac-md5|hmac-md5-96|hmac-ripemd160|hmac-sha1|hmac-sha1-96|umac-64@openssh\.com|hmac-md5-etm@openssh\.com|hmac-md5-96-etm@openssh\.com|hmac-ripemd160-etm@openssh\.com|hmac-sha1-etm@openssh\.com|hmac-sha1-96-etm@openssh\.com|umac-64-etm@openssh\.com|umac-128-etm@openssh\.com)\b' /etc/ssh/sshd_config
```

Nothing should be returned.

## Remediation

### Command Line

Edit the `/etc/ssh/sshd_config` file and add/modify the MACs line to contain a comma separated list of the site approved MACs.

Example:

```
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-256
```

## Default Value

MACs umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1

## References

1. More information on SSH downgrade attacks can be found here: http://www.mitls.org/pages/attacks/SLOTH
2. SSHD_CONFIG(5)

## Additional Information

Weak Protocol version 2 MAC algorithms:

```
hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1
hmac-sha1-96
umac-64@openssh.com
umac-128@openssh.com
hmac-md5-etm@openssh.com
hmac-md5-96-etm@openssh.com
hmac-ripemd160-etm@openssh.com
hmac-sha1-etm@openssh.com
hmac-sha1-96-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.com
```

"Strong" Protocol version 2 MACs currently FIPS 140-2 approved:

```
hmac-sha2-256 and hmac-sha2-512
```

## CIS Controls

Version 7

14.4 Encrypt All Sensitive Information in Transit - Encrypt all sensitive information in transit.

16.5 Encrypt Transmittal of Username and Authentication Credentials - Ensure that all account usernames and authentication credentials are transmitted across networks using encrypted channels.

## Profile Applicability

- Level 1 - Server
- Level 1 - Workstation

## Assessment Status

Automated