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kubesectips v1

Security tips for Kubernetes

Comming soon Kubernetes Pod PrivEsc…

PART 1 - ARCHITECTURE

What does Kubernetes do?

Architecture:

How pods communicate with each other.

PKI infrastructure - Certificate Authority CA:

PART 2 - VULNERABILITIES and some fixes.

Vulnerabilities - kubernetes secrets

A Secret is an object that contains a small amount of sensitive data such as a password, a token or a key. Such information might otherwise be put in a Pod specification or in an image. Users can create Secrets and the system also creates some Secrets. The name of a Secret object must be a valid DNS subdomain name.

Secrets can be things like:

Secret types:

Builtin Type Usage
Opaque arbitrary user-defined data
kubernetes.io/service-account-token service account token
kubernetes.io/dockercfg serialized ~/.dockercfg file
kubernetes.io/dockerconfigjson serialized ~/.docker/config.json file
kubernetes.io/basic-auth credentials for basic authentication
kubernetes.io/ssh-auth credentials for SSH authentication
kubernetes.io/tls data for a TLS client or server
bootstrap.kubernetes.io/token bootstrap token data

How secrets works:

https://kubernetes.io/docs/concepts/configuration/secret/#using-secrets-as-files-from-a-pod

Create a secret, commands:

kubectl create secret generic secret_01 --from-literal user=<user>
kubectl create secret generic secret_01 --from-literal password=<password>
kubectl run pod --image=nginx -oyaml --dry-run=client
kubectl run pod --image=nginx -oyaml --dry-run=client > <podName.yaml>

This is the generated file:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - name: mypod
    image: redis
    volumeMounts:
    - name: <secret_01>
      mountPath: "/etc/<secret_01>"
      readOnly: true
  volumes:
  - name: <secret_01>
    secret:
      secretName: <secret_01>
      items:
      - key: username
        path: my-group/my-username

Using Secrets as environment variables

If you want to use a secret in an environment variable to allow the rest of the pods to reference the same secret, you could use:

In the you could add the uncomment lines:

#apiVersion: v1
#kind: Pod
#metadata:
#  name: secret-env-pod
#spec:
#  containers:
#  - name: mycontainer
#    image: redis
    env:
      - name: SECRET_USERNAME
        valueFrom:
          secretKeyRef:
            name: mysecret
            key: username
#     - name: SECRET_PASSWORD
#        valueFrom:
#          secretKeyRef:
#            name: mysecret
#            key: password
#  restartPolicy: Never

The result is:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - name: mypod
    image: redis
    env:
      - name: PASSWORD
        valueFrom:
          secretKeyRef:
            name: <secret_02>
            key: <password>   
    volumeMounts:
    - name: <secret_01>
      mountPath: "/etc/<secret_01>"
      readOnly: true
  volumes:
  - name: <secret_01>
    secret:
      secretName: <secret_01>
      items:
      - key: username
        path: my-group/my-username

Save and:

kubectl -f <podName.yaml> delete --force
kubectl -f <podName.yaml> create

or:

kubectl -f <podName.yaml> replace --force

More info: https://kubernetes.io/docs/concepts/configuration/secret/#using-secrets-as-environment-variables

Discover secrets in docker:

To get the id of the container.

docker ps | grep <service>

Inspect:

docker inspect <docker_id>

Check env (environment variable section) for secrets and you will see:

If you want to copy:

docker cp <docket_id>:/etc/<secret_01> <secret_01>

Discover secrets in etcd:

Remember that etcd is a consistent and highly-available key-value store used as Kubernetes backing store for all cluster data. Let’s access to the secret in etcd:

cat /etc/kubernetes/manifests/kube-apiserver.yaml | grep etcd

You will see certs, keys and url’s were are located in the FS. Once you get it, you would be able to connect to etcd.

ETCDCTL_API=3 etcdctl --cert <path to client.crt> --key <path to client.ket> --cacert <path to CA.cert> endpoint=[<ip:port>] health
i.e:
ETCDCTL_API=3 etcdctl --cert /etc/kubernetes/pki/apiserver-etcd-client.crt --key /etc/kubernetes/pki/apiserver-etcd-client.key --cacert /etc/kubernetes/pki/etcd/etcd/ca.cert endpoint=[127.0.0.1:1234] health

Once you achieve establish communication you would be able to get the secrets:

ETCDCTL_API=3 etcdctl --cert <path to client.crt> --key <path to client.ket> --cacert <path to CA.cert> endpoint=[<ip:port>] get <path/to/secret>
i.e:
ETCDCTL_API=3 etcdctl --cert /etc/kubernetes/pki/apiserver-etcd-client.crt --key /etc/kubernetes/pki/apiserver-etcd-client.key --cacert /etc/kubernetes/pki/etcd/etcd/ca.cert endpoint=[127.0.0.1:1234] get /registry/secrets/default/secret_02

Adding encryption to the ETCD

So, by default all the secrets are in plain text unless you apply an encryption layer: If the identity provider is empty with the default value = {} so the secrets are in plain text. https://kubernetes.io/docs/tasks/administer-cluster/encrypt-data/

Encryption types

| Name | Encryption | Strength | Speed | Key Length | Other Considerations | |-|-|-|-|-|-| | identity | None | N/A | N/A | N/A | Resources written as-is without encryption. When set as the first provider, the resource will be decrypted as new values are written. | | aescbc | AES-CBC with PKCS#7 padding | Strongest | Fast | 32-byte | The recommended choice for encryption at rest but may be slightly slower than secretbox. | | secretbox | XSalsa20 and Poly1305 | Strong | Faster | 32-byte | A newer standard and may not be considered acceptable in environments that require high levels of review. | | aesgcm | AES-GCM with random nonce | Must be rotated every 200k writes | Fastest | 16, 24, or 32-byte | Is not recommended for use except when an automated key rotation scheme is implemented. | | kms | Uses envelope encryption scheme: Data is encrypted by data encryption keys (DEKs) using AES-CBC with PKCS#7 padding, DEKs are encrypted by key encryption keys (KEKs) according to configuration in Key Management Service (KMS) | Strongest | Fast | 32-bytes | The recommended choice for using a third party tool for key management. Simplifies key rotation, with a new DEK generated for each encryption, and KEK rotation controlled by the user. |

The secrets will be encrypted with the above algorithms and encoded by base64.

kubectl get secrets --all-namespaces -o json | kubectl replace -f -

How to encrypt the ETCD

Create a directory in /etc/kubernetes ; in this case you will name it as etcd, so you have:

/etc/kubernetes/etcd

You create a yaml file with the configuration.

vi <configFile.yaml>

You can copy the content of https://kubernetes.io/docs/tasks/administer-cluster/encrypt-data/

apiVersion: apiserver.config.k8s.io/v1
kind: EncryptionConfiguration
resources:
  - resources:
    - secrets
    providers:
    - aescbc:
        keys:
        - name: key1
          secret: <your pass in b64>
    - identity: {}

Generate pass in b64 (remember to use a pass character with lenght = 16 or = 24 or = 32) :

echo -n <password> | base64

You can see how the encryption provider is not setting.

After that, you have to edit the file /etc/kubernetes/manifest/kube-apiserver.yaml and add the following lines into the sections: And add the following line: spec:

  containers:
  - command:
    - kube-apiserver
    - --encriyption-provider-config=/etc/kubernetes/etcd/<configFile.yaml>

Scroll down in the volumeMounts:

- mountPath: /etc/kubernetes/etcd
    name: etcd
    readOnly: true

Scroll down in the volumeMounts to hostPath:

- hostPath:
    path: /etc/kubernetes/etcd
    type: DirectoryOrCreate
  name: etcd

Get information about the secrets.

kubectl get secret
kubectl get secret <secret_name> -oyaml
ETCDCTL_API=3 etcdctl get /registry/secrets/default/secret1 [...] | hexdump -C
kubectl create secret generic test-secret --from-literal='username=my-app' --from-literal='password=45tRf$we34rR'

With root access:

# kubectl get secret
kubectl get secret test-secret -oyaml

Do not forget to delete de secrets and re-create them again in order to apply the encryption layer.

Final tips:

Vulnerabilities - Container runtime sandboxes

How an attack with lateral movement and privesc could be done:

Getting inside the container:

kubectl get node
kubectl run pod --image=<image_name>
kubectl exec pod -it -- bash

Once inside the container:

root@pod01:/# uname -r

If you want to gather information you could use:

strace uname -r
ltrace uname -r

When the attack achieves discovering the kernel version, he could run exploiting techniques to gather information or escalate into the OS.

For secure sandboxes:

https://github.com/google/gvisor

https://katacontainers.io/

Vulnerabilities - OS

Is mandatory to keep in mind to define privilege and access control for container / pod:

More info at: https://kubernetes.io/docs/tasks/configure-pod-container/security-context/

userID and groupID

# kubectl run pod --image=busybox --command -oyaml --dry-run=client > <podName.yaml> -- sh -c 'sleep 1h'
# vi <podName>.yaml

Add the uncomment lines:

#apiVersion: v1
#kind: Pod
#metadata:
#  name: security-context-demo
spec:
  securityContext:
    runAsUser: 1000
    runAsGroup: 3000
    fsGroup: 2000
#  volumes:
#  - name: sec-ctx-vol
#    emptyDir: {}
#  containers:
#  - name: sec-ctx-demo
#    image: busybox
#    command: [ "sh", "-c", "sleep 1h" ]
   securityContext:
    runAsNonRoot: true
#    volumeMounts:
#    - name: sec-ctx-vol
#      mountPath: /data/demo
#    securityContext:
#      allowPrivilegeEscalation: true

Save and:

# kubectl -f <podName>.yaml delete --force
# kubectl -f <podName>.yaml create

Check permissions:

# kubectl exec -it <podName> -- sh

How to disable privilege escalation:

vi <podName>.yaml

Set this line to false

      allowPrivilegeEscalation: false

Save and:

kubectl -f <podName>.yaml delete --force
kubectl -f <podName>.yaml create

Modify PodSecurityPolicy

Pod security policies control the security policies about how a pod has to run. More info at: https://kubernetes.io/docs/concepts/policy/pod-security-policy/

Edit the kube-apiserver.yaml file:

vi /etc/kubernetes/manifests/kube-apiserver.yaml

Inside you add in

- --enable-admission-plugins=NodeRestriction,PodSecurityPolicy

Vulnerabilities - mTLS

Mutual authentication, two-way, pod to pod.

More info at: https://kubernetes.io/docs/tasks/configure-pod-container/security-context/

Create a sidecar proxy app

Create your .yaml

kubectl run app --image=bash --comand -oyaml --dry-run=client > <appName.yaml> -- shj -c 'ping google.com'

Edit your .yaml and add the uncomment lines:

#apiVersion: v1
#kind: Pod
#metadata:
#  name: security-context-demo
#spec:
#  securityContext:
#    runAsUser: 1000
#    runAsGroup: 3000
#    fsGroup: 2000
#  volumes:
#  - name: sec-ctx-vol
#    emptyDir: {}
#  containers:
#  - name: sec-ctx-demo
#    image: busybox
    command: [ "sh", "-c", "apt update && apt install iptables -y && iptables -L && sleep 1h" ]
    securityContext:
      capabilities:
        add: ["NET_ADMIN"]
 #   volumeMounts:
 #   - name: sec-ctx-vol
 #     mountPath: /data/demo
 #   securityContext:
 #     allowPrivilegeEscalation: true

See the logs of the proxy:

kubectl logs app -C proxy

More info at: https://kubernetes.io/docs/tasks/configure-pod-container/security-context/

PART 3 - HARDENING.

3.1 CLUSTER HARDENING - RBAC

https://kubernetes.io/docs/reference/access-authn-authz/rbac/ RBAC = Role-based access control (RBAC) is a method of regulating access to a computer or network resources based on the roles of individual users within your organization. RBAC authorization uses the rbac.authorization.k8s.io API group to drive authorization decisions, allowing you to dynamically configure policies through the Kubernetes API

To enable RBAC, start the API server with the –authorization-mode flag set to a comma-separated list that includes RBAC; for example:

kube-apiserver --authorization-mode=Example,RBAC --other-options --more-options

This is enabled by default. RBAC functions:

The principle of Least Privilege is the meaning of only access to data or information when is necessary for a legitimate purpose.

Types of resources:

https://kubernetes.io/docs/concepts/overview/working-with-objects/namespaces/

Concept of namespaces:

Kubernetes supports multiple virtual clusters backed by the same physical cluster. These virtual clusters are called namespaces. These are intended for use in environments with many users spread across multiple teams, or projects. For clusters with a few to tens of users, you should not need to create or think about namespaces at all. Start using namespaces when you need the features they provide.

Namespaces provide a scope for names. Names of resources need to be unique within a namespace, but not across namespaces. Namespaces cannot be nested inside one another and each Kubernetes resource can only be in one namespace.

Viewing namespaces:

You can list the current namespaces in a cluster using:

kubectl get namespace
NAME              STATUS   AGE
default           Active   1d
kube-node-lease   Active   1d
kube-public       Active   1d
kube-system       Active   1d
Setting the namespace preference

You can permanently save the namespace for all subsequent kubectl commands in that context.

kubectl config set-context --current --namespace=<insert-namespace-name-here>

Not All Objects are in a Namespace. Most Kubernetes resources (e.g. pods, services, replication controllers, and others) are in some namespaces. However, namespace resources are not themselves in a namespace. And low-level resources, such as nodes and persistentVolumes, are not in any namespace.

To see which Kubernetes resources are and aren’t in a namespace:

In a namespace
kubectl api-resources --namespaced=true
Not in a namespace
kubectl api-resources --namespaced=false

Difference between Role and ClusterRole:

Role:

RBAC allows setting different permissions for the same role with the independence of the namespace. Roles example:

/api/v1/namespaces/{namespace}/pods/{name}/log

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  namespace: defaultGreen
  name: pod-and-pod-logs-reader
rules:
- apiGroups: [""]
  resources: ["pods", "pods/log"]
  verbs: ["get", "list", "watch"]

Other example, same Role different nameSpace and permissions:

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  namespace: defaultYellow
  name: pod-and-pod-logs-reader
rules:
- apiGroups: [""]
  resources: ["pods", "pods/log"]
  verbs: ["watch"]
ClusterRole:

A ClusterRole can be used to grant the same permissions as a Role. Because ClusterRoles are cluster-scoped, you can also use them to grant access to:

For example you can use a ClusterRole to allow a particular user to run:

kubectl get pods --all-namespaces
ClusterRole example:
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  # "namespace" omitted since ClusterRoles are not namespaced
  name: secret-reader
rules:
- apiGroups: [""]
  #
  # at the HTTP level, the name of the resource for accessing Secret
  # objects is "secrets"
  resources: ["secrets"]
  verbs: ["get", "watch", "list"]

Role and ClusterRole Binding concept:

A role binding grants the permissions defined in a role to a user or set of users. It holds a list of subjects (users, groups, or service accounts), and a reference to the role being granted. A RoleBinding grants permissions within a specific namespace whereas a ClusterRoleBinding grants that access cluster-wide.

A RoleBinding may reference any Role in the same namespace. Alternatively, a RoleBinding can reference a ClusterRole and bind that ClusterRole to the namespace of the RoleBinding. If you want to bind a ClusterRole to all the namespaces in your cluster, you use a ClusterRoleBinding.

RoleBinding example:

apiVersion: rbac.authorization.k8s.io/v1
# This role binding allows "jane" to read pods in the "default" namespace.
# You need to already have a Role named "pod-reader" in that namespace.
kind: RoleBinding
metadata:
  name: read-pods
  namespace: default
subjects:
# You can specify more than one "subject"
- kind: User
  name: jane # "name" is case sensitive
  apiGroup: rbac.authorization.k8s.io
roleRef:
  # "roleRef" specifies the binding to a Role / ClusterRole
  kind: Role #this must be Role or ClusterRole
  name: pod-reader # this must match the name of the Role or ClusterRole you wish to bind to
  apiGroup: rbac.authorization.k8s.io

ClusterRoleBinding example:

apiVersion: rbac.authorization.k8s.io/v1
# This cluster role binding allows anyone in the "manager" group to read secrets in any namespace.
kind: ClusterRoleBinding
metadata:
  name: read-secrets-global
subjects:
- kind: Group
  name: manager # Name is case sensitive
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: secret-reader
  apiGroup: rbac.authorization.k8s.io

Permissions are additive so if you have a clusterRole with “list” and “delete” secrets you can add it with a Role with “get”. So be aware and test always your roles and permissions and specify what is ALLOWED, because everything is DENIED.

3.2 SERVICE ACCOUNTS HARDENING

ACCOUNTS

https://kubernetes.io/docs/reference/access-authn-authz/service-accounts-admin/ Users:

How it works:

Openssl –> CSR (CertificateSigningRequest) –> CertificateSignedRequest –> Kubernetes API <– CA

Be aware of the certificates because there is no way to invalidate them, you have to wait until the expiration date reaches. So what could you do in case you have to restrict the access?

ServiceAccounts:

3.3 KUBERNETES API HARDENING

API requests are always assigned to a User, ServiceAccount or Anonymous request. After the request must be authenticated. https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet-authentication-authorization/

Request process:

User or K8s ServiceAccount –> Authentication –> Authorization –> Admission Control.

TIPS:

3.4 KUBERNETES CLUSTER HARDENING

Upgrade it frecuently, you will receive:

Release cycles: Each 3 months there is a new minor release https://kubernetes.io/docs/setup/release/version-skew-policy/ 1.20.3 = 1(Major).20(Minor).3(patch)

Best way to update or upgrade a Kubernetes Cluster:

https://kubernetes.io/docs/tasks/administer-cluster/cluster-upgrade/