Mount Azure FileShare using terraform
Recently I've read a good book about Terraform that I think answers your question (Terraform Up &amp; Running 2nd edition. ISBN-13: 978-1492046905. ISBN-10: 1492046906). 

Basically it says that all data in Terraform state file is stored in plain text, including sensitive data. The author says that at the moment of the writing (May 2019) there is no real solution for this, but workarounds like encryption and IAM to control user access based on policies (the book talks about AWS, so you have to find the Azure equivalent and prevent unauthorized access to the state file).

You can find more details on Chapter 3, pages 78 and 79. There is also an open issue at Github about this: https://github.com/hashicorp/terraform/issues/516.