I’m a long-time Jenkins user. And I always thought that it’s only possible to get secrets via some kind of expose from runner.

However, there’s an easy way(even scary how easy it is). If you have access to Script Console /scripts, you can run following snippet to retrieve ALL secrets:

com.cloudbees.plugins.credentials.SystemCredentialsProvider.getInstance().getCredentials().forEach{
  it.properties.each { prop, val ->
    if (prop == "secretBytes") {
      println(prop + "=>\n" + new String(com.cloudbees.plugins.credentials.SecretBytes.fromString("${val}").getPlainData()) + "\n")
    } else {
      println(prop + ' = "' + val + '"')
    }
  }
  println("-----------------------")
}

Taken from here