The RHEL 8 operating system must implement DoD-approved encryption to protect the confidentiality of SSH server connections.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-230252	RHEL-08-010291	SV-230252r743940_rule		Medium

Description
Without cryptographic integrity protections, information can be altered by unauthorized users without detection. Remote access (e.g., RDP) is access to DoD nonpublic information systems by an authorized user (or an information system) communicating through an external, non-organization-controlled network. Remote access methods include, for example, dial-up, broadband, and wireless. Cryptographic mechanisms used for protecting the integrity of information include, for example, signed hash functions using asymmetric cryptography enabling distribution of the public key to verify the hash information while maintaining the confidentiality of the secret key used to generate the hash. RHEL 8 incorporates system-wide crypto policies by default. The SSH configuration file has no effect on the ciphers, MACs, or algorithms unless specifically defined in the /etc/sysconfig/sshd file. The employed algorithms can be viewed in the /etc/crypto-policies/back-ends/opensshserver.config file. The system will attempt to use the first hash presented by the client that matches the server list. Listing the values "strongest to weakest" is a method to ensure the use of the strongest hash available to secure the SSH connection. Satisfies: SRG-OS-000250-GPOS-00093, SRG-OS-000393-GPOS-00173, SRG-OS-000394-GPOS-00174, SRG-OS-000125-GPOS-00065

Description

Without cryptographic integrity protections, information can be altered by unauthorized users without detection. Remote access (e.g., RDP) is access to DoD nonpublic information systems by an authorized user (or an information system) communicating through an external, non-organization-controlled network. Remote access methods include, for example, dial-up, broadband, and wireless. Cryptographic mechanisms used for protecting the integrity of information include, for example, signed hash functions using asymmetric cryptography enabling distribution of the public key to verify the hash information while maintaining the confidentiality of the secret key used to generate the hash. RHEL 8 incorporates system-wide crypto policies by default. The SSH configuration file has no effect on the ciphers, MACs, or algorithms unless specifically defined in the /etc/sysconfig/sshd file. The employed algorithms can be viewed in the /etc/crypto-policies/back-ends/opensshserver.config file. The system will attempt to use the first hash presented by the client that matches the server list. Listing the values "strongest to weakest" is a method to ensure the use of the strongest hash available to secure the SSH connection. Satisfies: SRG-OS-000250-GPOS-00093, SRG-OS-000393-GPOS-00173, SRG-OS-000394-GPOS-00174, SRG-OS-000125-GPOS-00065

STIG	Date
Red Hat Enterprise Linux 8 Security Technical Implementation Guide	2021-06-14

Details

Check Text ( C-32921r743938_chk )
Verify the SSH server is configured to use only ciphers employing FIPS 140-2-approved algorithms with the following command: $ sudo grep -i ciphers /etc/crypto-policies/back-ends/opensshserver.config CRYPTO_POLICY='-oCiphers=aes256-ctr,aes192-ctr,aes128-ctr' If the cipher entries in the "opensshserver.config" file have any ciphers other than "aes256-ctr,aes192-ctr,aes128-ctr", the order differs from the example above, they are missing, or commented out, this is a finding.

Check Text ( C-32921r743938_chk )

Verify the SSH server is configured to use only ciphers employing FIPS 140-2-approved algorithms with the following command:

$ sudo grep -i ciphers /etc/crypto-policies/back-ends/opensshserver.config

CRYPTO_POLICY='-oCiphers=aes256-ctr,aes192-ctr,aes128-ctr'

If the cipher entries in the "opensshserver.config" file have any ciphers other than "aes256-ctr,aes192-ctr,aes128-ctr", the order differs from the example above, they are missing, or commented out, this is a finding.

Fix Text (F-32896r743939_fix)
Configure the RHEL 8 SSH server to use only ciphers employing FIPS 140-2-approved algorithms by updating the "/etc/crypto-policies/back-ends/opensshserver.config" file with the following line: -oCiphers=aes256-ctr,aes192-ctr,aes128-ctr A reboot is required for the changes to take effect.