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Access the Controller Services Directly

Services Available

Much like the initial configuration of the etcd server, the configuration of this controller node is also uncommonly insecure for the purposes of education.

Set the SSH Key, Region, and controller IP to variables:

$ export KEY_NAME="hkfs"
$ export AWS_DEFAULT_REGION="us-east-1"
$ export CONTROLLERIP=$(aws ec2 describe-instances \
  --region ${AWS_DEFAULT_REGION} \
  --filter 'Name=tag:Name,Values=controller' \
  --query 'Reservations[].Instances[].NetworkInterfaces[0].Association.PublicIp' \
  --output text)

Probe the ssh port:

Verify the port is responding:

$ nc -v $CONTROLLERIP 22
Connection to 54.89.108.72 port 22 [tcp/ssh] succeeded!
SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.4

Probe the cAdvisor service:

Verify the port is responding:

$ $ nc -vz $CONTROLLERIP 4194
Connection to 54.89.108.72 port 4194 [tcp/*] succeeded!

Hit tcp/4194 via curl:

$ curl $CONTROLLERIP:4194
<a href="/containers/">Temporary Redirect</a>.

Navigate to it using a browser:

$ open http://$CONTROLLERIP:4194/containers/

Hit tcp/4194 on the Prometheus formatted /metrics endpoint:

$ curl $CONTROLLERIP:4194/metrics

As you can see, there are many pieces of information that describe the infrastructure that should not be exposed to an attacker.

From the /containers/ endpoint:

  • How many CPU shares and how much RAM is available on this node.
  • How much CPU/RAM/Disk/FS is in use, and that usage over time.
  • A full process listing complete with base process name, PID, UID, GID, CPU usage, Memory usage, and time running.

From the /metrics endpoint:

  • Useful information about the Host OS and container runtime:

    cadvisor_version_info{cadvisorRevision="",cadvisorVersion="",dockerVersion="1.13.1",kernelVersion="4.4.0-1049-aws",osVersion="Ubuntu 16.04.3 LTS"} 1

  • When a container was created/started. In this case: Saturday, March 17, 2018 2:36:01 PM

    container_start_time_seconds{container_name="kubernetes-dashboard",id="/kubepods/podd60f089f-0a85-11e8-9462-06d7638bd978/a3c7cd6d8e0ebf603b431604ab7a1844f8ed0b651b231b10849010fd7cf37b17",image="gcr.io/google_containers/kubernetes-dashboard-amd64@sha256:2c4421ed80358a0ee97b44357b6cd6dc09be6ccc27dfe9d50c9bfc39a760e5fe",name="k8s_kubernetes-dashboard_kubernetes-dashboard-5b575fd4c-77fqr_kube-system_d60f089f-0a85-11e8-9462-06d7638bd978_3",namespace="kube-system",pod_name="kubernetes-dashboard-5b575fd4c-77fqr"} 1.521297361e+09

  • A full listing of the containers running on this host plus a lot of metadata about each one. For example, this single metric from one pod offers:

    ...snip...
container_cpu_load_average_10s{container_name="kubernetes-dashboard",id="/kubepods/podd60f089f-0a85-11e8-9462-06d7638bd978/a3c7cd6d8e0ebf603b431604ab7a1844f8ed0b651b231b10849010fd7cf37b17",image="gcr.io/google_containers/kubernetes-dashboard-amd64@sha256:2c4421ed80358a0ee97b44357b6cd6dc09be6ccc27dfe9d50c9bfc39a760e5fe",name="k8s_kubernetes-dashboard_kubernetes-dashboard-5b575fd4c-77fqr_kube-system_d60f089f-0a85-11e8-9462-06d7638bd978_3",namespace="kube-system",pod_name="kubernetes-dashboard-5b575fd4c-77fqr"} 0
...snip...
    • Container name: kubernetes-dashboard
    • Namespace: kube-system
    • Image: gcr.io/google_containers/kubernetes-dashboard-amd64
    • Image Version/Hash: sha256:2c4421ed80358a0ee97b44357b6cd6dc09be6ccc27dfe9d50c9bfc39a760e5fe
    • Pod name: kubernetes-dashboard-5b575fd4c-77fqr
    • Kubernetes Deployment UID: d60f089f-0a85-11e8-9462-06d7638bd978
    • Runtime (Docker) Container ID: a3c7cd6d8e0ebf603b431604ab7a1844f8ed0b651b231b10849010fd7cf37b17

Using just this information from cAdvisor, it's possible to gather a tremendous amount of information about the node's Host OS, the Network interface names, the CPU/RAM/Net/Disk utilization (over time), the processes running, and how long they've been running. What a helpful service!

Probe the "Insecure" Kubernetes API service:

Verify the port is responding:

$ nc -vz $CONTROLLERIP 8080
Connection to 54.89.108.72 port 8080 [tcp/http-alt] succeeded!

Curl the API Server directly:

$ $ curl $CONTROLLERIP:8080
{
  "paths": [
    "/api",
    "/api/v1",
    "/apis",
    "/apis/",
    "/apis/admissionregistration.k8s.io",
    "/apis/admissionregistration.k8s.io/v1beta1",
    "/apis/apiextensions.k8s.io",
    "/apis/apiextensions.k8s.io/v1beta1",
    "/apis/apiregistration.k8s.io",
    "/apis/apiregistration.k8s.io/v1beta1",
    "/apis/apps",
    "/apis/apps/v1",
    "/apis/apps/v1beta1",
    "/apis/apps/v1beta2",
    "/apis/authentication.k8s.io",
    "/apis/authentication.k8s.io/v1",
    "/apis/authentication.k8s.io/v1beta1",
    "/apis/authorization.k8s.io",
    "/apis/authorization.k8s.io/v1",
    "/apis/authorization.k8s.io/v1beta1",
    "/apis/autoscaling",
    "/apis/autoscaling/v1",
    "/apis/autoscaling/v2beta1",
    "/apis/batch",
    "/apis/batch/v1",
    "/apis/batch/v1beta1",
    "/apis/certificates.k8s.io",
    "/apis/certificates.k8s.io/v1beta1",
    "/apis/events.k8s.io",
    "/apis/events.k8s.io/v1beta1",
    "/apis/extensions",
    "/apis/extensions/v1beta1",
    "/apis/networking.k8s.io",
    "/apis/networking.k8s.io/v1",
    "/apis/policy",
    "/apis/policy/v1beta1",
    "/apis/rbac.authorization.k8s.io",
    "/apis/rbac.authorization.k8s.io/v1",
    "/apis/rbac.authorization.k8s.io/v1beta1",
    "/apis/storage.k8s.io",
    "/apis/storage.k8s.io/v1",
    "/apis/storage.k8s.io/v1beta1",
    "/healthz",
    "/healthz/autoregister-completion",
    "/healthz/etcd",
    "/healthz/ping",
    "/healthz/poststarthook/apiservice-openapi-controller",
    "/healthz/poststarthook/apiservice-registration-controller",
    "/healthz/poststarthook/apiservice-status-available-controller",
    "/healthz/poststarthook/bootstrap-controller",
    "/healthz/poststarthook/ca-registration",
    "/healthz/poststarthook/generic-apiserver-start-informers",
    "/healthz/poststarthook/kube-apiserver-autoregistration",
    "/healthz/poststarthook/start-apiextensions-controllers",
    "/healthz/poststarthook/start-apiextensions-informers",
    "/healthz/poststarthook/start-kube-aggregator-informers",
    "/healthz/poststarthook/start-kube-apiserver-informers",
    "/logs",
    "/metrics",
    "/swagger-2.0.0.json",
    "/swagger-2.0.0.pb-v1",
    "/swagger-2.0.0.pb-v1.gz",
    "/swagger.json",
    "/swaggerapi",
    "/ui",
    "/ui/",
    "/version"
  ]
}

Because the unencrypted request was successful and the list of available endpoints came back, it means we have full access to the API server and can get/update/delete any information stored inside the cluster.

Obtain the version of the Kubernetes API Server:

$ curl $CONTROLLERIP:8080/version
{
  "major": "1",
  "minor": "9",
  "gitVersion": "v1.9.2",
  "gitCommit": "5fa2db2bd46ac79e5e00a4e6ed24191080aa463b",
  "gitTreeState": "clean",
  "buildDate": "2018-01-18T09:42:01Z",
  "goVersion": "go1.9.2",
  "compiler": "gc",
  "platform": "linux/amd64"
}

List the services available via the API's built-in /ui proxy:

$ curl $CONTROLLERIP:8080/ui/
<a href="/api/v1/namespaces/kube-system/services/https:kubernetes-dashboard:/proxy/">Temporary Redirect</a>.

Because we have sufficient access to the cluster API, visiting this URL will likely result in the ability to access the Kubernetes dashboard in this cluster.

Obtain logs from containers, pods, and system logging endpoints on the underlying host:

$ curl $CONTROLLERIP:8080/logs/
<pre>
<a href="apt/">apt/</a>
<a href="auth.log">auth.log</a>
<a href="auth.log.1">auth.log.1</a>
<a href="auth.log.2.gz">auth.log.2.gz</a>
<a href="auth.log.3.gz">auth.log.3.gz</a>
<a href="btmp">btmp</a>
<a href="btmp.1">btmp.1</a>
<a href="cloud-init-output.log">cloud-init-output.log</a>
<a href="cloud-init.log">cloud-init.log</a>
<a href="containers/">containers/</a>
<a href="dist-upgrade/">dist-upgrade/</a>
<a href="dpkg.log">dpkg.log</a>
<a href="dpkg.log.1">dpkg.log.1</a>
<a href="fsck/">fsck/</a>
<a href="kern.log">kern.log</a>
<a href="kern.log.1">kern.log.1</a>
<a href="lastlog">lastlog</a>
<a href="lxd/">lxd/</a>
<a href="pods/">pods/</a>
<a href="syslog">syslog</a>
<a href="syslog.1">syslog.1</a>
<a href="syslog.2.gz">syslog.2.gz</a>
<a href="syslog.3.gz">syslog.3.gz</a>
<a href="syslog.4.gz">syslog.4.gz</a>
<a href="syslog.5.gz">syslog.5.gz</a>
<a href="syslog.6.gz">syslog.6.gz</a>
<a href="syslog.7.gz">syslog.7.gz</a>
<a href="unattended-upgrades/">unattended-upgrades/</a>
<a href="wtmp">wtmp</a>
<a href="wtmp.1">wtmp.1</a>
</pre>

View, for example, the host auth.log:

$ curl $CONTROLLERIP:8080/logs/auth.log
...snip...
Apr 10 02:17:01 ip-10-1-0-10 CRON[1571]: pam_unix(cron:session): session opened for user root by (uid=0)
Apr 10 02:17:01 ip-10-1-0-10 CRON[1571]: pam_unix(cron:session): session closed for user root
Apr 10 02:20:53 ip-10-1-0-10 sudo: pam_unix(sudo:session): session closed for user root
Apr 10 02:20:54 ip-10-1-0-10 sshd[32757]: Received disconnect from x.x.x.x port 55139:11: disconnected by user
Apr 10 02:20:54 ip-10-1-0-10 sshd[32757]: Disconnected from x.x.x.x port 55139
Apr 10 02:20:54 ip-10-1-0-10 sshd[32718]: pam_unix(sshd:session): session closed for user ubuntu
Apr 10 02:20:54 ip-10-1-0-10 systemd-logind[1231]: Removed session 630.
Apr 10 02:20:54 ip-10-1-0-10 systemd: pam_unix(systemd-user:session): session closed for user ubuntu

View the kubernetes-dashboard pod logs:

$ curl $CONTROLLERIP:8080/logs/pods/d60f089f-0a85-11e8-9462-06d7638bd978/kubernetes-dashboard_2.log
...snip...
{"log":"Using apiserver-host location: http://10.1.0.10:8080\n","stream":"stdout","time":"2018-02-05T15:08:17.444915083Z"}
{"log":"Skipping in-cluster config\n","stream":"stdout","time":"2018-02-05T15:08:17.444920926Z"}
{"log":"Using random key for csrf signing\n","stream":"stdout","time":"2018-02-05T15:08:17.444923968Z"}
{"log":"No request provided. Skipping authorization header\n","stream":"stdout","time":"2018-02-05T15:08:17.44492672Z"}
{"log":"Successful initial request to the apiserver, version: v1.9.2\n","stream":"stdout","time":"2018-02-05T15:08:17.44887775Z"}

Mimic the kubectl get pods command:

$ curl $CONTROLLERIP:8080/api/v1/namespaces/default/pods?limit=500
{
  "kind": "PodList",
  "apiVersion": "v1",
  "metadata": {
    "selfLink": "/api/v1/namespaces/default/pods",
    "resourceVersion": "1633976"
  },
  "items": [
    {
      "metadata": {
        "name": "subpath",
        "namespace": "default",
        "selfLink": "/api/v1/namespaces/default/pods/subpath",
        "uid": "3fb02fa6-2e9e-11e8-9d04-06d7638bd978",
        "resourceVersion": "418041",
        "creationTimestamp": "2018-03-23T13:29:44Z"
      },
      "spec": {
        "volumes": [
...snip...

While not terribly practical, it is possible to interact with the API using curl as shown above. However, having the kubectl binary makes things much more user-friendly.

Probe the Kubelet Healthz service:

Verify the port is responding:

$ nc -vz $CONTROLLERIP 10248
nc: connectx to 54.89.108.72 port 10248 (tcp) failed: Connection refused

It's only running on the localhost address:

$ ssh -i ${KEY_NAME}.pem ubuntu@$CONTROLLERIP
ubuntu@ip-10-1-0-10:~$ curl localhost:10248/
404 page not found
ubuntu@ip-10-1-0-10:~$ curl localhost:10248/healthz
ok

Probe the Kube-Proxy Metrics service:

Verify the port is responding:

$ nc -vz $CONTROLLERIP 10249
nc: connectx to 54.89.108.72 port 10249 (tcp) failed: Connection refused

It's only running on the localhost address:

$ ssh -i ${KEY_NAME}.pem ubuntu@$CONTROLLERIP
ubuntu@ip-10-1-0-10:~$ curl localhost:10249/healthz
ok
ubuntu@ip-10-1-0-10:~$ curl localhost:10249/metrics

ubuntu@ip-10-1-0-10:~$ curl localhost:10249/metrics

It can tell us how kube-proxy reaches the API server on 10.1.0.10:8080 without encryption necessary.

Probe the Kubelet Read/Write service:

Verify the port is responding:

$ nc -vz $CONTROLLERIP 10250
Connection to 54.89.108.72 port 10250 [tcp/*] succeeded!

Hit tcp/10250 via curl:

$ curl $CONTROLLERIP:10250

$ curl -sk https://$CONTROLLERIP:10250
404 page not found

The lack of an authn/authz error means this port is unprotected, and it provides an extremely useful attack path to leverage the kubelet to have remote command execution inside nearly any pod/container, access to any pod log on that system, and access to any secret available to that node at a minimum.

So, the Kubelet is always listening on a TLS port, but by default, it's not authenticating or authorizing access to it. The -s is to be "silent" and the -k tells curl to allow connections without certificates.

According to the source code, the following endpoints are available on both the Kubelet "read-only" API and "read/write" API:

  • /metrics
  • /metrics/cadvisor
  • /spec/
  • /stats/

The following endpoints are only available on the Kubelet's "read/write" API:

  • /logs/ - Get logs from a pod/container.
  • /run/ - Alias for /exec/
  • /exec/ - Exec a command in a running container
  • /attach/ - Attach to the stdout of a running container
  • /portForward/ - Forward a port directly to a container
  • /containerLogs/ - Get logs from a pod/container.
  • /runningpods/ - Lists all running pods in short JSON form
  • /debug/pprof/ - Various go debugging performance endpoints

Directly leveraging the unprotected kubelet API to:

As you can see, the kubelet is essentially a remote API running as root on your system that /always/ needs additional hardening to prevent seriously useful avenues for escalation.

Probe the Kubernetes Scheduler HTTP service:

Verify the port is responding:

$ nc -vz $CONTROLLERIP 10251
Connection to 54.89.108.72 port 10251 [tcp/*] succeeded!

Hit tcp/10251 via curl:

$ curl $CONTROLLERIP:10251
404 page not found
$ curl $CONTROLLERIP:10251/healthz
ok

Probe the Kubernetes Controller Manager service:

Verify the port is responding:

$ nc -vz $CONTROLLERIP 10252
Connection to 54.89.108.72 port 10252 [tcp/apollo-relay] succeeded!

Hit tcp/10252 via curl:

$ curl $CONTROLLERIP:10252
404 page not found
$ curl $CONTROLLERIP:10252/healthz
ok

Probe the Kubelet Read-Only service:

Verify the port is responding:

$ nc -vz $CONTROLLERIP 10255
Connection to 54.89.108.72 port 10255 [tcp/*] succeeded!

According to the source code, the following endpoints are available on the Kubelet "read-only" API:

  • /metrics
  • /metrics/cadvisor
  • /spec/
  • /stats/

Hit tcp/10255 via curl:

$ curl $CONTROLLERIP:10255
404 page not found

The /healthz endpoint reports the health of the Kubelet.

$ curl $CONTROLLERIP:10255/healthz
ok

The /metrics from the Kubelet indicate how busy the node is in terms of the docker runtime and how many containers "churn" on this node.

$ curl $CONTROLLERIP:10255/metrics

The /metrics/cadvisor endpoint passes through the metrics from the cAdvisor port.

$ curl $CONTROLLERIP:10255/metrics/cadvisor

The /spec/ endpoint writes the cAdvisor MachineInfo() output, and this gives a couple hints that it runs on AWS, the instance type, and the instance ID:

$ curl $CONTROLLERIP:10255/spec/

The /pods endpoint provides the near-equivalent of kubectl get pods -o json for the pods running on this node:

$ curl -s $CONTROLLERIP:10255/pods

Probe the Kube-Proxy Healthcheck service:

Verify the port is responding:

$ nc -vz $CONTROLLERIP 10256
Connection to 54.89.108.72 port 10256 [tcp/*] succeeded!

Hit tcp/10256 via curl:

$ curl $CONTROLLERIP:10256
404 page not found
$ curl $CONTROLLERIP:10256/healthz
{"lastUpdated": "2018-03-27 21:17:09.14039841 +0000 UTC m=+888081.767665429","currentTime": "2018-03-27 21:17:36.679120214 +0000 UTC m=+888109.306387163"}

Access to the Kubernetes API or the Kubelet read/write API from anywhere is almost a guaranteed full compromise of the cluster including all of its data, secrets, and source code.

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