Reasonably so, but not cryptographically secure, at least in Kubernetes 1.8. If you are looking at the secrets from a security perspective, the constraints on secrets are better than leaving values in ConfigMap, but the security profile has significant limits.
At the heart, the data for secrets is stored in plain-text (albiet encoded text) in etcd 3.0, which underpins Kubernetes 1.8. It does not use encryption at rest, or symmetric keys to preserve (and access) the secrets. If you are running your own Kubernetes cluster, be aware that an unsecured etcd represents a significant weakness in the overall security of the cluster.
For many applications and use cases, this is perfectly acceptable, ...
