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With increased focus on security and governance in today’s digital economy, I want to highlight a simple but important use case that demonstrates how to use AWS Identity and Access Management (IAM) with Security Token Service (STS) to give trusted AWS accounts access to resources that you control and manage.

Security Token Service is an extension of IAM and is one of several web services offered by AWS that does not incur any costs to use.  But, unlike IAM, there is no user interface on the AWS console to manage and interact with STS. Rather all interaction is done entirely through one of several extensive SDKs or directly using common HTTP protocol.  I will be using Terraform to create some simple resources in my sandbox account and .NET Core SDK to demonstrate how to interact with STS.

The main purpose and function of STS is to issue temporary security credentials for AWS resources to trusted and authenticated entities.  These credentials operate identically to the long-term keys that typical IAM users have, with a couple of special characteristics:

  • They automatically expire and become unusable after a short and defined period of time elapses
  • They are issued dynamically

These characteristics offer several advantages in terms of application security and development and are useful for cross-account delegation and access.  STS solves two problems for owners of AWS resources:

  • Meets the IAM best-practices requirement to regularly rotate access keys
  • You do not need to distribute access keys to external entities or store them within an application

One common scenario where STS is useful involves sharing resources between AWS accounts.  Let’s say, for example, that your organization captures and processes data in S3, and one of your clients would like to push large amounts of data from resources in their AWS account to an S3 bucket in your account in an automated and secure fashion.

While you could create an IAM user for your client, your corporate data policy requires that you rotate access keys on a regular basis, and this introduces challenges for automated processes.  Additionally, you would like to limit the distribution of access keys to your resources to external entities.  Let’s use STS to solve this!

To get started, let’s create some resources in your AWS cloud.  Do you even Terraform, bro?

Let’s create a new S3 bucket and set the bucket ACL to be private, meaning nobody but the bucket owner (that’s you!) has access.  Remember that bucket names must be unique across all existing buckets, and they should comply with DNS naming conventions.  Here is the Terraform HCL syntax to do this:

Great! We now have a bucket… but for now, only the owner can access it.  This is a good start from a security perspective (i.e. “least permissive” access).

What an empty bucket may look like

Let’s create an IAM role that, once assumed, will allow IAM users with access to this role to have permissions to put objects into our bucket.  Roles are a secure way to grant trusted entities access to your resources.  You can think about roles in terms of a jacket that an IAM user can wear for a short period of time, and while wearing this jacket, the user has privileges that they wouldn’t normally have when they aren’t wearing it.  Kind of like a bright yellow Event Staff windbreaker!

For this role, we will specify that users from our client’s AWS account are the only ones that can wear the jacket. This is done by including the client’s AWS account ID in the Principal statement.  AWS Account IDs are not considered to be secret, so your client can share this with you without compromising their security.  If you don’t have a client but still want to try this stuff out, put your own AWS account ID here instead.


Great, now we have a role that our trusted client can wear.  But, right now our client can’t do anything except wear the jacket.  Let’s give the jacket some special powers, such that anyone wearing it can put objects into our S3 bucket.  We will do this by creating a security policy for this role.  This policy will specify what exactly can be done to S3 buckets that it is attached to. Then we will attach it to the bucket we want our client to use.  Here is the Terraform syntax to accomplish this:

A couple things to note about this snippet – First, we are using Terraform interpolation to inject values from previous terraform statements into a couple of places in the policy – specifically the ARN from the role and bucket we created previously.  Second, we are specifying a condition for the s3 policy – one that requires a specific object ACL for the action s3:PutObject, which is accomplished by including the HTTP request header x-amz-acl to have a value of bucket-owner-full-control with the PUT object request.  By default, objects PUT in S3 are owned by the account that created them, even if it is stored in someone else’s bucket.  For our scenario, this condition will require your client to explicitly grant ownership of objects placed in your bucket to you, otherwise the PUT request will fail.

So, now we have a bucket, a policy in place on our bucket, and a role that assumes that policy.  Now your client needs to get to work writing some code that will allow them to assume the role (wear the jacket) and start putting objects into your bucket.  Your client will need to know a couple of things from you before they get started:

  1. The bucket name and the region it was created in (the example above created a bucket named d4h2123b9-xaccount-bucket in us-west-2)
  2. The ARN for the role (Terraform can output this for you). It will look something like this but will have your actual AWS Account ID: arn:aws:iam::123456789012:role/sts-delegate-role

They will also need to create an IAM User in their account and attach a policy allowing the user to assume roles via STS.  The policy will look similar to this:


Let’s help your client out a bit and provide some C# code snippets for .NET Core 2.0 (available for Windows, macOS and LinuxTo get started, install the .NET SDK for your OS, then fire up a command prompt in a favorite directory and run these commands:

The first command will create a new console app in the subdirectory s3cli.  Then switch context to that directory and import the AWS SDK for .NET Core, and then add packages for SecurityToken and S3 services.
Once you have the libraries in place, fire up your favorite IDE or text editor (I use Visual Studio Code), then open Program.cs and add some code:

This snippet sends a request to STS for temporary credentials using the specified ARN.  Note that the client must provide IAM user credentials to call STS, and that IAM user must have a policy applied that allows it to assume a role from STS.

This next snippet takes the STS credentials, bucket name, and region name, and then uploads the Program.cs file that you’re editing and assigns it a random key/name.  Also note that it explicitly applies the Canned ACL that is required by the sts-delegate-role:

So, to put this all together, run this code block and make the magic happen!  Of course, you will have to define and provide proper variable values for your environment, including  securely storing your credentials.

Try it out from the command prompt:

If all goes well, you will have a copy of Program.cs in the bucket. Not very useful itself, but it illustrates how to accomplish the task.

What a bucket with something in it may look like

Here is a high-level document of what we put together:

Putting it all together

Steps:

  1. Your client uses their IAM user to call AWS STS and requests the role ARN you gave them
  2. STS authenticates the client’s IAM user and verifies the policy for the ARN role, then issues a temporary credential to the client.
  3. The client can use the temporary credentials to access your S3 bucket (they will expire soon), and since they are now wearing the Event Staff jacket, they can successfully PUT stuff in your bucket!

There are many other use-cases for STS. This is just one very simplistic example. However, with this brief introduction to the concepts, you should now have a decent idea of how STS works with IAM roles and policies, and how you can use STS to give access to your AWS resources for trusted entities. For more tips like this, contact us.

-Jonathan Eropkin, Cloud Consultant

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