Cis Ubuntu1604 V200 5 3 16

---
name: cis-ubuntu1604-v200-5-3-16
description: "Ensure only strong Key Exchange 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: "5.3.16"
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 5.3.16

## Description

Key exchange is any method in cryptography by which cryptographic keys are exchanged between two parties, allowing use of a cryptographic algorithm. If the sender and receiver wish to exchange encrypted messages, each must be equipped to encrypt messages to be sent and decrypt messages received.

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

## Rationale

Key exchange methods that are considered weak should be removed. A key exchange method may be weak because too few bits are used or the hashing algorithm is considered too weak. Using weak algorithms could expose connections to man-in-the-middle attacks.

## 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*kexalgorithms\s+([^#]+,)?(diffie-hellman-group1-sha1|diffie-hellman-group14-sha1|diffie-hellman-group-exchange-sha1)\b'
```

Nothing should be returned.

Run the following command and verify the output:

```bash
grep -Ei '^\s*kexalgorithms\s+([^#]+,)?(diffie-hellman-group1-sha1|diffie-hellman-group14-sha1|diffie-hellman-group-exchange-sha1)\b' /etc/ssh/sshd_config
```

Nothing should be returned.

## Remediation

### Command Line

Edit the /etc/ssh/sshd_config file add/modify the KexAlgorithms line to contain a comma separated list of the site approved key exchange algorithms.

Example:

```
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
```

## Default Value

kexalgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1

## References

1. SSHD_CONFIG(5)

## Additional Information

Weak Key Exchange Algorithms:

```
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
```

Key Exchange algorithms supported by OpenSSH 7.2p2:

```
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
```

"Strong" Key Exchange Algorithms currently FIPS 140-2 approved:

```
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256
```

## CIS Controls

Version 7

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

## Profile Applicability

- Level 1 - Server
- Level 1 - Workstation

## Assessment Status

Automated