Skipper Ingress Controller Allows Unauthorized Access to Internal Services via ExternalName

Impact

When running Skipper as an Ingress controller, users with permissions to create an Ingress and a Service of type ExternalName can create routes that enable them to use Skipper's network access to reach internal services.

Patches

https://github.com/zalando/skipper/releases/tag/v0.24.0 disables Kubernetes ExternalName by default.

Workarounds

Developers can allow list targets of an ExternalName by using -kubernetes-only-allowed-external-names=true and allow list via regular expressions -kubernetes-allowed-external-name '^[a-z][a-z0-9-.]+[.].allowed.example$'

References

https://kubernetes.io/docs/concepts/services-networking/service/#externalname