What is the safest way to include sensitive variables in a CI/CD pipeline for Terraform?

Passing variables to Terraform with a -var flag is a common method utilized in CI/CD pipelines to securely handle sensitive data. This technique allows you to provide variables at runtime without hardcoding them into your configuration files or source code repository. By using the -var flag, sensitive information can be dynamically supplied from a secure environment, such as a CI/CD system's secret management tool or environment variables, thus reducing the risk of exposing sensitive data.

In contrast, storing sensitive variables as plain text in a source code repository and copying them directly into Terraform code increases the risk of accidental leaks or unauthorized access. Using a secure_vars.tf file can seem secure, but if that file is checked into version control or if access isn't tightly controlled, it can still expose sensitive information. Hence, passing variables with the -var flag represents a more flexible and safer approach for managing sensitive data within a CI/CD environment.