Rooting a Googlebook Breaks Cast My Apps, Magisk Developer Finds

If you root your Googlebook, Cast My Apps stops working. John Wu, the developer behind the popular Android rooting tool Magisk, found that an Android phone will refuse to cast its apps to a Googlebook that fails a security check called hardware key attestation. A rooted Googlebook fails that check, so the phone says no. Android Authority picked up the find on Monday, November 2.

1. What Cast My Apps does

The short version: Cast My Apps lets you open supported apps from your Android phone in a window on your Googlebook, so you can use a phone-only app on the big screen without picking up your phone. It needs a phone running Android 17.

Why it matters: it is one of the headline features of Googlebook OS, and one of the main ways Googlebooks feel tied to your phone. That makes it a big thing to lose.

2. What John Wu found

The discovery: Wu has been running a Magisk rooted Googlebook and noticed app casting never worked on it. On Sunday he shared why:

A quick note on Wu: he is the creator of Magisk and, as Android Authority notes, also works at Google. He has been open about liking Googlebooks for tinkering, and in October he called them “the ideal platform for Android modding.”

3. Why attestation blocks it

What attestation is: Googlebook OS is built on Android. Like Android phones, a Googlebook keeps a key in secure hardware it can use to prove to other devices that it is running unmodified, verified software. That proof is called hardware key attestation.

Why a rooted Googlebook fails: rooting a Googlebook means unlocking its bootloader and changing its system software, as Wu has explained, so it can no longer give that proof. When you start Cast My Apps, the phone asks for it, doesn’t get it, and refuses to cast.

Why a phone would care: your phone apps can hold messages, banking details and other private data. As Android Authority points out, casting them to a device whose software has been changed could be a risk for your phone, which is likely why the check is there.

4. What we know so far

How to read it: Yes and No are based on Wu’s own posts and the Magisk release notes. Not tested means no one has reported results yet.

5. Magisk now supports Googlebooks

Official support: Wu spent weeks adapting Magisk to Googlebooks, which run Android in a different way from phones. Magisk 31.1 arrived on October 25 with official support for Googlebooks, which run what the release notes call Aluminium OS.

What changed: to make it work, Magisk now asks you to grant root access to apps ahead of time instead of showing a pop-up prompt, and its app hiding feature is gone.

The trade-off: Wu’s find is a reminder that rooting a Googlebook is not free. You gain full control of the laptop, which is great for things like turning it into a dev machine with the Linux terminal, but you lose Cast My Apps, at least for now.

Our take

We love seeing people push Googlebooks further, and tinkerers are a big part of what made Chromebooks and Android great. If Cast My Apps is a feature you use every day, think twice before rooting, since there is no known way around this check right now. If you root for the Linux terminal or full control and rarely cast phone apps, you may not miss it, and Wu’s point still stands: unlike a rooted phone, a rooted Googlebook can still reach banking and payment sites in the browser. We’ll update this post if Google says more.