Cis K8S V1110 4.2.12

4.2.12 Ensure that the Kubelet only makes use of Strong Cryptographic Ciphers (Manual)

Profile Applicability

• Level 1 - Worker Node

Description

Ensure that the Kubelet is configured to only use strong cryptographic ciphers.

Rationale

TLS ciphers have had a number of known vulnerabilities and weaknesses, which can reduce the protection provided by them. By default Kubernetes supports a number of TLS ciphersuites including some that have security concerns, weakening the protection provided.

Impact

Kubelet clients that cannot support modern cryptographic ciphers will not be able to make connections to the Kubelet API.

Audit

The set of cryptographic ciphers currently considered secure is the following:

• TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
• TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
• TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305
• TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
• TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305
• TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
• TLS_RSA_WITH_AES_256_GCM_SHA384
• TLS_RSA_WITH_AES_128_GCM_SHA256

Run the following command on each node:

ps -ef | grep kubelet

If the --tls-cipher-suites argument is present, ensure it only contains values included in this set. If it is not present check that there is a Kubelet config file specified by --config, and that file sets tlsCipherSuites: to only include values from this set.

Remediation

If using a Kubelet config file, edit the file to set tlsCipherSuites: to TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,TLS_RSA_WITH_AES_256_GCM_SHA384,TLS_RSA_WITH_AES_128_GCM_SHA256 or to a subset of these values.

If using executable arguments, edit the kubelet service file /etc/kubernetes/kubelet.conf on each worker node and set the --tls-cipher-suites parameter as follows, or to a subset of these values.

--tls-cipher-suites=TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,TLS_RSA_WITH_AES_256_GCM_SHA384,TLS_RSA_WITH_AES_128_GCM_SHA256

Based on your system, restart the kubelet service. For example:

systemctl daemon-reload
systemctl restart kubelet.service

Default Value

By default the Kubernetes API server supports a wide range of TLS ciphers

Additional Information

The list chosen above should be fine for modern clients. It's essentially the list from the Mozilla "Modern cipher" option with the ciphersuites supporting CBC mode removed, as CBC has traditionally had a lot of issues

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