How should I pass sensitive environment variables to Amazon ECS tasks?

Approach 1:

You can use Parameter Store to store the variables. If you store them as SecureString, the values will be encrypted.

You can reference them as environment variables in the task definition.

You need to retrieve them in the container startup script

value_from_parameter_store =`aws ssm get-parameter --name $parameter_store_key --with-decryption --output text --query Parameter.Value --region $REGION `

You can also mention parameter_store_key as an environment variable. so that you can use $parameter_store_key

Example

Dockerfile:

FROM ubuntu
//some other steps
CMD ["sh","/startup.sh"]

startup script:

#! /bin/bash
export db_password =`aws ssm get-parameter --name $parameter_store_key --with-decryption --output text --query Parameter.Value --region $REGION `
// Please note that above line has `(backtick) 
// Do other stuff and use this password 

Put parameter in SSM:

aws ssm put-parameter --name 'db_password' --type "SecureString" --value 'P@ssW%rd#1'

Docker run command:

docker run -e parameter_store_key=db_password -e REGION=us-east-1 <docker_image>

Approach 2:

Recently AWS announced secrets support in ContainerDefinition for ECS Using Secrets in ECS

In the task definition link you posted there is an "environment" section that allows you to do this. They become environment variables inside the container.

If you mean you would like to keep information outside of the task definition and that task could reference it, you cannot. Your best bet in that case is to have your container pull that information from an outside source and not have the ECS task config try to reference it.

Edit: Im getting downvoted at this point because the parameter store is now the right way to do it. At the time this answer was the most correct way, but the other solutions using SSM are the right way now.

Parameter store is the way to go, it stores the variables encrypted using a KMS key.

Amazon has just announced support for specifying secrets in the task definition. Reference the parameter value from the SSM and the environment variable to set with the task.

{
        ....
        "secrets": [
            {
                "name": "environment_variable_name",
                "valueFrom": "arn:aws:ssm:region:aws_account_id:parameter/parameter_name"
            }
        ]
    }

See the official docs here.

There's also a project called chamber that can load all parameters from a given path in SSM and set them as environment variables.