The bitnami/pgpool Docker image, and the bitnami/postgres-ha k8s chart, under default configurations, comes with an repmgr user that allows unauthenticated access to the database inside the cluster. The PGPOOL_SR_CHECK_USER is the user that Pgpool itself uses to perform streaming replication checks against nodes, and should not be at trust level. This allows to log into a PostgreSQL database using the repgmr user without authentication. If Pgpool is exposed externally, a potential attacker could use this user to get access to the service. This is also present within the bitnami/postgres-ha Kubernetes Helm chart.
The product initializes or sets a resource with a default that is intended to be changed by the administrator, but the default is not secure.
Developers often choose default values that leave the product as open and easy to use as possible out-of-the-box, under the assumption that the administrator can (or should) change the default value. However, this ease-of-use comes at a cost when the default is insecure and the administrator does not change it.