CVE-2023-32196: Rancher's External RoleTemplates can lead to privilege escalation

A vulnerability has been identified whereby privilege escalation checks are not properly enforced for RoleTemplateobjects when external=true, which in specific scenarios can lead to privilege escalation.

The bug in the webhook rule resolver ignores rules from a ClusterRole for external RoleTemplates when its context is set to either project or is left empty. The fix introduces a new field to the RoleTemplate CRD named ExternalRules. The new field will be used to resolve rules directly from the RoleTemplate. Additionally, rules from the backing ClusterRole will be used if ExternalRules is not provided. The new field will always take precedence when it is set, and serve as the source of truth for rules used when creating Rancher resources on the local cluster.

Please note that this is a breaking change for external RoleTemplates, when context is set to project or empty and the backing ClusterRole does not exist, as this was not previously required.