AWS protect SSRF vulnerabilities with new EC2 Instance Metadata feature

Geoffroy RENAUD — Wed, 20 Nov 2019 21:39:50 +0000

After the annonce on the AWS Security blog for protecting AWS instance from SSRF attack, we should test and implement this new security feature: https://aws.amazon.com/fr/blogs/security/defense-in-depth-open-firewalls-reverse-proxies-ssrf-vulnerabilities-ec2-instance-metadata-service/

AWS documentation about this feature : https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html#configuring-instance-metadata-options

There is 2 methods in order to use it :

Add a policy to your users/geoups with this example policy :

{
    "Version": "2012-10-17",
    "Statement": [{
        "Sid": "RunInstanceWithImdsV2Only",
        "Effect": "Allow",
        "Action": "ec2:RunInstances",
        "Resource": "*",
        "Condition": {
            "StringEquals": {
                "ec2:MetadataHttpTokens": "required"
            }
        }
    }]
}

Modify a running instance metadata :

aws ec2 modify-instance-metadata-options --instance-id i-1234567898abcdef0 --http-token required

We need to enforce this best practice if possible by design using AWS Organization SCP.

Here a first try for a policy to Enforce EC2 Metadata Token :

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "EnforceEC2metadataTOKEN",
            "Effect": "Deny",
            "Action": "ec2:RunInstances",
            "Resource": [
                "*"
            ],
            "Condition": {
                "StringNotEquals": {
                    "ec2:MetadataHttpTokens": "required"
                }
            }
        }
    ]
}

Seems working for me, I need to try more when Terraform or CDK will implement the feature to completely validate:

@ejcx_ wrote an article about this feature with a incomplete satisfaction, here why with explanation: https://ejj.io/blog/fixing-capital-one

Forem: Geoffroy RENAUD

AWS protect SSRF vulnerabilities with new EC2 Instance Metadata feature