Cis GCP Cos 5.1.13

5.1.13 Ensure only strong Ciphers are used (Automated)

Description

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

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.

The mm_newkeys_from_blob function in monitor_wrap.c, when an AES-GCM cipher is used, does not properly initialize memory for a MAC context data structure, which allows remote authenticated users to bypass intended ForceCommand and login-shell restrictions via packet data that provides a crafted callback address.

Audit Procedure

Run the following command and verify that output does not contain any of the listed weak ciphers:

# sshd -T | grep ciphers

Weak Ciphers:

3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
rijndael-cbc@lysator.liu.se

Expected Result

Output should not contain any of the listed weak ciphers.

Remediation

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 chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

Default Value

ciphers aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr

References

1. https://nvd.nist.gov/vuln/detail/CVE-2016-2183
2. https://nvd.nist.gov/vuln/detail/CVE-2015-2808
3. https://www.kb.cert.org/vuls/id/565052
4. https://www.openssh.com/txt/cbc.adv
5. https://nvd.nist.gov/vuln/detail/CVE-2008-5161
6. https://nvd.nist.gov/vuln/detail/CVE-2013-4548
7. https://www.kb.cert.org/vuls/id/565052
8. https://www.openssh.com/txt/cbc.adv
9. SSHD_CONFIG(5)

Additional Information

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

CVE-2013-4548 referenced above applies to OpenSSH versions 6.2 and 6.3. If running these versions of Open SSH, Please upgrade to version 6.4 or later to fix the vulnerability, or disable AES-GCM in the server configuration.

The following are the supported ciphers in openSSH:

3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
rijndael-cbc@lysator.liu.se
chacha20-poly1305@openssh.com

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