cis-gcp-cos-5.1.15
DevOps & SecurityEnsure only strong Key Exchange algorithms are used
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5.1.15 Ensure only strong Key Exchange algorithms are used (Automated)
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.
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
Run the following command and verify that output does not contain any of the listed weak Key Exchange algorithms:
# sshd -T | grep kexalgorithms
Weak Key Exchange Algorithms:
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
Expected Result
Output should not contain any of the listed weak Key Exchange algorithms.
Remediation
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,curve25519-sha256@libssh.org,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
Default Value
kexalgorithms 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
Additional Information
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 140-2 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
The Key Exchange algorithms supported by OpenSSH 7 are:
curve25519-sha256
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
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 |
Profile
- Level 1 - Server