RootMyTV

RootMyTV is a "remote" root exploit chain, and Jailbreak, for LG webOS smart TVs.

All you need is an internet-connected smart TV, and a TV remote (or alternatively, an Arduino and an IR LED!).

How it works

In summary, we use a chain of exploits to get persistent root code execution. As root, we install the webOS Homebrew Channel app, and disable various security/sandboxing/jailing anti-features.

Rooting

Background

webOS, as the name suggests, is a Smart TV operating system mostly based on web technologies. Applications, both system and external are either run in a stripped down web browser ("WebAppMgr") or in Qt QML runtime. Almost all system and external applications run in chroot-based jails as an additional security layer.

"Web apps", outside of standard web technologies, also get access to an API for communicating with "Luna Service Bus". This is a bus, similar to D-Bus, used to exchange messages and provide various services across different security domains. Bus clients can expose some RPC methods to other applications (identified by URIs luna://service-name/prefix-maybe/method-name) which accept JSON object message as their call parameters, and then can return one or many messages. (depending on the call being "subscribable" or not)

While Luna bus seems to have extensive ACL handling, considering the history of webOS IP transfers, seems like not many engineers fully understand its capabilities. Part of the bus is marked as "private", which is only accessible by certain system applications, while most of the other calls are "public" and can be accessed by all apps.

Unexpectedly, one of the "public" services exposed on a bus is "LunaDownloadMgr" which provides a convenient API for file download, progress tracking, etc... Said service has been researched in the past and an identity confusion bug leading to an arbitrary unjailed root file write vulnerability has been publicly documented.

This in of itself was not very helpful in production hardware, thus we needed to find a way of calling an arbitrary Luna service from an application with com.webos. / com.palm. / com.lge. application ID.

Step #0 - Getting in (stage1.html)

In order to gain initial programmatic control of the TV user interface an interface of "LG Connect Apps" can be used. Its protocol called "SSAP" is a simple websocket-based RPC mechanism that can be used to indirectly interact with Luna Service bus and has been extensively documented in various home-automation related contexts. We use that to launch a vulnerable system application which is not easily accessible with plain user interaction.

Step #0.1 - Escaping the origins

SSAP API is meant to be used from an external mobile app. For the sake of simplicity, though, we wanted to serve our exploit as a web page. This lead us to notice, that, understandably, SSAP server explicitly rejects any connections from HTTP origins. However, there was an additional exception from that rule, and seemingly authors wanted to allow file:// origins, which present themselves to the server as null. Turns out there's one other origin that can be used that is also reprted as null and that is data: URIs.

In order to exploit this, we've created a minimal WebSocket API proxy implementation that opens a hidden iframe with a javascript payload (which is now running in a data:/null origin) and exchanges the messages with the main browser frame. This has been released as a separate library.

Step #0.2 - General Data Protocol Redirection

There's a minor problem with establishing the connection with SSAP websocket server. While we all believe in utter chaos, we don't feel very comfortable with serving our exploit over plain HTTP, which would be the only way of avoiding Mixed Content prevention policies. (by default https origins are not allowed to communicate with plain http endpoints)

While some newer Chromium versions do allow Mixed Content communication with localhost, that was not the case when Chromium 38 was released (used in webOS 3.x). Thankfully, it seems like the system browser on webOS 3.x is also vulnerable to something that has been considered a security issue in most browsers for a while now - navigation to data: URIs. Thus, when applicable, our exploits attempts to open itself as a data: base64-encoded URI. This makes our browser no longer consider the origin being secure, and we can again access the plain-http WebSocket server.

Mitigation note

An observant reader may have noticed that the service we use is meant to be used remotely. While the connection itself needs a confirmation using a remote we highly recommend to disable LG Connect Apps functionality in order to prevent remote exploitation, or at least to keep the TV on a separate network.

Step #1 - Social login escape (stage1.html)

Having some initial programmatic control of the TV via SSAP we can execute any application present on the TV. All cross-application launches can contain an extra JSON object called launchParams. This is used to eg. open a system browser with specific link open, or launch a predetermined YouTube video. Turns out this functionality is also used to select which social website to use in com.webos.app.facebooklogin, which is the older sibling of com.webos.app.iot-thirdparty-login used in initial exploit, present on all webOS versions up until (at least) 3.x.

When launching social login via LG Account Management this application accepts an argument called server. This turns out to be a part of URL that "web app" browser is navigated to. Thus, using properly prepared launchParams we are able to open an arbitrary web page (with the only requirement being it served over https) running as a system app that is considered by LunaDownloadMgr a "system" app.

Step #2 - Download All The Things (stage2.html)

Since we are already running as a system application, we can download files (securely over https!) into arbitrary unjailed filesystem locations as root.

We use that to download following files:

• stage3.sh → /media/cryptofs/apps/usr/palm/services/com.palmdts.devmode.service/start-devmode.sh - this is the script executed at startup by /etc/init/devmode.conf as root, in order to run developer mode jailed SSH daemon.
• hbchannel.ipk → /media/internal/downloads/hbchannel.ipk - since our end goal is intalling the Homebrew Channel app, we can also just download it during the earlier stages of an exploit and confirm it's actually downloaded.
• devmode_enabled → /var/luna/preferences/devmode_enabled - this is the flag checked before running start-devmode.sh script, and is just a dummy file.

Step #3 - Homebrew Channel Deployment (stage3.sh)

stage3.sh script is a minimal tool that, after opening an emergency telnet shell and removing itself (in case something goes wrong and the user needs to reboot a TV - script keeps running but will no longer be executed on next startup), installs the homebrew channel app via standard devmode service calls and elevates its service to run unjailed as root as well.

Legacy

"LG ThinQ Login" is a privileged app, which is used to sign in to various "smart" services. If we use the option to sign in with an Amazon account, we can click web links, and ultimately end up on google.com. From there, we can search and navigate to RootMy.TV, which hosts the next stage of the exploit. Any javascript that we run has privileged access to various "private" Luna IPC APIs, including DownloadManager, which has a publicly documented arbitrary-root-file-write vulnerability.

Using DownloadManager, we download the Homebrew Channel app, force-enable the developer mode setting, and then download a shell script to /media/cryptofs/apps/usr/palm/services/com.palmdts.devmode.service/start-devmode.sh.

Then, we use another Luna API call to reboot the TV. When the TV boots back up, and on every subsequent boot, our code in start-devmode.sh script gets run as root.

Jailbreaking

Normally, the only way to run your own code on webOS is to enable Developer Mode, which is an officially supported feature. There are four big problems with LG's Developer Mode:

1. It requires creating an online account with LG, which in turn requires accepting oppressive ToS agreements.

2. When developer mode times out, all developer-installed apps are removed.

3. Apps run inside a restricted chroot jail, under the unprivileged user account "prisoner".

4. Apps can only access "public" Luna APIs, which significantly restricts their potential functionality.

The start-devmode.sh startup script contains code to overcome these limitations, as follows:

• It starts a telnet server, allowing full remote root access to the TV, for debugging, research etc.

• It patches sam (System and Application Manager) at runtime, to allow installing and launching (non-devmode) apps from unofficial sources. Apps installed in this way can access "private" Luna APIs.

• It remounts the app data paritions without the nosuid flag, enabling native apps with the setuid filesystem permission bit to run as root. Apps with root privileges can trivially escape from the chroot jail.

• System telemetry is disabled by setting the "immutable" filesystem permission bit, on various telemetry log directories.

Homebrew Channel

To take full advantage of these new features, we created the "webOS Homebrew Channel" app. This app allows users and developers to easily "sideload" their own apps.

It also provides some Luna IPC services which may be useful for jailbroken app development, including the ability to run shell commands as root. We also provide a user-friendly interface to manage various configuration options, like locking software update nagging, early boot user scripts with some fallback in case of system crashes or exposing root SSH daemon.