5.3.14 Ensure only strong Ciphers are used - approved ciphers

Information

This variable limits the ciphers that SSH can use during communication.

Notes:

Some organizations may have stricter requirements for approved ciphers. Ensure that ciphers used are in compliance with site policy.

The only 'strong' ciphers currently FIPS 140-2 compliant are: aes256-ctr,aes192-ctr,aes128-ctr

Rationale:

Weak ciphers that are used for authentication to the cryptographic module cannot be relied upon to provide confidentiality or integrity, and system data may be compromised.

The DES, Triple DES, and Blowfish ciphers, as used in SSH, have a birthday bound of approximately four billion blocks, which makes it easier for remote attackers to obtain cleartext data via a birthday attack against a long-duration encrypted session, aka a 'Sweet32' attack

The RC4 algorithm, as used in the TLS protocol and SSL protocol, does not properly combine state data with key data during the initialization phase, which makes it easier for remote attackers to conduct plaintext-recovery attacks against the initial bytes of a stream by sniffing network traffic that occasionally relies on keys affected by the Invariance Weakness, and then using a brute-force approach involving LSB values, aka the 'Bar Mitzvah' issue

The passwords used during an SSH session encrypted with RC4 can be recovered by an attacker who is able to capture and replay the session

Error handling in the SSH protocol; Client and Server, when using a block cipher algorithm in Cipher Block Chaining (CBC) mode, makes it easier for remote attackers to recover certain plaintext data from an arbitrary block of ciphertext in an SSH session via unknown vectors

Solution

Edit the /etc/ssh/sshd_config file add/modify the Ciphers line to contain a comma separated list of the site approved ciphers
Example:

Ciphers aes256-ctr,aes192-ctr,aes128-ctr

Default Value:

Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]

See Also

https://workbench.cisecurity.org/files/3152

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-8, CSCv7|14.4

Plugin: Unix

Control ID: 78a21e6dcfda25c5021b5191fedcf9fd1466bd4d25b48a7c7f675a6bac74a22c