Search this site





XSendEvent + LD_PRELOAD == win

As far as feature requests come, for xdotool, one of the more common ones is to have the ability to send key or mouse events to a specific window, not just the active one. XTEST (what xdotool uses for key/mouse currently) doesn't let you specify a window to send events. XSendEvent(3) lets you send hand-crafted events to a specific window, but most applications ignore these sent events.

The XEvent struct has a member 'send_event' which is true if the event came from an XSendEvent call and false otherwise. Programs like firefox and xterm (by default) ignore many events that have 'send_event' set to true.


Writing a custom shared library that overrides the default XNextEvent and XPeekEvent functions allows us to force 'send_event' to always be false, so an application with this library loaded will happily handle keyboard/mouse events generated with XSendEvent. I already have a helpful project that lets me write such a shared library: liboverride.

#include <stdio.h>
#include <X11/Xlib.h>

void hack_send_event(XEvent *ev) {
  switch (ev->type) {
    case KeyPress: case KeyRelease: case ButtonPress: case ButtonRelease:
      ev->xany.send_event = False;

override(`XNextEvent', `
    real_func(display, event_return);
This small bit of liboverride code will give me a shared library I can preload with LD_PRELOAD. Doing so will ensure that send_event is false for any key or mouse button events.

Works well. Now that we have a reliable way to allow XSendEvent I think it's worth putting this into xdotool.

ssh honeypot.

Using slight variations on the techniques mentioned in my previous post, I've got a vmware instance running Fedora 8 that permits any and all logins. These login sessions are logged with script(1).

Fedora 8 comes with selinux enabled by default. This means sshd was being denied permission to execute my special logging shell. The logs in /var/log/audit/ explained why, and audit2allow even tried to help make a new policy entry for me. However, I couldn't figure out (read: be bothered to search for more than 10 minutes) how to install this new policy. In searching, I found out about chcon(1). A simple command fixed my problems:

chcon --reference=/bin/sh /bin/sugarshell
The symptoms prior to this fix were that I could authenticate, but upon login I would get a '/bin/sugarshell: Permission Denied' that wasn't logged by sshd.

There are plenty of honeypot software tools out there, but I really wasn't in the mood for reading piles of probably-out-of-date documentation about how to use them. This hack (getpwnam + pam_permit + logging shell) took only a few minutes.

As a bonus, I found a feature in Fedora's yum tool that I like about freebsd's packaging system: It's trivial to ask "Where did this file come from?" Doing so made me finally look into how to do it in Ubuntu.

FreeBSD: pkg_info -W /usr/local/bin/ssh
/usr/local/bin/ssh was installed by package openssh-portable-4.7.p1,1
Fedora: yum whatprovides /usr/bin/ssh
openssh-server.x86_64 : The OpenSSH server daemon
Ubuntu: dpkg -S /usr/bin/ssh
openssh-client: /usr/bin/ssh

Let's see what I catch.

liboverride project page is up.

I finally got around to putting up a project page for liboverride.

Location: /projects/liboverride

Tracking and Analyzing SSH Bots.

I've posted previously about what can be done about ssh bots. In this same context, I've just finished working on a new idea: Tracking the username/passwords used by the bots.

To track the login attempts, I wrote a new pam module: pam_logfailure. The goal of pam_logfailure is to log the passwords used by bots attempting to bruteforce logins. However, when I installed the module, I found that it wasn't working properly:

Dec 20 12:24:50 kenya2 pam_logfailure: host: user:john pass:^H ^M^?INCORRECT
I saw line after line of these, and couldn't figure out why the bots were using this as a password. Turns out they aren't. This password is what OpenSSH forces upon pam for users that do not exist. This is apparently by design:
auth-pam.c: static char badpw[] = "\b\n\r\177INCORRECT";
If you are an invalid user, or are trying to login as root while root login is disabled, the password you sent is replaced with 'badpw' above. This makes it kind of hard to track what passwords bots are using...

Thankfully, I was already one step ahead of myself when I wrote a function injection tool back in September (liboverride). So, all I had to do was inject my own 'getpwnam' function to spoof data when a user did not exist to trick OpenSSH into passing the password through.

After injecting my own getpwnam(), pam_logfailure started working just fine:

Dec 22 11:17:47 kenya2 pam_logfailure: host: user:admin pass:admins
So where will I go next with these ssh-bot games?
  • Reverse-hack. I picked 3 random ssh bot hosts from my logs, and all of them run sshd. It would be pretty trivial to take the password attempts used against my machine and try them on the host the bot is coming from. Seems likely that turning the bot's actions on itself will grant me access to the infected machine.
  • Redirect to a honeypot. We could detect when a bot is trying to login, and add a firewall rule that would put future ssh attempts from these hosts into a honeypot which accepts all logins to see what happens.
  • Fingerprint ssh bots by behavior.

The usage of getpwnam.over is like any other liboverride code. 'make' and then use "LD_PRELOAD=/path/to/ ". In this case, I added this line to /usr/local/etc/rc.d/openssh (my sshd start script):

export LD_PRELOAD=/path/to/

Here is the code:

New project: liboverride (20070903)

Last month, I wrote about overriding shared library functions. I spent time today working on that project and it's to the point where I want to put it out for consumption. It's not perfect, but I've used it to easily override both libc and libX11 functions with great results.

Download: liboverride-20070903.tar.gz