Kubernetes Certificates: A Practical Guide

What Are Certificates in Kubernetes? 

In Kubernetes, certificates are digital documents that establish trust between the components of the cluster and its users. They use Public Key Infrastructure (PKI) to verify the identity of different components like nodes, services, and users.

Kubernetes certificates enable secure communication within the cluster by encrypting the data transmitted between entities, ensuring that sensitive information remains confidential and intact.

These certificates help in implementing Transport Layer Security (TLS) for Kubernetes clusters. TLS certificates are used for encrypting traffic and for authentication purposes, allowing only authorized entities to communicate within the network. They help protect against security threats such as man-in-the-middle attacks and unauthorized data access.

This is part of a series of articles about Kubernetes management

The Importance of PKI and TLS Certificates in Kubernetes 

PKI provides a framework for managing digital certificates and encryption keys, which are essential for establishing secure communications and verifying the identity of cluster components. This mechanism helps ensure that only authenticated nodes, services, and users can interact within the Kubernetes ecosystem, preventing unauthorized access and security breaches.

TLS certificates further improve security by enabling encrypted connections across the cluster. This ensures that data transmitted between nodes, pods, and services is protected from eavesdropping and tampering by malicious actors. The use of TLS certificates is important for preserving the confidentiality and integrity of sensitive data as it moves within the cluster.

How Kubernetes Manages Certificates 

Kubernetes manages certificates through a built-in Certificate Authority (CA) that automates the generation, signing, and distribution of certificates required for cluster operations. This CA is responsible for issuing certificates to nodes, services, and users based on their roles within the cluster, ensuring secure and authenticated communications. 

The process is largely transparent to users but can be manually overseen and customized via Kubernetes’ command-line tools and APIs. The system also supports automatic certificate renewal, which helps in maintaining the security posture of the cluster without manual intervention. 

Kubernetes periodically checks for certificates nearing expiration and automatically reissues them, minimizing the risk associated with expired certificates. This automation is crucial for large-scale or dynamic environments where manual certificate management would be impractical or prone to errors.

Certificates for Kubernetes Servers 

Here are some of the components in Kubernetes that use certificates to enable secure operations:

• KubeAPI server: Part of the Kubernetes control plane, acting as the primary interface for cluster management and interaction. It uses TLS certificates to secure communications with clients. 
• etcd server: The main datastore for all cluster data, including configuration and state information. It uses TLS certificates to secure communication with other cluster components. 
• Kubelet server: Runs on every node in the Kubernetes cluster and manages pods and containers. It uses TLS certificates for secure communication with the Kubernetes API server, verifying its identity and encrypting data. 

Certificates for Kubernetes Clients 

Certificates can also be used by Kubernetes clients:

• Admin: Responsible for cluster administration tasks, including deploying applications, managing resources, and configuring network policies. To securely perform these operations, the admin client uses certificates for authentication with the Kubernetes API server. 
• Kube scheduler: Responsible for assigning pods to nodes within the Kubernetes cluster, based on criteria such as resource availability, constraints, and affinities. It uses certificates to securely communicate its scheduling decisions to the Kubernetes API server. 
• Kube controller: Oversees the state of the cluster and ensures that it matches the desired state specified by users. It uses certificates for secure communication with the Kubernetes API server.
• Kube proxy: A network proxy that runs on each node in the Kubernetes cluster, enabling communication to and from pods across and outside the cluster. It relies on certificates to establish secure channels for this communication.

Working with Certificates in Kubernetes

Trusting TLS in a Cluster

To trust TLS in a Kubernetes cluster, the cluster must be configured to recognize the Certificate Authority (CA) that issued the TLS certificates. This involves distributing the CA’s certificate to all nodes within the cluster so that they can verify the authenticity of the certificates presented by other nodes, services, and clients. By doing this, the cluster ensures that any communication over TLS is both encrypted and authenticated, protecting against unauthorized access and ensuring data integrity.

Requesting a Certificate

Requesting a certificate in Kubernetes involves generating a CertificateSigningRequest (CSR). This request contains information about the entity requesting the certificate, such as its identity and the public key that will be associated with the certificate. The CSR is then sent to the Kubernetes CA for approval and signing.

Creating a Certificate Signing Request

A CertificateSigningRequest can be created using a tool like OpenSSL or Kubernetes’ own command-line tools to generate a CSR file. This file includes the public key and the entity’s identification information that needs the certificate.

For example:

cat <<EOF | cfssl genkey - | cfssljson -bare server
{
  "hosts": [
    "example-svc.example-namespace.svc.cluster.local",
    "example-pod.example-namespace.pod.cluster.local",
    "192.10.8.15",
    "192.10.8.16"
  ],
  "CN": "example-pod.example-namespace.pod.cluster.local",
  "key": {
    "algo": "ecdsa",
    "size": 256
  }
}
EOF

Creating a CertificateSigningRequest Object to Send to the Kubernetes API

Once the CSR file is created, the next step is to encapsulate this request into a Kubernetes CSR object. This object is then submitted to the Kubernetes API server. The CSR object must include the base64-encoded CSR file and specify the type of request being made.

For example:

cat <<EOF | kubectl apply -f -
apiVersion: certificates.k8s.io/v1
kind: CertificateSigningRequest
metadata:
  name: example-svc.example-namespace
spec:
  request: $(cat server.csr | base64 | tr -d '\n')
  signerName: examplewebsite.com/serving
  usages:
  - digital signature
  - key encipherment
  - server auth
EOF

Getting the CertificateSigningRequest Approved

After submitting the CertificateSigningRequest object, it must be approved by a cluster administrator or an automated process configured to handle such approvals. Approval signifies that the requestor’s identity and intent have been verified and that the certificate can be issued.

The admin can use kubectl certificate approve example-svc.example-namespace to approve the CSR.

Signing the CertificateSigningRequest

Once approved, the Kubernetes CA signs the CertificateSigningRequest, generating a valid certificate for the requestor. This certificate is then made available to the requesting entity, allowing it to engage in secure, authenticated communications within the cluster. The signed certificate, along with the CA’s certificate, ensures the integrity and trustworthiness of the interactions within the Kubernetes environment.

Challenges of Managing Digital Certificates in Kubernetes and Solutions 

Here are some of the main challenges associated with certificate management in Kubernetes and how to address them.

Security and Storage of Private Keys

Private keys are crucial for the security of digital certificates, as they are used to decrypt sensitive information and sign data to verify authenticity. In Kubernetes, the security of these private keys is a significant challenge due to the distributed and dynamic nature of the environment.

To address this, Kubernetes administrators should:

• Use Secure Storage Solutions: Store private keys in secure, encrypted storage such as HashiCorp Vault or a cloud provider’s Key Management Service (KMS).
• Limit Access: Implement strict access controls and policies to restrict who can access the private keys.
• Use Hardware Security Modules (HSMs): For highly sensitive environments, HSMs can provide an additional layer of security by storing keys in hardware rather than software.

Certificate Rotation and Renewal

Certificates have a limited lifespan and must be renewed periodically to maintain secure communication. In Kubernetes, managing the timely rotation and renewal of certificates can be challenging, especially in large clusters.

Solutions include:

• Automated Certificate Management: Kubernetes supports automated certificate renewal through its built-in CA, reducing the manual effort required. Tools like cert-manager can also automate this process, ensuring certificates are renewed before they expire.
• Monitoring and Alerts: Implement monitoring solutions to track certificate expiry dates and set up alerts to notify administrators when a certificate is approaching expiration.
• Graceful Rotation: Use features like dual certificate support, where both the old and new certificates are valid for a transition period, ensuring seamless rotation without downtime.

Lack of Visibility and Governance

Without proper visibility and governance, it can be difficult to track and manage the various certificates deployed across a Kubernetes cluster. This can lead to unauthorized certificates being issued or expired certificates going unnoticed, increasing the risk of security breaches.

To overcome this:

• Centralized Certificate Management: Utilize centralized tools like cert-manager or external certificate management platforms to maintain an inventory of all certificates, monitor their status, and enforce governance policies.
• Audit Logs: Enable and regularly review Kubernetes audit logs to track certificate-related actions, such as issuance, approval, and rotation, to ensure compliance with security policies.
• Policy Enforcement: Implement policies that enforce strict controls over who can issue and approve certificates, reducing the risk of unauthorized certificates.

TLS Outages and Cyberattacks

TLS outages can occur due to expired certificates, misconfigurations, or attacks, leading to service disruptions or security vulnerabilities. In a Kubernetes environment, where services are interdependent, such outages can have widespread impacts.

Mitigation strategies include:

• Preemptive Renewal: Regularly renew certificates well before their expiration date to prevent outages caused by expired certificates.
• Redundant Paths: Configure redundant TLS paths and failover mechanisms to maintain service availability in case of a TLS failure.
• Regular Testing: Conduct regular testing and simulations of TLS certificate expirations and renewals to identify potential issues before they impact production.

Cryptographic Key Compromise

If a private key is compromised, an attacker could impersonate legitimate services or decrypt sensitive information, leading to severe security breaches.

To mitigate this risk:

• Key Rotation: Regularly rotate keys, even if there is no indication of compromise, to limit the potential impact of a key being exposed.
• Key Revocation: Implement processes to quickly revoke certificates associated with compromised keys and replace them with new ones.

Multi-Factor Authentication (MFA): Use MFA for accessing systems where private keys are stored, adding an additional layer of security against unauthorized access.

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