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5.1.6 Ensure sshd KexAlgorithms is configured

Information

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

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

Solution

The key exchange algorithms available to the SSH server are governed by the system-wide crypto policy. The weak key exchange algorithms in scope are SHA1-based; disable them by disabling SHA1 hash and signature support in the system-wide crypto policy as described in the "Configure system wide crypto policy" section.

Once the crypto policy has been updated, reload the SSH daemon:

# systemctl reload-or-restart sshd

Impact:

A too restrictive list of key exchange algorithms can lead to a "no matching key exchange method found" error during connection establishment, when a client and the server share no common algorithm. Verify that peers required to connect to this host support at least one of the retained algorithms before applying.

See Also

https://workbench.cisecurity.org/benchmarks/27891

Item Details

Category: ACCESS CONTROL, IDENTIFICATION AND AUTHENTICATION, SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|AC-17(2), 800-53|IA-5, 800-53|IA-5(1), 800-53|SC-8, 800-53|SC-8(1), CSCv7|14.4

Plugin: Unix

Control ID: 6780529c644040ae03a88f32e04dab6829171960c88a03aefaa18f3089db8a1b