ingress-nginx mirror-target self-DOS

A security issue was discovered in ingress-nginx where a user with permission to create an Ingress may be able to cause denial of service of the ingress-nginx data plane, or possibly redirect traffic from other Ingresses. Kubernetes clusters are only affected if they are using ingress-nginx. Due to a missing check, the mirror-target Ingress annotation can be used to target an unauthenticated configuration API within the ingress-nginx controller, allowing write commands to be submitted. Corrupting the data within that configuration API results in all traffic through ingress-nginx being dropped until an Ingress configuration change or a controller restart causes the data to be recreated correctly. In principle, this write access could also be used to redirect traffic from other Ingresses controlled by the same ingress-nginx instance. In practice, there is negligible additional risk: such redirection would require privileged information about internal ingress-nginx state, and users can already redirect traffic from other Ingresses controlled by the same ingress-nginx anyway.