Canarytokens' new member: AWS API key Canarytoken

This is the fourth post in a series highlighting bits from our recent BlackHat USA 2017 talk. An index of all the posts in the series is here.

Introduction

In this blog post, we will introduce you to the newest member of our Canarytoken’s family, the Amazon Web Services API key token. This new Canarytoken allows you to sprinkle AWS API keys around and then notifies you when they are used. (If you stick around to the end, we will also share some of the details behind how we built it).

Background

Amazon Web Services offers a massive range of services that are easily integratable with each other. This encourages companies to build entire products and product pipelines using the AWS suite. In order to automate and manipulate AWS services using their API, we are given access keys which can be restricted by AWS policies. Access keys are defined on a per user basis which means there are a few moving parts in order to lock down an AWS account securely.

Take it for a spin - using an AWS API key Canarytoken

Using the AWS API key Canarytoken is as simple as can be. Simply make use of the free token server at http://canarytokens.org or use the private Canarytoken server built into your Canary console. Select the ‘AWS Keys’ token from the drop down list.



Enter an email and a token reminder (Remember: The email address is the one we will notify when the token is tripped, and the reminder will be attached to the alert. Choose a unique reminder, nothing sucks more than knowing a token is tripped, but being unsure where you left it). Then click on “Create my Canarytoken”.



You will notice that we arrange your credentials in the same way as the AWS console usually does, so you can get straight down to using (or testing) them. So lets get to testing. Click “Download your AWS Creds” and save the file somewhere you will find it.

For our tests, we are going to use the AWS Commandline tool (if you don’t have it yet, head over to http://docs.aws.amazon.com/cli/latest/userguide/installing.html). Below is a simple bash script that will leverage the AWS command line tool to create a new user named TestMePlease using your new-almost-authentic AWS API keys.

Simply go to your command line, navigate to the same location as the script and type, ./test_aws_creds.sh <access_key_id> <secret_access_key> . If all went to plan, you should be receiving an alert notifying you that your AWS API key Canarytoken was used.

NB: Due to the way these alerts are handled (by Amazon) it can sometimes take up to 20 minutes for the alert to come through.

Waiting...waiting...waiting (0-20mins later). Ah we got it!


Check...it...out! This is what your AWS API key Canarytoken alert will look like, delivered by email. The email will contain some useful details such as User Agent, Source IP and a reminder of where you may have placed this Canarytoken (we always assumed you not going to use only one! Why would you? They are free!!).

The simple plan then should be: Create a bunch of fake keys. Keep one on the CEO’s laptop. (He will never use it, but the person who compromises him will). Keep one on your webserver (again, no reason for it to be used, except by the guy who pops a shell on that box, etc)

Under the hood - steps to creating an AWS API key Canarytoken

The AWS API key Canarytoken makes use of a few AWS services to ensure that the Canarytoken is an actual AWS API key - indistinguishable from a real working AWS API key. This is important because we want to encourage attackers to have to use the key to find out how juicy it actually is - or isn’t. We also want this to be dead simple to use. Enter your details and click a button. If you want to see how the sausage is made, read on:


Creation - And on the 5th day…


The first service necessary for creating these AWS API key Canarytokens, is an AWS Lambda that is triggered by an AWS API Gateway event. Let’s follow the diagram’s flow. Once you click the ‘Create my Canarytoken’ button, a GET request is sent to the AWS API Gateway. This request contains query parameters for the domain (of the Canarytokens server), the username (if we want to specifiy one, otherwise a random one is generated) and the actual Canarytoken that will be linked to the created AWS API key. This is where the free version and commercial versions diverge slightly.

Our free version of Canarytokens (canarytokens.org), does not allow you to specify your own username for the AWS API key Canarytoken. The domain of the Canarytoken server is used in conjunction with the Canarytoken to create the AWS user on the account. (This is still completely useful, because the only way an attacker is able to obtain the username tied to the token, is to make an API call, and this call itself will trigger the alert). Our private Canary consoles enjoy a slightly different implementation. This uses an AWS Dynamo Database that links the users to their tokens and allowing clients the opportunity to specify what the user name for your AWS user should be. 

If the AWS API Gateway determines that sufficient information is included in the request, it triggers the lambda responsible for creating the AWS API key Canarytoken. This lambda creates a new user with no privileges on the AWS account, generates AWS API keys for that user and responds to the request with a secret access key and an access key id.


We should note that the newly created user has no permissions (to anything), so anyone with this AWS API key can’t do anything of importance. (Even if they did, its a user on our infrastructure, not yours!). Of course, before the attacker is able to find out how impotent her key is, she first has to use it and this is when we catch them out (detection time!).

Detection - I see you! 

Now that the AWS API key has been created and returned to the user, lets complete the loop and figure out when these AWS API keys are being used. The first service in our detection process, spoken about in our previous posts, is CloudTrail. CloudTrail is super useful when monitoring anything on an AWS account because it logs all important (not all) API calls recording the username, the keys used, the methods called, the user-agent information and a whole lot more. 

We configure CloudTrail to send its logs to another AWS logging service known as CloudWatch. This service allows subscriptions and filtering rules to be applied. This means that if a condition in the logs from CloudTrail is met in the CloudWatch service, it will trigger whichever service you configure it to - in our case another AWS Lambda function. In pure AWS terms, we have created a subscription filter which will send logs that match the given filter to our chosen lambda.

For the AWS API key Canarytoken, we use a subscription filter such as

  • "FilterPattern": "{$.userIdentity.type = IAMUser}"

This filter will check the incoming logs from CloudTrail and only send logs that contain the user identity as an IAM User - this is different from using root credentials as the user is then ‘root’.

Alert - Danger Will Robinson, danger!

All thats left now is for us to generate our Alert. We employ an AWS Lambda (again) to help us with this. This lambda receives the full log of the attempted AWS API call and bundles it into a custom HTTP Request that trips the Canarytoken. Our Canarytoken Server receives the request with all this information and relays the alert to you with all the information formatted neatly.

Summary - TLDR;

Amazon Web Services is a massive collection of easily integratable services which enables companies of all sizes to build entire products and services with relative ease. This makes AWS API keys an attractive target for many attackers.

The AWS API key Canarytoken allows the creation of real AWS API keys which can be strewn around your environment. An attacker using these credentials will trigger an alert informing you of his presence (and other useful meta information).. It’s quick, simple, reliable and a high quality indicator of badness.

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