1. Challenges and Solutions of Window
Remote Shellcode @若渴
2017.11.19
<ajblane0612@gmail.com>
AjMaChInE

2. Outline
• Overview of window remote shellcode
• Some challenges and solutions
– Antivirus
– EMET
– Firewall
– Intrusion-Detection System (IDS)/ Intrusion-
Prevention System (IPS)
• Reference

3. Remote Shellcode [0][1]
pipeprotocol process terminal process
command
sh

4. Windows Shellcode Skeleton in
Assembly [12]
• Getting EIP
• Decoder
• Getting addresses of required functions
• Setup socket
• Spawning a shell

5. Getting EIP – Why [12][13]
• What is the problem with such a hardcoded
address?
Compiler
move that code
to another
address space
????

6. Getting EIP [12]

7. Getting Addresses of Required
Functions [2]
Finding kernel32.dll
LoadLibraryA
GetProcAddress
System calls
not reliable

8. How to Finding kernel32.dll in ASLR?
[2][3][4]
Process Environment
Block(PEB)
fs:[0x30]
Structured Exception
Handling(SEH)
fs:[0]
Thread Environment Block
+0x4 TOPSTACK
fs:[0x18] not reliable

9. Finding kernel32.dll- PEB
“The process of determining the kernel32.dll base address
involves making use of the Process Environment Block (PEB). The
operating system allocates a structure for every running process
that can always be found at fs:[0x30] from within the process.
The PEB structure holds information about the process’ heaps,
binary image information, and, most importantly, three linked
lists regarding loaded modules that have been mapped into
process space. The linked lists themselves differ in purposes
from showing the order in which the modules were loaded to
the order in which the modules were initialized. The
initialization order linked list is of most interest as the order in
which kernel32.dll is initialized is always constant as the second
module to be initialized.” [2]

10. Finding kernel32.dll- SEH
“Windows NT based versions the top-most entry in the SEH list
can always be found at fs:[0] from within the process. With this
in mind, one can walk the list of installed exception handlers
until they reach the last one. When the last one is reached the
address of the function pointer can be used as a starting point
for walking down in increments of 64KB, or 16 × 4096 byte
pages. In Windows, DLL’s will only align on 64KB boundaries. At
each 64KB boundary a check can be performed to see if the two
characters at that point are ‘MZ’. These two characters mark the
MSDOS header that is prepended to portable executables.” [2]

11. Address Resolution of Required
Functions
• PEB parsing [5]
– This method uses the Process Environment Block(PEB)
data structure to locate the base addresses of loaded DLLs
and finding their function addresses with parsing the
Export Address Table(EAT)
• Hash API search [5]
– For quickly finding required functions
EAT
IAT
DLL
Required Functions
hash hash?=

12. Functions You maybe Want to [6]
• WinExec
• CreateProcessW
• CreateProcessA
• LoadLibraryExA
• LoadLibraryExW
• OpenFile
• CreateThread
• CreateRemoteThread
• GetProcAddress
• LoadModule
• CreateFileA
• CreateFileW
• _lopen
• _lcreat
• CopyFileA
• CopyFileW
• CopyFileExA
• CopyFileExW
• MoveFileA
• MoveFileExW
• LockFile
• GetModuleHandleA
• VirtualProtect
• OpenProcess
• GetModuleHandleW
• MoveFileWithProgressA
• MoveFileWithProgressW
• DeleteFileA

13. Challenges of Shellcode for Antivirus
[7][8]
• Static signature analysis
– Signature analysis is based on a blacklist method
– EX: YARA [9]
• Static heuristic analysis
– In this case the AV will check the code for patterns which are known
to be found in malwares. There are a lot of possible rules, which
depends on the vendor
• Dynamic analysis
– These days most AV will rely on a dynamic approach. When an
executable is scanned, it is launched in a virtual environment for a
short amount of time. Combining this with signature verification and
heuristic analysis allows detecting unknown malwares even those
relying on encryption. Indeed, the code is self-decrypted in AV
sandbox; then, analysis of the “new code” can trigger some suspicious
behavior.

14. Bypassing Challenges of Shellcode for
Antivirus [7][8]
• Bypassing static signature analysis/ static
heuristic analysis
– Decryption [10][11]
– Obfuscation [7]
– Non-standard languages for windows binaries
[25]
• Bypassing dynamic analysis

15. Obfuscation

16. The Veil-Framework [25]
• Obfuscated code
• Encrypted code
• Non-standard languages for windows binaries
– Python, Ruby, Perl, Go, etc.

17. Bypassing Dynamic Analysis [7][8]
• Allocate and fill 100M memory
• Hundred million increments
• Attempt to open a system
process
• Attempt to open a non-existing
URL
• Action which depends on local
username
• What the fuck is NUMA?
• What the fuck are FLS?
• Check process memory
• Time distortion
• What is my name?
• I am my own father
• First open a mutex
• Load fake library
• Is debugger present
• Number of Cores
• Trap flag manipulation

18. Bypassing Dynamic Analysis - Hundred
Million Increments [8]
AV detection
emulator

19. Proper Ways To Execute Shellcodes
[7][8]
• HeapCreate/HeapAlloc
• LoadLibrary/GetProcAddress
• GetModuleHandle/GetProcAddress
• Multi-Threading

20. Challenges of Shellcode for EMET
• Preventing EAT parsing techniques

21. Bypassing Challenges of Shellcode for
EMET
• IAT parsing [23]
– Also holding the WIN API function addresses by
the application

22. Challenges of Shellcode for Firewalls
• Inbound detection
• Outbound detection
• Usually, firewall allow connection to popular
services like port 25(SMTP), 53(DNS),
80(HTTP), etc.

23. Bypassing Challenges of Shellcode for
Firewalls
• Bypassing inbound detection
– Reverse remote shellcode
• Bypassing outbound detection (進去了要怎出
來)，EX [12] :

24. Bypassing Outbound Detection
• DLL/PE Injection to iexplore.exe, telnet, ftp,
SSH and alike [13]
• One-way shellcode [2][12]
• Meterpreter HTTP, HTTPS and DNS stagers [21]

25. DLL Injection Overview – Step 1/2 [17]

26. DLL Injection Overview – Step 3/4 [17]

27. Execution Methods of DLL Injection
[15]
• CreateRemoteThread()
• NtCreateThreadEx()
• QueueUserAPC()
• SetWindowsHookEx()
• RtlCreateUserThread()
• Code cave via SetThreadContext()
• Reflective DLL

28. DLL/PE Injection to iexplore.exe [13]
• Querying the register key, rather than referring to
“c:...iexplore.exe”
• CreateProcess() to open and keep browser
windows hidden
• WaitForInputIdle() to give processes time for
initialization
• WaitProcessMemory() to copy networking code
• CreateRemoteThread() to run code
• The injected procedure connects the web site and
sends HTTP request

29. Reflective DLL Injection [16][18]
Reflective DLL (= DLL-format PE file loader)
reflective DLL
is loaded by
reflective DLL

30. DLL/PE/Process Hollowing
Injection[19][20]

31. One-way Shellcode – Find Socket [12]
(using anonymous pipe)

32. One-way Shellcode – Reuse Socket [12]
The problem of the “Find Socket” method:
• If the socket already been closed
(the SO_REUSEADDR socket option)

33. One-way Shellcode – Rebind Socket
[12]
The problem of the “Rebind Socket” method:
• using SO_EXCLUSIVEADDRUSE, thus reusing
the address is not possible

34. The Meterpreter:
a stager, and and
stage [21][22][24]

35. An Up-to-Standards Secure Corporate Environment
with the meterpreter/reverse_winhttp Payload [14][22]

36. Bypassing An Up-to-Standards Secure Corporate
Environment with the meterpreter/reverse_winhttp
Payload [14][22]
Thread 1Thread 2
local proxy with port 8080
reverse_winhttp
LHOST=127.0.0.1
LPORT=8080
NTLM authentication
+ HTTP requests
trust local proxy and go
through the corporate
proxy

37. Reference
• [0] How To Make A Reverse TCP Backdoor In Python - Part 1
– https://0x00sec.org/t/how-to-make-a-reverse-tcp-backdoor-in-python-part-1/1038
• [1] How To Make A Reverse TCP Backdoor In Python - Part 2
– https://0x00sec.org/t/how-to-make-a-reverse-tcp-backdoor-in-python-part-2/1040
• [2] Understanding Windows Shellcode
– http://www.hick.org/code/skape/papers/win32-shellcode.pdf
• [3] Windows Reverse Shell Shellcode I.
– http://sh3llc0d3r.com/windows-reverse-shell-shellcode-i/
• [4] Windows Reverse Shell Shellcode II.
– http://sh3llc0d3r.com/windows-reverse-shell-shellcode-ii/
• [5] Art of Anti Detection 3 – Shellcode Alchemy
– https://pentest.blog/art-of-anti-detection-3-shellcode-alchemy/
• [6] NT shellcodes prevrntion Demystified
– http://www.phrack.org/issues/63/15.html#article
• [7] Art of Anti Detection – 1 Introduction to AV and Detection Techniques
– https://www.exploit-db.com/docs/40900.pdf
• [8] Bypass Antivirus Dynamic Analysis - Limitations of the AV Model and How to Exploit Them
– https://wikileaks.org/ciav7p1/cms/files/BypassAVDynamics.pdf
• [9] YARA
– http://virustotal.github.io/yara/

38. • [10] Code Segment Encryption
– http://blog.sevagas.com/?Code-segment-encryption
• [11] Hide Meterpreter Shellcode in Executable
– http://blog.sevagas.com/Hide-meterpreter-shellcode-in-executable
• [12] History and Advances in Windows Shellcode
– http://phrack.org/issues/62/7.html
– https://www.blackhat.com/presentations/bh-asia-03/bh-asia-03-chong.pdf
• [13] Using Process Infection to Bypass Windows Software Firewalls
– http://phrack.org/issues/62/7.html
• [14] Evade Egress Restrictions with Staged Payloads
– https://blog.cobaltstrike.com/2013/11/15/evade-egress-restrictions-with-staged-payloads/
• [15] Inject All the Things
– http://blog.deniable.org/blog/2017/07/16/inject-all-the-things/
– https://github.com/fdiskyou/injectAllTheThings/
– Microsoft Visual Studio Express 2013 for Windows Desktop
• [16] Reflective DLL Injection
– https://www.dc414.org/wp-content/uploads/2011/01/242.pdf
– https://github.com/stephenfewer/ReflectiveDLLInjection
• [17] Windows DLL Injection Basics
– http://blog.opensecurityresearch.com/2013/01/windows-dll-injection-basics.html

39. • [18] DOUBLEPULSAR Usermode Analysis: Generic Reflective DLL Loader
– https://countercept.com/our-thinking/doublepulsar-usermode-analysis-generic-reflective-dll-
loader/
• [19] Ten Process Injection Techniques: A Technical Survey Of Common And Trending Process
Injection Techniques
– https://www.endgame.com/blog/technical-blog/ten-process-injection-techniques-technical-survey-
common-and-trending-process
– https://github.com/secrary/InjectProc
• [20] Process Hollowing
– https://github.com/m0n0ph1/Process-Hollowing
• [21] Metasploit - The Exploit Learning Tree
– https://www.exploit-db.com/docs/27935.pdf
• [22] Meterpreter Stage AV/IDS Evasion with Powershell
– https://arno0x0x.wordpress.com/2016/04/13/meterpreter-av-ids-evasion-powershell/
– https://github.com/Arno0x/PowerShellScripts/blob/master/proxyMeterpreterHideout.ps1
• [23] Teaching Old Shellcode New Tricks
– https://recon.cx/2018/brussels/resources/slides/RECON-BRX-2017-
Teaching_Old_Shellcode_New_Tricks.pdf
– https://github.com/secretsquirrel/fido
• [24] Deep Dive Into Stageless Meterpreter Payloads
– https://blog.rapid7.com/2015/03/25/stageless-meterpreter-payloads/
• [25] The Art of AV Evasion - or Lack Thereof
– https://www.slideshare.net/CTruncer/the-art-of-av-evasion-or-lack-thereof
– https://github.com/Veil-Framework/Veil