diff --git a/cluster/common.sh b/cluster/common.sh
index 10a547fe40e..2a20e26e6c1 100755
--- a/cluster/common.sh
+++ b/cluster/common.sh
@@ -753,6 +753,7 @@ NUM_NODES: $(yaml-quote ${NUM_NODES})
 STORAGE_BACKEND: $(yaml-quote ${STORAGE_BACKEND:-etcd3})
 STORAGE_MEDIA_TYPE: $(yaml-quote ${STORAGE_MEDIA_TYPE:-})
 ENABLE_GARBAGE_COLLECTOR: $(yaml-quote ${ENABLE_GARBAGE_COLLECTOR:-})
+ENABLE_LEGACY_ABAC: $(yaml-quote ${ENABLE_LEGACY_ABAC:-})
 MASTER_ADVERTISE_ADDRESS: $(yaml-quote ${MASTER_ADVERTISE_ADDRESS:-})
 ETCD_CA_KEY: $(yaml-quote ${ETCD_CA_KEY_BASE64:-})
 ETCD_CA_CERT: $(yaml-quote ${ETCD_CA_CERT_BASE64:-})
diff --git a/cluster/gce/config-default.sh b/cluster/gce/config-default.sh
index b7097139bc8..0386c80b227 100755
--- a/cluster/gce/config-default.sh
+++ b/cluster/gce/config-default.sh
@@ -204,6 +204,9 @@ SCHEDULING_ALGORITHM_PROVIDER="${SCHEDULING_ALGORITHM_PROVIDER:-}"
 # Optional: install a default StorageClass
 ENABLE_DEFAULT_STORAGE_CLASS="${ENABLE_DEFAULT_STORAGE_CLASS:-true}"
 
+# Optional: Enable legacy ABAC policy that makes all service accounts superusers.
+ENABLE_LEGACY_ABAC="${ENABLE_LEGACY_ABAC:-true}" # true, false
+
 # TODO(dawn1107): Remove this once the flag is built into CVM image.
 # Kernel panic upon soft lockup issue
 SOFTLOCKUP_PANIC="${SOFTLOCKUP_PANIC:-false}" # true, false
diff --git a/cluster/gce/config-test.sh b/cluster/gce/config-test.sh
index b9c4c7780a5..57cd7835701 100755
--- a/cluster/gce/config-test.sh
+++ b/cluster/gce/config-test.sh
@@ -241,6 +241,17 @@ SCHEDULING_ALGORITHM_PROVIDER="${SCHEDULING_ALGORITHM_PROVIDER:-}"
 # Optional: install a default StorageClass
 ENABLE_DEFAULT_STORAGE_CLASS="${ENABLE_DEFAULT_STORAGE_CLASS:-true}"
 
+# Optional: Enable legacy ABAC policy that makes all service accounts superusers.
+if [[ "${E2E_UPGRADE_TEST:-}" == "true" ]]; then
+  # Enable (match the regular default) when running upgrade tests (E2E_UPGRADE_TEST=true is set by upgrade CI jobs).
+  # This ensures the combination of legacy ABAC and default RBAC policies work properly for upgrade scenarios.
+  ENABLE_LEGACY_ABAC="${ENABLE_LEGACY_ABAC:-true}" # true, false
+else
+  # Disable by default when running regular e2e tests.
+  # This ensures default RBAC policies alone are sufficient for e2e tests from 1.6+
+  ENABLE_LEGACY_ABAC="${ENABLE_LEGACY_ABAC:-false}" # true, false
+fi
+
 # TODO(dawn1107): Remove this once the flag is built into CVM image.
 # Kernel panic upon soft lockup issue
 SOFTLOCKUP_PANIC="${SOFTLOCKUP_PANIC:-true}" # true, false
diff --git a/cluster/gce/container-linux/configure-helper.sh b/cluster/gce/container-linux/configure-helper.sh
index 8cafc811a91..8d9015d20f7 100755
--- a/cluster/gce/container-linux/configure-helper.sh
+++ b/cluster/gce/container-linux/configure-helper.sh
@@ -888,10 +888,27 @@ function start-kube-apiserver {
   fi
 
   local authorization_mode="RBAC"
-  if [[ -n "${ABAC_AUTHZ_FILE:-}" && -e "${ABAC_AUTHZ_FILE}" ]]; then
-    params+=" --authorization-policy-file=${ABAC_AUTHZ_FILE}"
+  local -r src_dir="${KUBE_HOME}/kube-manifests/kubernetes/gci-trusty"
+
+  # Enable ABAC mode unless the user explicitly opts out with ENABLE_LEGACY_ABAC=false
+  if [[ "${ENABLE_LEGACY_ABAC:-}" != "false" ]]; then  
+    echo "Warning: Enabling legacy ABAC policy. All service accounts will have superuser API access. Set ENABLE_LEGACY_ABAC=false to disable this."
+    # Create the ABAC file if it doesn't exist yet, or if we have a KUBE_USER set (to ensure the right user is given permissions)
+    if [[ -n "${KUBE_USER:-}" || ! -e /etc/srv/kubernetes/abac-authz-policy.jsonl ]]; then
+      local -r abac_policy_json="${src_dir}/abac-authz-policy.jsonl"
+      remove-salt-config-comments "${abac_policy_json}"
+      if [[ -n "${KUBE_USER:-}" ]]; then
+        sed -i -e "s/{{kube_user}}/${KUBE_USER}/g" "${abac_policy_json}"
+      else
+        sed -i -e "/{{kube_user}}/d" "${abac_policy_json}"
+      fi
+      cp "${abac_policy_json}" /etc/srv/kubernetes/
+    fi
+
+    params+=" --authorization-policy-file=/etc/srv/kubernetes/abac-authz-policy.jsonl"
     authorization_mode+=",ABAC"
   fi
+
   local webhook_config_mount=""
   local webhook_config_volume=""
   if [[ -n "${GCP_AUTHZ_URL:-}" ]]; then
@@ -900,7 +917,6 @@ function start-kube-apiserver {
     webhook_config_mount="{\"name\": \"webhookconfigmount\",\"mountPath\": \"/etc/gcp_authz.config\", \"readOnly\": false},"
     webhook_config_volume="{\"name\": \"webhookconfigmount\",\"hostPath\": {\"path\": \"/etc/gcp_authz.config\"}},"
   fi
-  local -r src_dir="${KUBE_HOME}/kube-manifests/kubernetes/gci-trusty"
   params+=" --authorization-mode=${authorization_mode}"
 
   local container_env=""
diff --git a/cluster/gce/gci/configure-helper.sh b/cluster/gce/gci/configure-helper.sh
index b59f098ad26..bdd9fac92dc 100644
--- a/cluster/gce/gci/configure-helper.sh
+++ b/cluster/gce/gci/configure-helper.sh
@@ -1091,8 +1091,10 @@ function start-kube-apiserver {
   local authorization_mode="RBAC"
   local -r src_dir="${KUBE_HOME}/kube-manifests/kubernetes/gci-trusty"
 
-  # Create the ABAC file only if it's explicitly requested.
-  if [[ -n "${ENABLE_LEGACY_ABAC_16_ONLY:-}" ]]; then
+  # Enable ABAC mode unless the user explicitly opts out with ENABLE_LEGACY_ABAC=false
+  if [[ "${ENABLE_LEGACY_ABAC:-}" != "false" ]]; then  
+    echo "Warning: Enabling legacy ABAC policy. All service accounts will have superuser API access. Set ENABLE_LEGACY_ABAC=false to disable this."
+    # Create the ABAC file if it doesn't exist yet, or if we have a KUBE_USER set (to ensure the right user is given permissions)
     if [[ -n "${KUBE_USER:-}" || ! -e /etc/srv/kubernetes/abac-authz-policy.jsonl ]]; then
       local -r abac_policy_json="${src_dir}/abac-authz-policy.jsonl"
       remove-salt-config-comments "${abac_policy_json}"
@@ -1103,15 +1105,11 @@ function start-kube-apiserver {
       fi
       cp "${abac_policy_json}" /etc/srv/kubernetes/
     fi
-  fi
 
-  # Load existing ABAC policy files written by versions < 1.6 of this script
-  # TODO: only default to this legacy path when in upgrade mode
-  local abac_authorization_file="${ABAC_AUTHZ_FILE:-/etc/srv/kubernetes/abac-authz-policy.jsonl}"
-  if [[ -n "${abac_authorization_file:-}" && -e "${abac_authorization_file}" ]]; then
-    params+=" --authorization-policy-file=${abac_authorization_file}"
+    params+=" --authorization-policy-file=/etc/srv/kubernetes/abac-authz-policy.jsonl"
     authorization_mode+=",ABAC"
   fi
+
   local webhook_config_mount=""
   local webhook_config_volume=""
   if [[ -n "${GCP_AUTHZ_URL:-}" ]]; then