Code inject via nginx.ingress.kubernetes.io/permanent-redirect annotation

How it was found

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Summary:

The value of the nginx.ingress.kubernetes.io/permanent-redirect annotation will be not sanitized and passed into the nginx configuration. This leads into a code inject from any user that is allowed to create ingress objects.

Kubernetes Version:

v1.26.3 (minikube)

Component Version:

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NGINX Ingress controller
  Release:       v1.8.0
  Build:         35f5082ee7f211555aaff431d7c4423c17f8ce9e
  Repository:    https://github.com/kubernetes/ingress-nginx
  nginx version: nginx/1.21.6

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Steps To Reproduce:

1. Install ingress-nginx, using latest version and default values. For demo purpose, I set allow-snippet-annotations=false

   helm upgrade -i ingress-nginx ingress-nginx/ingress-nginx -f values.yaml # values.yaml is attached
   

2. apply service and ingress object from attachments

   k apply -f ingress.yaml #ingress.yaml is attached
   

3. Optional: If ingress-nginx is not exposed, run kubectl port-forward deploy/ingress-nginx-controller 8080:80 and continue step 4 in a separate shell.

4. Validate, if the code is injected. This demo uses the hostname kubernetes.api, use the --resolve parameter of curl to do an request for the hidden server instance. The code below expect that ingress-nginx is accessible trough 127.0.0.1:8080

   curl -v --resolve "kubernetes.api:8080:127.0.0.1" http://kubernetes.api:8080/api/v1/namespaces/kube-system/secrets/
   

Supporting Material/References:

• values.yaml - Used in step 1
• ingress.yaml - Used in step 2

Impact

All users with access to create or update ingress objects, are able to running commands on ingress-nginx-controller pod. Since the token of the ServiceAccount is mounted on filesystem, a user can call the Kubernetes API and fetch all secrets or config maps from the cluster. Additionally, the user can read or write files to the filesystem.