Minimal `basti` IAM Policy Allows Shell Access

Summary

### Summary

The provided Minimal IAM Policy for `bastic connect` does not include `ssm:SessionDocumentAccessCheck`. This results in the ability to get a shell session on the bastion, not just the intended access for Port Forwarding.

### Details

`basti connect` is designed to "securely connect to your RDS/Aurora/Elasticache/EC2 instances", using a bastion instance "with [AWS Session Manager](https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager.html) port forwarding capability to make the target available on your localhost."

The [Minimal IAM Policy](https://github.com/BohdanPetryshyn/basti#minimal-iam-permissions) allows port forwarding via the following statement:

``` { "Effect": "Allow", "Action": "ssm:StartSession", "Resource": [ "arn:aws:ssm:*:*:document/AWS-StartPortForwardingSessionToRemoteHost", "arn:aws:ec2:<your-region>:<your-account-id>:instance/<your-basti-instance-id>" ] } ```

This statement does not include the following condition:

``` "Condition": { "BoolIfExists": { "ssm:SessionDocumentAccessCheck": "true" } } ```

As a result, the `basti connect` minimal policy is logically identical to:

``` { "Effect": "Allow", "Action": "ssm:StartSession", "Resource": [ "arn:aws:ssm:*:*:document/AWS-StartPortForwardingSessionToRemoteHost", "arn:aws:ssm:*:*:document/SSM-SessionManagerRunShell", "arn:aws:ec2:<your-region>:<your-account-id>:instance/<your-basti-instance-id>" ] } ```

A `basti` admin would expect users under the minimal policy to be able to port forward. However, they could also get a shell on the bastion.

For more details on this footgun, see: https://ramimac.me/ssm-iam

### PoC _Complete instructions, including specific configuration details, to reproduce the vulnerability._

### Impact

Impact would depend on configuration/hardening of the bastion. I've seen examples where bastions have credentials to downstream systems in configuration or memory that would be exposed.