Cis Ubuntu2004 V300 5 1 12

---
name: cis-ubuntu2004-v300-5-1-12
description: "Ensure sshd KexAlgorithms is configured"
category: cis-networking
version: "3.0.0"
author: cyberstrike-official
tags: [cis, ubuntu, linux, ubuntu-20.04, ssh]
cis_id: "5.1.12"
cis_benchmark: "CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0"
tech_stack: [ubuntu, linux]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# Ensure sshd KexAlgorithms is configured (Automated)

## Profile Applicability

- Level 1 - Server
- Level 1 - Workstation

## 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.

Notes:

- Kex algorithms have a higher preference the earlier they appear in the list
- Some organizations may have stricter requirements for approved Key exchange algorithms
- Ensure that Key exchange algorithms used are in compliance with site policy
- The only Key Exchange Algorithms currently FIPS 140 approved are:
  - ecdh-sha2-nistp256
  - ecdh-sha2-nistp384
  - ecdh-sha2-nistp521
  - diffie-hellman-group-exchange-sha256
  - diffie-hellman-group16-sha512
  - diffie-hellman-group18-sha512
  - diffie-hellman-group14-sha256

## 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 to verify none of the "weak" Key Exchange algorithms are being used:

```bash
# sshd -T | grep -Pi -- 'kexalgorithms\h+([^#\n\r]+,)?(diffie-hellman-group1-sha1|diffie-hellman-group14-sha1|diffie-hellman-group-exchange-sha1)\b'
```

Nothing should be returned.

- IF - A line is returned, review the list of Key Exchange Algorithms. The following are considered "weak" Key Exchange Algorithms, and should not be used:

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

## Expected Result

Nothing should be returned (no weak KexAlgorithms in use).

## Remediation

### Command Line

Edit the `/etc/ssh/sshd_config` file and add/modify the `KexAlgorithms` line to contain a comma separated list of the site unapproved (weak) KexAlgorithms preceded with a `-` above any `Include` entries:
Example:

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

Note: First occurrence of an option takes precedence. If Include locations are enabled, used, and order of precedence is understood in your environment, the entry may be created in a file in Include location.

## Default Value

KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256

## References

1. https://ubuntu.com/server/docs/openssh-crypto-configuration
2. NIST SP 800-53 Rev. 5: SC-8
3. SSHD(8)
4. SSHD_CONFIG(5)

## CIS Controls

v8 - 3.10 Encrypt Sensitive Data in Transit
Encrypt sensitive data in transit. Example implementations can include: Transport Layer Security (TLS) and Open Secure Shell (OpenSSH).

v7 - 14.4 Encrypt All Sensitive Information in Transit
Encrypt all sensitive information in transit.

Additional Information:
The supported algorithms are:
curve25519-sha256, curve25519-sha256@libssh.org, diffie-hellman-group1-sha1, diffie-hellman-group14-sha1, diffie-hellman-group14-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, diffie-hellman-group-exchange-sha1, diffie-hellman-group-exchange-sha256, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, sntrup459761x25519-sha512@tinyssh.org

MITRE ATT&CK Mappings: T1040, T1040.000, T1557, T1557.000 | TA0006 | M1041