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Jeff McJunkin's thoughts on Penetration Testing, Systems Administration, and Network Defense

Category: Systems Administration

Step-by-step Implementation of Local Administrator Password Randomization Script

Since the documentation was a bit sparse on my script in my previous post, I thought I’d post clearer instructions, for those not as familiar with Group Policy. This post is going live with my guest Tech Segment on PaulDotCom today.

Assumptions

For this implementation guide, I assume you have an Active Directory domain and several clients to manage. You’ll also need to do your work from a machine with Group Policy Editor, and either the correct delegated permissions or Domain Admin privileges. If there are any instructions that are unclear, leave a comment and I’ll update the post.

1. Download script

Please download the randomize-local-admin.vbs script from my GitHub (right-click, Save Link As…) and save it your Desktop or another accessible location. You’ll need this shortly.

2. Create the Group Policy Object

Open up Group Policy Management Console, browse to the Group Policy Objects folder, then right-click on it and create a new Group Policy Object.

Name it something recognizable such as “Local Administrator Password Randomization”, then right-click and Edit it.

Now browse to Computer Configuration -> Policies -> Windows Settings and double-click on Startup. This is where we’ll set the script to run on boot.

Once the new window pops up, click Show Files to open the GPO’s directory and copy the VBScript (randomize-local-admin.vbs) inside. Make sure it has the right extension, or Windows won’t recognize it as a VBScript.

Now add the script to the Group Policy Object by clicking Add and selecting the script.

3. Create the WMI Filter (optional)

The intent of this script is to randomize local Administrator accounts on desktops and member servers, but domain controllers don’t have local accounts. So as to not randomize the builtin Domain Admin account, we’ll need to exclude DC’s, either via the Organization Units (OU’s) we target or by WMI Filters. If your Active Directory OU structure isn’t built with separate areas for Domain Controllers, or you want to link the entire domain, we can use a WMI Filter to exclude all machines classified as DC’s.

Under WMI Filters, right-click and click New. Right a name and description (“Exclude Domain Controllers” seems reasonable) and then click Add. You’ll need the following WMI Filter:

select * from Win32_OperatingSystem where ProductType <> “2”

4. Link the GPO to the proper OU’s

Now that we’ve created the GPO and its WMI Filter, we can link it to an Organizational Unit. First, though, you’ll need to associate the WMI filter if you created one. After clicking on the Group Policy Object, select the WMI filter from the lower side of the right pane, under WMI Filtering.

Right-click an existing OU which has systems you’d like to target and click “Link an Existing GPO…”. Select the GPO you just created, and it will take effect on the next reboot on all Computer objects in that OU. You can select other OU’s in the same way.

To make an acceptable level of overkill, this script creates 120-character passwords using the full ASCII character set (1-255).

Again, if you have any issues feel free to leave a comment or send me an email.

Creating a Network Boot Menu including Kon-Boot to Bypass Local Authentication

In a former post I referred to Kon-Boot, but didn’t go into much detail. Here, I’ll expand on my use of Kon-Boot and how to set it up for your own network. Specifically, to keep things as easy as possible I add it to my PXE menu, so that you’re simply a reboot and a few keystrokes away from logging on to an otherwise locked machine.

What is Kon-Boot?

Kon-Boot is a bootable shim which bypasses local account authentication on Windows machines. In other words, by booting into Kon-Boot (via floppy, CD, or over the network via PXE) you can bypass local passwords, such as for the built-in Administrator account.

As an aside, note that it cannot bypass domain authentication. This makes sense, as domain-joined machines contact domain controllers for domain user accounts, unless the client machine is offline and has cached credentials.

Kon-Boot is the reason I randomize the local Administrator password without disabling the account. For whatever reason, the author of Kon-Boot didn’t bypass enabled/disabled checks on local accounts, just their passwords.

Creating a PXE Boot Server

I’ll assume you already have a DHCP server on your network. The FOG Project, which I heartily recommend for anyone looking for a free Ghost replacement, has a wonderful page on setting DHCP options for many types of DHCP servers. The same options will apply for us — option 066 (“Next Server”) will be the IP address of the PXE server, and option 067 (“Boot File”) will be “pxelinux.0”. In fact, if FOG itself is interesting, they have great setup guides that will get a FOG menu installed on Ubuntu, to which you can simply add Kon-Boot as an additional option.

Next, you’ll need a trivial file transfer protocol (TFTP) server. For this tutorial, I’ll go through the setup of tftpd-hpa and syslinux on Ubuntu 12.04 Server. I’d recommend making this machine a VM, as the resources required are laughably low. I won’t, however, go into the installation of Ubuntu Server, as there are plenty of fantastic guides for that piece already. If you use the previous link, keep track of the IP address you statically assigned, or make a static DHCP reservation for the PXE server.

Once you’re logged in, there’s a few pieces of software to install.

apt-get install tftpd-hpa syslinux vim-nox openssh-server

This will install the TFTP server, syslinux (think of it as GRUB for PXE), a minimal version of vim to edit text files, and an SSH server. We’ll also need to create the directory structure and copy over some of the syslinux files to the TFTP root directory.

mkdir /var/lib/tftpboot/pxelinux.cfg

cp /usr/lib/syslinux/memdisk /var/lib/tftpboot # used to boot a floppy image

cp /usr/lib/syslinux/pxelinux.0 /var/lib/tftpboot # the PXE boot file

cp /usr/lib/syslinux/vesamenu.c32 /var/lib/tftpboot # for the PXE menu

You’ll also have a choice between setting a root password and a fair number of sudo commands. For the purposes of this post, I’ll recommend setting a strong password (as an example, this password generator site is xkcd-approved!) for root.

sudo passwd # Enter your user password, then enter a strong password for root, twice

Next, we’ll install the free version of Kon-Boot. Earlier this year, the author came out with a new paid version, but maintained the original version for free download. The major restriction is the free version (v1.1) doesn’t support 64-bit Windows systems, while the paid version (v2.0) does. Go to the Kon-Boot download page, download from the first mirror, and save the zip locally. Inside kon-boot-all.zip you’ll find FD0-konboot-v1.1-2in1.zip (password is “kon-boot”), and in that you’ll find FD0-konboot-v1.1-2in1.img. Save it to another location locally, such as your desktop.

If you don’t already have it installed, you’ll also need a program to copy the Kon-Boot floppy image over to the PXE server. I recommend installing WinSCP (skip the “sponsored” version if you prefer) for those on a Windows platform.

Use your PXE server’s address instead

Copy the FD0-konboot-v1.1-2in1.img file to the /var/lib/tftpboot directory.

Now you’ll need to write the pxelinux configuration file at /var/lib/tftpboot/pxelinux.cfg/default. You can either use the command below, or a text editor like vim or nano to write the text below:

echo -n ”

prompt 0
DEFAULT vesamenu.c32
timeout 50

label local
localboot 0
MENU DEFAULT
MENU LABEL Boot from hard disk
TEXT HELP
Boot from the local hard drive.
If you are unsure, select this option.
ENDTEXT

label Kon-Boot
kernel memdisk
append initrd=FD0-konboot-v1.1-2in1.img
MENU LABEL Kon-Boot Floppy Image
TEXT HELP
Kon-Boot will bypass local authentication for
local administrative purposes.
ENDTEXT

” > /var/lib/tftpboot/pxelinux.cfg/default

If you’ve set up your DHCP server options as mentioned before, you should now be able to boot a client to the network and see the menu we’ve created. You may have to press a specific key during boot or change your BIOS options. For VMware virtual machines, pressing F12 during boot attempts a PXE boot.

Booting into Kon-Boot will take a few moments showing a fancy startup image, and then boot into Windows.

Kon-Boot Startup

Screenshot from paid version (v2.0)

Now that you’re inside Windows, you can log in as the local Administrator account (or any other local account) without worrying about the password. You can leave the password field blank, or type in any particular password you want. It makes no difference, as the password check is more-or-less replaced with “return true”. Whereas before you’d see this:

Failed Login

Now you’ll be able to log on and take any actions you wish.

Successful Login

What do you think? As I don’t have need for remote access to computers without using domain privileges, I see this as a useful way to log in to infected or suspicious machines locally. In terms of return on investment, I’ve been extremely happy with further customizing my PXE boot menu with SpinRite (drive recovery), Recovery is Possible (live Linux over PXE!), and even Memtest86+.