Cis Gcp Cos 5.1.14

---
name: cis-gcp-cos-5.1.14
description: "Ensure only strong MAC algorithms are used"
category: cis-gcp-cos
version: "1.2.0"
author: cyberstrike-official
tags: [cis, gcp, container-optimized-os, ssh, sshd, sshd-config, encryption, mac]
cis_id: "5.1.14"
cis_benchmark: "CIS Google Container-Optimized OS Benchmark v1.2.0"
tech_stack: [gcp, linux]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# 5.1.14 Ensure only strong MAC algorithms are used (Automated)

## Description

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

## 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

Run the following command and verify that output does not contain any of the listed weak MAC algorithms:

```bash
# sshd -T | grep -i "MACs"
```

Weak 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
```

## Expected Result

Output should not contain any of the listed weak MAC algorithms.

## Remediation

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

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

The only "strong" MACs currently FIPS 140-2 approved are hmac-sha2-256 and hmac-sha2-512.

The Supported MACs are:

```
hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
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
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.com
```

## CIS Controls

| Controls Version | Control                                                             | IG 1 | IG 2 | IG 3 |
| ---------------- | ------------------------------------------------------------------- | ---- | ---- | ---- |
| v8               | 3.10 Encrypt Sensitive Data in Transit                              |      | x    | x    |
| v7               | 14.4 Encrypt All Sensitive Information in Transit                   |      | x    | x    |
| v7               | 16.5 Encrypt Transmittal of Username and Authentication Credentials |      | x    | x    |

## Profile

- Level 2 - Server