Terraform does not have a native data type or input mechanism called "secure string." While other systems like AWS Systems Manager Parameter Store use this terminology, it is not a feature within the Terraform language (HCL) for defining or passing variables. Terraform can mark variables as sensitive to redact their values from CLI output, but the underlying type remains a standard string. The other options represent valid, documented methods for providing sensitive data to a Terraform configuration without hardcoding it.

A. A Terraform provider: Providers like HashiCorp Vault, AWS Secrets Manager, or Azure Key Vault are specifically designed to fetch secrets dynamically at runtime using data sources, which is a recommended practice.

B. Environment variables: Terraform automatically reads environment variables prefixed with TFVAR to populate input variables, providing a common method for injecting secrets from a CI/CD system.

C. A -var flag: The -var 'foo=bar' command-line flag is a direct way to pass variable values, including secrets, during plan or apply operations, keeping them out of the configuration files.

1. HashiCorp Terraform Documentation

"Input Variables": This document details the primary ways to assign values to variables. It explicitly lists command-line flags (-var and -var-file) and Environment Variables (TFVAR...) as valid methods. It does not mention a "secure string" mechanism.

Source: HashiCorp

Terraform Language Documentation

"Assigning Values to Root Module Variables".

URL: https://developer.hashicorp.com/terraform/language/values/variables#assigning-values-to-root-module-variables

2. HashiCorp Terraform Documentation

"Sensitive Values": This page explains the sensitive argument for variables and outputs. It clarifies that this is a boolean flag (sensitive = true) applied to a variable to suppress its value in UI output

not a distinct data type.

Source: HashiCorp

Terraform Language Documentation

"Suppressing Values in CLI Output".

URL: https://developer.hashicorp.com/terraform/language/values/variables#suppressing-values-in-cli-output

3. HashiCorp Vault Provider Documentation: This official provider documentation demonstrates the use of data sources (e.g.

data "vaultgenericsecret" "...") to retrieve secrets from an external system (Vault)

confirming that providers are a valid method for secret management.

Source: HashiCorp

Terraform Registry

"Vault Provider - vault\generic\secret Data Source".

URL: https://registry.terraform.io/providers/hashicorp/vault/latest/docs/data-sources/genericsecret