1. Patching Windows Executables
with the Backdoor Factory
Joshua Pitts
DerbyCon 2013

2. Other Potential Titles
• I’M DOWN WITH APT (yeah you know me)
• Lassie, did Timmy fall in a Code Cave again??
• Why I Cyber and How You Can Cyber too
• When ET met EMET (A Love Story)
• Hugging Your Way to the Top, One Hug at a
Time
• How I Owned Your Mother

3. About Me
• US Marine, Pre-911: SIGINT
• Past: IT and Physical Security Auditor,
Operational Security Lead, Malware and
Forensic Analyst
• Current: Reverse Engineer, Pentester
• Python, C/C++, ASM (INTEL)
• I have certs.. Serious inquires only :P
• Currently work at Leviathan Security Group

4. Urban Dictionary

5. Overview
• History of Patching
• How I learned to patch binaries
• The Backdoor Factory
– Features
– Capabilities
• Demos (Live and Video)
• Mitigations
• Going forward

6. What is Patching
Definition (Wikipedia):
A patch is a piece of software designed to fix
problems with, or update a computer program
or its supporting data. This includes fixing
security vulnerabilities and other bugs, and
improving the usability or performance.

7. For This Presentation
My Definition:
Adding or taking away content or functionality
to a compiled binary.

8. Security Pros and Patching
• Red Teaming
– persistence in plain sight
• Pentesting/Social Engineering
– Salt all the parking lots!
• Research
– Just because :D
• Malware/Code analysis
– Break the anti-analysis code

9. History of Patching
Good thing we don’t do this with operating systems.

10. The MS Method
• MSP – Windows install patch file.
• Contains at least two data transforms
– One updates the install database
– One has info that the installer uses for ‘patching
files’
• The installer will then use this info to apply
the patch from the cabinet file
• Yada Yada Yada…

11. What does this mean?
MS definition of patching is replacing old
registry entries, dlls, and exes with new items
Not the patching that we’re taking about today

12. How Key Gens/Crackers do it
• Find code branch(es) that validate(s) the
software’s protection mechanism(s)
• Make it return to a value that meets a valid
condition for approved/continued operation
• Sometimes its a function that returns a
True/False (0/1) value after the check.

13. How Metasploit Patches
• msfvenom –p windows/shell_reverse_tcp –x
psexec.exe … The overwrite program entry
method
• msfvenom –p windows/shell_reverse_tcp –x
psexec.exe –k … The allocate and create
thread and return to original program entry
method (Keep)

14. MSF Overwrite program Entry - Before

15. MSF Overwrite program Entry - After

16. Pros/Cons
• PRO: Attacker receives a shell (or 17) :D
• PRO: Size of EXE not changed
• CON: No continued execution of program
– WIN - LOSE

17. MSF Create Thread Method (Keep)
When debugging in Immunity Debugger

18. MSF Create Thread Method (Keep)
Immunity is telling the truth, memory sections:
Original memory sections:

19. Msfvenom x32 Keep Method
Explained
• Two separate functions or stubs
• Part Two: The shellcode payloads that we all
know
• Part One: Is not new, not so well known, but is
awesome
• Looks like an un-named section of code that
has RWE attributes (very suspicious)
• Very important for stager payloads

21. Pros And Cons of the Msfvenom Keep
Method
• PRO: Attacker receives shell and the binary
works ‘normally’ for the user
– That’s a WIN-WIN
• CON: Should be easy for AV to catch
• CON: Size of binary has increased

22. MSFVenom Win64 Patching Support
Only supports x32
– Submitted a bug post on security street
– A feature request was submitted (#8383):

23. PE/COFF

24. The Portable Executable Format
MS-DOS 2.0 HEADER
and unused space
OEM ID/INFO
OFFSET to PE header
MS-DOS 2.0 Stub and Reloc
table
And unused Space
PE Header
Section Headers
Import pages
(import/export info
Base reloc info
Resource info)
• Not much has changed in the last 20 or so years
• Must be backwards compatible (win8 must read
the header)
• Easy to automatically manipulate
• http://msdn.microsoft.com/library/windows/har
dware/gg463125

25. The Common Object File Format
(COFF) Format
Microsoft COFF Header
(machine type, number
of sections, etc..)
Section Headers
Raw Data
Code
Data
Debug Info
Relocations
• This is included in the PE File Format
• The most important section for RE
• Includes:
- Machine Type
- Number of Sections
- Size and Access (RWE) of Sections
• Typically includes the rest of the file Code,
Data, Debug, Reloc (the actual sections)

26. End of PE/COFF

27. How I learned to Backdoor Windows
Binaries
• Though the Offensive Security Cracking the Perimeter
course
– Duh
• Manual Labor
• Time Consuming
– At first hours
– Now about 10-15 mintues
• Missing some important concepts:
– Working with the import table
– Working with staged payloads (stagers)
– Multi cave jumping
– win32 only (slight differences in x64 asm)

28. CTP Methods
• Append a new section of code
– Similar to the Metasploit msfvenom keep
– Named RWX section (e.g. .sdata, .moo, .what,
etc…)
• Use existing code caves for encoding/decoding
shellcode in the appended section
– We looked at XOR encoding
– XOR encoding is no longer effective against AV

29. CTP Method

30. Code Caves?

31. Code Caves?
A code cave is an area of bytes in a binary that
contain a null byte pattern (x00) greater than
two bytes.

32. How are code caves created?
• Not sure, so I did some research…
• Or went on a quest…

33. How are code caves created?
• Starting out, I assumed that a unicorn would
know everything.
• So I went to defcon.
• And what’s better than a unicorn at
DEFCON!!!

34. How are code caves created?
Hi Unicorn!
Hi.. err, human!
How are code
caves made?
I don’t know.

Aww. Want a
beer?

Pssst… Check
compliers…
o/

35. How are code caves created?
Tested the following x32 compilers:
• G++ - GNU C++
• Cl.exe – MS compiler linker.
• Lcc-win32
• Mingw32-c++

36. How are code caves created?
Against this code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <windows.h>
int array[600] = {0};
int main(int argc, char **argv)
{
printf("hello world");
return 0;
}
This Code is FREE as in BEER.

37. Find Code Caves Demo
Code: http://github.com/secretsquirrel/the-backdoor-factory
Binaries: http://live.sysinternals.com

38. How Are Code Caves Created?
Results for code caves greater than 200 bytes in
named sections (e.g. .text, .data, .idata, etc…):
• Cl.exe : 7
• G++: 4
• Mingw32-c++: 3
• Lcc-win32: 0

39. How Are Code Caves Created?
• Remember this line of code:
– int array[600] = {0};
• Not one had a cave of at least 600 bytes.

40. How Are Code Caves Created?
Lesson:
If you want to minimize code caves in your code, carefully
pick your compiler.**
**More research could be done in this area.***
***I don’t write compilers****
****Nor do I want too…
…yet :P

42. Some Ideas
• Automation
• Split shellcode into smaller sections
• Use multiple code caves
• Non-sequential cave jumping
• Use user provided shellcode
• Combine the Metasploit Stager solution with
the CTP code cave use

43. Solution: BDF
• x32 version released in March 2013
– Supported only single cave jumping
– No x32 stagers support
• Python27 - Single Script
• Supports win32/64
– Supports x32 stagers
– No stagers payloads for x64 yet. (Remember that bug feature
request?)
• Code Cave injection (single and multiple)
• Support user-provided shellcode
• Some Randomization (different hash every time an EXE is created)
– Through random one’s compliment in the code
– And different types of nops
• Injector Module

44. How BDF works
• Enumerates PE/COFF Header format
• Determines if binary is supported (win32/64
intel)
• Locates code caves that correspond to size of
shellcode
• Patches executable in an attempt to return
registers/flags to the original state for continued
execution
– Patches entry location with a JMP to the first selected
code cave/appended section
– Patches each user selected code cave

45. How BDF works
• Very primitive disassembler to do just what
we need
• Reworked the x32 msfvenom stager ‘keep’
stub to work in code caves and with user
provided shellcode
-x64 stager support is in the works
• Reworked a handful of useful metasploit
payloads to allow for multiple code cave
jumping

47. Original Way BDF Worked

48. Now You Can Do This

49. Or This

50. Demos
• Support Check (Live)
• Backdooring win32/64 binaries (Live)
– Append code cave
– Single code caving
– Code Cave jumping
• Mass backdooring (directory) - Live
• Provide your own shellcode - Live
• Prototyping shellcode (video)
• The injector module (video)

51. DEMO – Support Check
• If not supported, then what?
• Email me with the disassembly of the entry
function (from a legitimate email)
• I’ll send you the opcode update

52. DEMO - Patch a file with shellcode
win32/64
• Append
• Pick a single Cave
• Multi-Jumps
• Note – Non-stager payloads will ‘hang’ if C2 is
not available
– Payloads are patched ‘in-line’ and not run in a
separate thread

53. DEMO - Mass backdooring (directory)

54. DEMO - Prototyping shellcode

55. DEMO – Injector Module
• Injector is the hunt and backdoor binary of
doom.
• Use responsibly

56. DEMO - Provide your own shellcode
• Can be anything, just make sure it matches
the process type (x32 for x32)
• Make sure you use ExitFunction=Thread or
you will kill the parent process (not good)

57. Attack Scenarios or Methods
• Salting Parking Lots with USBs
• Hosting Rouge Exes
• Attacking system services
• Linux Setuid Attack for Windows
– Patching binaries that require elevated perms but
might be in non-admin directories
• Sysinternal tools
• Setup files

58. Mitigations
• UPX encoding
• Self Validation
• AVs
• EMET 4.0+
• Hash verification
• White Listing

59. Mitigations - UPX Encoding
• Not supported by BDF
• Unpack – Patch – Pack
• UPX is not protection against patching
• Could opens your up your Exe to potential
weaknesses
– Are you unpacking to unprotected memory space?

60. Mitigations - Self Validation
• Team Viewer
• Round Robin Checking
• Find the check, patch it

61. Mitigations – Anti-Virus’
• I broke the “rule” and uploaded my samples to
Virustotal for this presentation
• It really doesn’t matter
• AV is dead

62. MSFVENOM keep vs MSVENOM non-keep vs
BDF Cave Jumping
MSFVENOM –k –t exe
Hash: 6d0cb53a4fa983287310260f5c8659ab6c3a761ef8dbd147cf2cfd9e971f4295

63. MSFVENOM keep vs MSVENOM non-keep vs
BDF Cave Jumping
MSFVENOM –t exe
Hash: 6d0cb53a4fa983287310260f5c8659ab6c3a761ef8dbd147cf2cfd9e971f4295

64. MSFVENOM keep vs MSVENOM non-keep vs
BDF Cave Jumping
BDF Cave jumping
Hash: 5620ba8c64ff0d9cde65eda4156fbb85186f2bd0f16636cded5c9a0d8024d4e9

65. win32 BDF vs win64 BDF
ZoomIt.exe vs ZoomIt64.exe

66. EMET 4.0+ FTW?
• If you use position-independent shellcode
(metasploit)
• And the target binary is protected by EMET…
• And the Caller protection setting is enabled…
• And the application is running as win32…
• EMET will stop this type of attack!

67. EMET 4.0+ FTW?
• If the binary executes as a win64 process
• EMET will not stop this type of attack!
From the EMET 4.0 User Guide:

68. Mitigations - Whitelisting
• Based on what you’ve seen today, why would
you use trust AV.
• There are whitelisting vendors, I’m not
endorsing any of them
• I did not test it, but they “should” work – if
based on hashing verification and not name
• Not the end game solution (e.g. powershell
memory injection)

69. Enterprise Mitigations
• Don’t let end users download binaries
• Verify Your Binaries before Deploying
– Verify hashes
– Conduct forensic analysis and testing
– Look at network traffic
– Etc…

70. Road Map
• x64 stub to support staged payloads
• Support Mach-O and ELF formats
• Patch the IAT and api pointers to shorten
required shellcode and elimiate ROP-like calls
• MITM patching of binaries during download

71. Progress on x64 Stager

72. Questions?