How to get certifiates checked on elasticsearch-output-plugin?

Hi,

I installed logstash on centos via rpm and installed via system-install as service.

When starting logstash I get the following warning:

** WARNING ** You have enabled encryption but DISABLED certificate verification.
** WARNING ** To make sure your data is secure change :ssl_certificate_verification to true

My output config looks like this:

output
{
  elasticsearch
  {
    #hosts                          => ["${ES_HOST}:${ES_PORT}"]
    hosts                           => ["${ES_HOSTS}"]
    ssl                             => "${USE_ES_SSL}"
    cacert                          => "${ES_CA_CERT_PATH}"
    ssl_certificate_verification    => "${USE_ES_OUTPUT_SSL_CERT_VERIFICATION}"

    # credentials are fetched from envrionment or logstash-keystore

    user            => "${LOGSTASH_USER}"
    password        => "${LOGSTASH_PASSWORD}"

    index           => "%{[@metadata][indexName]}"
  }
}

The Environment variable USE_ES_OUTPUT_SSL_CERT_VERIFICATION is set via /etc/sysconfig/<service_name>:

cat /etc/sysconfig/logstash-plx-0 | grep USE_ES_OUTPUT_SSL_CERT_VERIFICATION
USE_ES_OUTPUT_SSL_CERT_VERIFICATION=true

Using logstash 7.4.1

Any idea? Since I condigure cacert I would like to have the certificate checked.

Thanks, Andreas