cyberstrikeus Ensure sshd Ciphers are configured (Automated)
Profile Applicability
- Level 1 - Server
- Level 1 - Workstation
Description
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 compliant are:
- aes256-gcm@openssh.com
- aes128-gcm@openssh.com
- 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 Triple DES ciphers, as used in SSH, have a birthday bound of approximately four billion blocks, which makes it easier for remote attackers to obtain clear text data via a birthday attack against a long-duration encrypted session, aka a "Sweet32" attack.
- 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 plain text data from an arbitrary block of cipher text in an SSH session via unknown vectors.
Audit Procedure
Command Line
Run the following command to verify none of the "weak" ciphers are being used:
# sshd -T | grep -Pi -- '^\h*ciphers\h+\"?([^#\n\r]+,)?(3des|blowfish|cast128|aes(128|192|256)-cbc|arcfour(128|256)?|rijndael-cbc@lysator\.liu\.se|chacha20-poly1305@openssh\.com)\b'
- IF - a line is returned, review the list of ciphers. If the line includes
chacha20-poly1305@openssh.com, review CVE-2023-48795 and verify the system has been patched. No ciphers in the list below should be returned as they're considered "weak":
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
Expected Result
Nothing should be returned (no weak ciphers in use).
Remediation
Command Line
Edit the /etc/ssh/sshd_config file and add/modify the Ciphers line to contain a comma separated list of the site unapproved (weak) Ciphers preceded with a - above any Include entries:
Example:
Ciphers -3des-cbc,aes128-cbc,aes192-cbc,aes256-cbc,chacha20-poly1305@openssh.com
- IF - CVE-2023-48795 has been addressed, and it meets local site policy,
chacha20-poly1305@openssh.commay be removed from the list of excluded ciphers. 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
Ciphers chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
References
- https://nvd.nist.gov/vuln/detail/CVE-2023-48795
- https://nvd.nist.gov/vuln/detail/CVE-2019-1543
- https://nvd.nist.gov/vuln/detail/CVE-2016-2183
- https://nvd.nist.gov/vuln/detail/CVE-2008-5161
- https://www.openssh.com/txt/cbc.adv
- SSHD_CONFIG(5)
- NIST SP 800-53 Rev. 5: SC-8
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.
MITRE ATT&CK Mappings: T1040, T1040.000, T1557 | TA0006 | M1041