Enforcing IMDSv2…

…is a quick and easy way of earning Schrute bucks from your security team.

Essentially, IMDSv2 lowers the risk of getting caught up in an SSRF-type cybersecurity incident. The AWS docs do a better job of explaining the nitty gritty, so you can read more about it here.

During my exploration of enforcing IMDSv2 on a bunch of EC2 instances, I noticed that the CDK construct that creates EC2 instances (ec2.Instance) has an optional parameter called require_imdsv2.

Easy! So all I need to do is flick the switch to true and all will be well!

Whomp whomp 👎️

Not so fast. Unfortunately, flicking that switch in the standard construct causes EC2 instance replacement on existing instances!

That’s ok-ish for brand new instances being deployed, but it’s not really ideal when you have lots of pets running production (and soon to be) workloads 🙃

Custom Resources to the rescue!

Thanks to the AWS SDK, we have heaps of APIs available that we can consume within our CDK stack to help do things that are a little fiddly out of the box with CDK.

For this task, I created a new construct. It had two functions:

  • on_create: Execute an SDK API call to modifyInstanceMetadataOptions, setting HttpTokens to required, when an optional parameter in the CDK stack is set to true.
  • on_delete: Execute the same SDK API call reverting the modifyInstanceMetadataOptions HttpTokens parameter back to optional (allowing IMDSv1 calls). This is to cover removal of the optional parameter in the CDK stack.

In the end, this is what my new construct looks like -

from constructs import Construct
import aws_cdk as cdk
from aws_cdk import (

class EnforceImdsv2(Construct):
    """ This construct enforces the use of IMDSv2, without causing the EC2 instance to be replaced on update.
    def __init__(
        context: Construct,
        construct_id: str,
        instance_id: str,
    ) -> None:

        super().__init__(context, construct_id)

                    "InstanceId": instance_id,
                    "HttpTokens": "required",
                    "InstanceMetadataTags": "enabled"
            # Revert the changes if this custom resource is ever removed
                    "InstanceId": instance_id,
                    "HttpTokens": "optional",
                    "InstanceMetadataTags": "disabled"

Can we really see what impact we’ve made by enforcing IMDSv2?

Well yeah, kind of. We can see how many instances are interacting with IMDS with no token by looking at MetadataNoToken metric in CloudWatch.

This is a snippet of CloudWatch before I enforced IMDSv2 on a bunch of instances: img1

… and this is a day later, showing a vast decrease of calls to IMDS without a token: img1

Here’s the CloudWatch metric graph source for the above (or click here for even less effort):

    "metrics": [
        [ { "expression": "SELECT SUM(MetadataNoToken) FROM \"AWS/EC2\"", "label": "MetadataNoToken", "id": "q1", "period": 60, "stat": "Sum" } ]
    "view": "timeSeries",
    "stacked": true,
    "region": "ap-southeast-2",
    "stat": "Sum",
    "period": 60,
    "legend": {
        "position": "bottom"
    "yAxis": {
        "left": {
            "min": 0,
            "max": 280

Ciao for now! 👋