Pentesting Kubernetes Services

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Kubernetes uses several specific network services that you might find exposed to the Internet or in an internal network once you have compromised one pod.

Finding exposed pods with OSINT

One way could be searching for Identity LIKE "k8s.%.com" in crt.sh to find subdomains related to kubernetes. Another way might be to search "k8s.%.com" in github and search for YAML files containing the string.

How Kubernetes Exposes Services

It might be useful for you to understand how Kubernetes can expose services publicly in order to find them:

Exposing Services in Kubernetes

Finding Exposed pods via port scanning

The following ports might be open in a Kubernetes cluster:

Nmap

nmap -n -T4 -p 443,2379,6666,4194,6443,8443,8080,10250,10255,10256,9099,6782-6784,30000-32767,44134 <pod_ipaddress>/16

Kube-apiserver

This is the API Kubernetes service the administrators talks with usually using the tool kubectl.

Common ports: 6443 and 443, but also 8443 in minikube and 8080 as insecure.

curl -k https://<IP Address>:(8|6)443/swaggerapi
curl -k https://<IP Address>:(8|6)443/healthz
curl -k https://<IP Address>:(8|6)443/api/v1

Check the following page to learn how to obtain sensitive data and perform sensitive actions talking to this service:

Kubernetes Enumeration

Kubelet API

This service run in every node of the cluster. It's the service that will control the pods inside the node. It talks with the kube-apiserver.

If you find this service exposed you might have found an unauthenticated RCE.

Kubelet API

curl -k https://<IP address>:10250/metrics
curl -k https://<IP address>:10250/pods

If the response is Unauthorized then it requires authentication.

If you can list nodes you can get a list of kubelets endpoints with:

kubectl get nodes -o custom-columns='IP:.status.addresses[0].address,KUBELET_PORT:.status.daemonEndpoints.kubeletEndpoint.Port' | grep -v KUBELET_PORT | while IFS='' read -r node; do
    ip=$(echo $node | awk '{print $1}')
    port=$(echo $node | awk '{print $2}')
    echo "curl -k --max-time 30 https://$ip:$port/pods"
    echo "curl -k --max-time 30 https://$ip:2379/version" #Check  also for etcd
done

kubelet (Read only)

curl -k https://<IP Address>:10255
http://<external-IP>:10255/pods

etcd API

curl -k https://<IP address>:2379
curl -k https://<IP address>:2379/version
etcdctl --endpoints=http://<MASTER-IP>:2379 get / --prefix --keys-only

Tiller

helm --host tiller-deploy.kube-system:44134 version

You could abuse this service to escalate privileges inside Kubernetes:

cAdvisor

Service useful to gather metrics.

curl -k https://<IP Address>:4194

NodePort

When a port is exposed in all the nodes via a NodePort, the same port is opened in all the nodes proxifying the traffic into the declared Service. By default this port will be in in the range 30000-32767. So new unchecked services might be accessible through those ports.

sudo nmap -sS -p 30000-32767 <IP>

Vulnerable Misconfigurations

Kube-apiserver Anonymous Access

Anonymous access to kube-apiserver API endpoints is not allowed. But you could check some endpoints:

Checking for ETCD Anonymous Access

The ETCD stores the cluster secrets, configuration files and more sensitive data. By default, the ETCD cannot be accessed anonymously, but it always good to check.

If the ETCD can be accessed anonymously, you may need to use the etcdctl tool. The following command will get all the keys stored:

etcdctl --endpoints=http://<MASTER-IP>:2379 get / --prefix --keys-only

Kubelet RCE

The Kubelet documentation explains that by default anonymous access to the service is allowed:

Enables anonymous requests to the Kubelet server. Requests that are not rejected by another authentication method are treated as anonymous requests. Anonymous requests have a username of system:anonymous, and a group name of system:unauthenticated

To understand better how the authentication and authorization of the Kubelet API works check this page:

Kubelet Authentication & Authorization

The Kubelet service API is not documented, but the source code can be found here and finding the exposed endpoints is as easy as running:

curl -s https://raw.githubusercontent.com/kubernetes/kubernetes/master/pkg/kubelet/server/server.go | grep 'Path("/'

Path("/pods").
Path("/run")
Path("/exec")
Path("/attach")
Path("/portForward")
Path("/containerLogs")
Path("/runningpods/").

All of them sound interesting.

You can use the Kubeletctl tool to interact with Kubelets and their endpoints.

/pods

This endpoint list pods and their containers:

kubeletctl pods

/exec

This endpoint allows to execute code inside any container very easily:

kubeletctl exec [command]

Checking Kubelet (Read Only Port) Information Exposure

When a kubelet read-only port is exposed, it becomes possible for information to be retrieved from the API by unauthorized parties. The exposure of this port may lead to the disclosure of various cluster configuration elements. Although the information, including pod names, locations of internal files, and other configurations, may not be critical, its exposure still poses a security risk and should be avoided.

An example of how this vulnerability can be exploited involves a remote attacker accessing a specific URL. By navigating to http://<external-IP>:10255/pods, the attacker can potentially retrieve sensitive information from the kubelet:

References

Kubernetes Pentest Methodology Part 2

Threats & Research Archives - F-Secure Blog