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Avast Antivirus JavaScript Interpreter

NOTE: On 03/11/2020 Avast announced they had decided to disable this interpreter globally!

The main Avast antivirus process is called AvastSvc.exe, which runs as SYSTEM.

AvastSvc.exe

That service loads the low level antivirus engine, and analyzes untrusted data received from sources like the filesystem minifilter or intercepted network traffic.

Despite being highly privileged and processing untrusted input by design, it is unsandboxed and has poor mitigation coverage. Any vulnerabilities in this process are critical, and easily accessible to remote attackers.

So.. maybe not great that it includes a custom JavaScript interpreter....???? 🙃

screenshot

This repository contains an interactive shell that lets you test the interpreter on Linux for vulnerability research.

Building

Here's how to try it out, first install the dependencies.

Ubuntu

$ sudo apt install libreadline-dev:i386 libc6-dev-i386 gcc-multilib

Fedora

$ sudo yum install readline-devel.i686 glibc-devel.i686 libgcc.i686

Now you can clone this repository.

$ git clone https://github.com/taviso/avscript.git
$ cd avscript
$ git submodule update --init --recursive

If everything looks good, build it and avscript should be ready.

$ make

Notes

Reproducing Vulnerabilities on Windows

For performance reasons, Avast do not interpret every JavaScript file they encounter, they use a heuristic to determine if it's necessary. I've found that appending the file javascript.txt included in this repository is enough to always trigger the heuristic.

For example, if you have found a vulnerability and want to reproduce it on Windows, you would first do this:

$ cat yourtestcase.js javascript.txt > ReproForWindows.js

Now verify that it still does what you expect, e.g.

$ ./avscript ReproForWindows.js
main(): File ReproForWindows.js` loaded, about to initialize interpreter...
Segmentation fault (core dumped)

Then verify the crash reproduced on Windows.

Protected Process

The Avast service is a protected process, which means debugging it from userspace is tricky. If you have kd configured, you can simply undo this and then debugging in userspace works fine.

A quick and easy solution that works on 32-bit is to do this (note that PatchGuard won't permit this on x64, but you can do something similar with breakpoint commands).

screenshot

There is also a setting under "Troubleshooting" called "Enable Self-Defense" that should be disabled. I believe this setting disables hooking OpenProcess() in the SSDT, where they normally blacklist their own process.

You may need to temporarily disable "shields" in the Avast UI while you attach so that filesystem operations don't deadlock while the service is suspended.

Debugging

Avast does not publish any symbols for their engine, but debugging with IDA or gdb is still possible. There are some notes on debugging Windows code from Linux in the loadlibrary documentation.

If you want to use IDA, I would recommend using the gdbserver backend.

Simply use something like this, you might need to install the gdbserver package first:

$ gdbserver 0.0.0.0:23946 ./avscript

This works surprisingly well, even pseudocode breakpoints work.

IDA screenshot

Vulnerabilities

If you find a vulnerability, it is likely critical and wormable.

🔥 Please report it urgently. 🔥

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Avast JavaScript Interactive Shell

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