Explanation
When the principal is an AWS service, the ARN of the specific resource within that service to grant permission to.
Without this, any resource from principal will be granted permission – even if that resource is from another account.
For S3, this should be the ARN of the S3 Bucket. For CloudWatch Events, this should be the ARN of the CloudWatch Events Rule. For API Gateway, this should be the ARN of the API
Insecure Example
The following example will fail the AWS058 check.
resource "aws_lambda_permission" "bad_example" {
statement_id = "AllowExecutionFromSNS"
action = "lambda:InvokeFunction"
function_name = aws_lambda_function.func.function_name
principal = "sns.amazonaws.com"
}Secure Example
The following example will pass the AWS058 check.
resource "aws_lambda_permission" "good_example" {
statement_id = "AllowExecutionFromSNS"
action = "lambda:InvokeFunction"
function_name = aws_lambda_function.func.function_name
principal = "sns.amazonaws.com"
source_arn = aws_sns_topic.default.arn
}