microg/GmsCore · error · SecurityException

Caller is not Services Core

Error message

Caller is not Services Core

What it means

WearableListenerService.onBind/onStartCommand (post) validates that the calling UID belongs to Google Play services (GMS package) before dispatching wearable events. If the caller's packages, looked up via getPackagesForUid, do not include GMS_PACKAGE_NAME, a SecurityException is thrown. This protects the listener from spoofed Intents sent by other apps.

Source

Thrown at play-services-wearable/src/main/java/com/google/android/gms/wearable/WearableListenerService.java:145

    }

    @Override
    public void onPeerDisconnected(Node peer) {
    }

    private class Listener extends IWearableListener.Stub {
        private int knownGoodUid = -1;

        private boolean post(Runnable runnable) {
            int callingUid = Binder.getCallingUid();
            if (callingUid != knownGoodUid) {
                // TODO: Verify Gms is calling
                String[] packagesForUid = getPackageManager().getPackagesForUid(callingUid);
                if (packagesForUid != null) {
                    if (Arrays.asList(packagesForUid).contains(GMS_PACKAGE_NAME)) {
                        knownGoodUid = callingUid;
                    } else {
                        throw new SecurityException("Caller is not Services Core");
                    }
                }
            }
            synchronized (lock) {
                if (disconnected) {
                    return false;
                }
                serviceHandler.post(runnable);
                return true;
            }
        }

        @Override
        public void onDataChanged(final DataHolder data) throws RemoteException {
            post(new Runnable() {
                @Override
                public void run() {
                    WearableListenerService.this.onDataChanged(new DataEventBuffer(data));

View on GitHub (pinned to 157c9d86ac)

Solutions

  1. Ensure wearable events come only from the platform/Play services routing (use the Wearable DataClient/MessageClient APIs instead of manual Intents).
  2. On custom ROMs, install a properly signed GmsCore (e.g. microG signed variant) so the caller UID maps to the GMS package.
  3. In tests, mock the wearable clients rather than delivering Intents directly to WearableListenerService.
  4. Check getPackagesForUid(callingUid) in a debugging session to identify which package actually sent the Intent.

Example fix

// before
context.startService(new Intent(context, MyListenerService.class)
    .setAction(DataMapListenerIntentAction));

// after
Wearable.getDataClient(context).addListener(myListener); // let Play services deliver events
Defensive patterns

Strategy: try-catch

Validate before calling

String[] pkgs = getPackageManager().getPackagesForUid(callingUid);
boolean fromGms = pkgs != null && Arrays.asList(pkgs).contains("com.google.android.gms");

Try / catch

try {
    listenerService.onMessageReceived(event);
} catch (SecurityException e) {
    Log.w(TAG, "Untrusted wearable event caller", e);
}

Prevention

When it happens

Trigger: Another app (or a test harness) sends an explicit Intent to WearableListenerService (action com.google.android.gms.wearable.DATA_CHANGED etc.) with a UID not owned by Google Play services; the service handles it via onDataChanged/onMessageReceived/onPeerConnected/onPeerDisconnected/onConnectedNodes/onConnectedCapabilityChanged and post() throws.

Common situations: Developers manually broadcasting fake wearable Intents to test the listener; running the app on a device without a properly installed/signed Play Services package; custom ROMs with unsigned GmsCore; instrumentation tests binding to the service directly.

Understand the failure class

Background: Permission denied / not authorized / 403 Forbidden: access-control rejections when the caller lacks the required role, grant, or ownership — this error's family across 18 libraries.

Related errors

AI-assisted analysis of microg/GmsCore@157c9d86ac (2026-09-06). Data as JSON: /api/errors/4a8e1275d1a7a50a. Report an issue: GitHub.