cyberstrikeus 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:
# 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
- https://ubuntu.com/server/docs/openssh-crypto-configuration
- NIST SP 800-53 Rev. 5: SC-8
- SSHD(8)
- 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