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Governments have been secretly tracking the app activity of an unknown number of people using Apple and Google smartphones, US Senator Ron Wyden (D-Ore.) revealed today.

In a letter demanding that the Department of Justice update or repeal policies prohibiting companies from informing the public about these covert government requests, Wyden warned that “Apple and Google are in a unique position to facilitate government surveillance of how users are using particular apps.”

Push notifications are used to provide a wide variety of alerts to app users. A friendly ding or text alert on the home screen notifies users about new text messages, emails, social media comments, news updates, packages delivered, gameplay nudges—basically any app activity where notifications have been enabled could be tracked by governments, Wyden said.