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Introduction
This post is the second in a series about alternatives to bastion hosts in each of the major cloud providers. The first post covered an introduction to bastion hosts, the SSH multiplexing attack, some disadvantages to managing your own bastions, and an alternative solution in GCP. In this post, we’ll cover the Session Manager service provided by AWS. Although there are other methods for accessing EC2 instances in AWS, Session Manager is the best match for our requirements, which are:
- Does not require public IP addresses for the VMs.
- Eliminates the possibility of SSH multiplexing attacks.
- Consolidates access (such as SSH or RDP) to IAM credentials.
- Captures metadata and full session logs (when possible) for remote access.
The Session Manager service is provided as a component of AWS Systems Manager. To use Session Manager, you must set up AWS Systems Manager. While Session Manager supports Linux and Windows instances, we will focus on using it with Elastic Compute Cloud (EC2) Linux instances running in AWS. If you have a hybrid environment, which also contains VMs in your own facility, then you can still use this solution with some additional steps that we do not cover in this post.
Why AWS Session Manager?
Session Manager can be used to access instances within private subnets that allow no ingress from the internet. This is made possible by the Systems Manager (SSM) agent running on the EC2 instances, which pushes traffic to the Session Manager service. The configuration shown in the diagram below provides an example of how it can be configured, which will be explained in more detail in the sections below.
The figure shows that the EC2 instances are not at all accessible from the internet, even by our authorized client. The SSM agent starts the session for us, and pushes traffic to the interface endpoints we’ve created in the VPC for the SSM services. The Session Manager service acts as the intermediary, providing the client with a shell for an EC2 instance.
The client machine is outside of AWS, and is being used by someone with valid user credentials for the AWS environment. The client can create a session in two different parts of the AWS console, or via the AWS Command Line Interface (CLI). It is possible to restrict how users launch sessions, so you could choose to only allow them to invoke a connection via the AWS CLI (which requires the installation of the Session Manager Plugin) if your users don’t have the ability to log into the console, or the other way around.
AWS SSM provides the ability to establish a shell on your systems through its native service, or by using it as a tunnel for other protocols, such as Secure Shell (SSH). The advantages of using SSM without tunneling SSH are:
- It will log the commands issued during the session, as well as the results. If you use SSH within Session Manager, then this will all be encrypted.
- No need to manage SSH keys.
- Shell access is completely contained within Identity and Access Management (IAM) policies, so there is one central choke point to control that access.
Now that you have an idea of how AWS SSM works in general, we’ll take a closer look at it by breaking down the details into the following categories:
- Networking Considerations
- Virtual Machine Configuration
- Identity Management
- Logging
Networking Considerations
As shown in our diagram above, the SSM agent actually uses HTTPS. The amount of network traffic that needs to be allowed is minimal. If you follow the steps below, then it is not necessary to open port 22 to ingress traffic, even within the VPC:
- Create VPC interface endpoints in the target VPCs. This makes use of AWS PrivateLink, so the connection traffic between the target EC2 instances and the Session Manager service does not traverse the internet. Create the endpoints for the following service names (substitute your own region in the names below, such as ‘us-west-2’):
- com.amazonaws.[region].ssm
- com.amazonaws.[region].ssmmessages
- Configure the Security Groups and Network ACLs that protect the target EC2 instances to allow HTTPS egress traffic (443) from the instances to the Systems Manager endpoints. It is not necessary to allow any ingress traffic to the EC2 instances. The way we recommend to configure this is to add the endpoints and EC2 instances to a security group, and only allow the traffic within the members of that security group: