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Minimal `basti` IAM Policy Allows Shell Access

Low severity GitHub Reviewed Published Aug 23, 2023 in basti-app/basti

Package

npm basti-cdk (npm)

Affected versions

< 1.0.1

Patched versions

1.0.1

Description

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 port forwarding capability to make the target available on your localhost."

The Minimal IAM Policy 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.

References

@BohdanPetryshyn BohdanPetryshyn published to basti-app/basti Aug 23, 2023
Published to the GitHub Advisory Database Aug 24, 2023
Reviewed Aug 24, 2023

Severity

Low

Weaknesses

No CWEs

CVE ID

No known CVE

GHSA ID

GHSA-q4pp-j36h-3gqg

Source code

Credits

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