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ETCSS: Into the Mind of a Hacker
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ETCSS: Into the Mind of a Hacker


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  • 1. Anatomy of a Buffer Overflow Attack Rob Gillen @argodev
  • 2. Don’t Be Stupid The following presentation describes real attacks on real systems. Please note that most of the attacks described would be considered ILLEGAL if attempted on machines that you do not have explicit permission to test and attack. I assume no responsibility for any actions you perform based on the content of this presentation or subsequent conversations. Please remember this basic guideline: With knowledge comes responsibility.
  • 3. Disclaimer The content of this presentation represents my personal views and thoughts at the present time. This content is not endorsed by, or representative in any way of my employer nor is it intended to be a view into my work or a reflection on the type of work that I or my group performs. It is simply a hobby and personal interest and should be considered as such.
  • 4. Credits The vulnerability that we’ll be discussing was initially discovered by C4SS!0 G0M3S ( and was published on June 17, 2011. James Fitts created a MetaSploit module that I also reviewed while building this module
  • 5. Overview • Scenario – Machine 1: Kali Linux (BackTrack) – Machine 2: • Windows 7 Professional x64, SP1 • Freefloat FTP Server v1.0 • Tasks – Discover a vulnerability exists – Craft & test an exploit • Goal: Obtain reverse shell
  • 6. Attack Process • Identify target of interest • Identify software/versions being used • Setup local Instance • Fuzz to identify vulnerability • Design/Develop Exploit • Test • Package/Weaponize
  • 7. Terminology • • • • • • • CPU Registers Debugger Buffer Overflows Fuzzing Shellcode Encoding Bind Shell/Reverse Shell
  • 8. CPU Registers (8086) • • • • • • • • EAX EBX ECX EDX ESI EDI EBP ESP – – – – – – – – Accumulator Register Base Register Counter Register Data Register Source Index Destination Index Base Pointer Stack Pointer Content from:
  • 9. CPU Registers (8086) • EIP – program counter or commonly “instruction pointer” – a processor register that indicates where a computer is in its program sequence. • Holds the memory address of (“points to”) the next instruction that would be executed. • Any thoughts on why this specific register is particularly interesting? Content from:
  • 10. Debugger
  • 11. Buffer Overflow Content from:
  • 12. Fuzzing • Identify points where application or service accepts data • Send varying lengths/types of data until we crash the service and/or overwrite key buffers. • Increase buffer length until no longer successful (identify upper bounds of memory space available for exploit)
  • 13. Shellcode • Small piece of code used as the payload in the exploitation of a software vulnerability • Name comes from the purpose – usually spawns a shell and performs some action • Often written in assembly code • Types: – “normal”, Staged, Egg-hunt, Omelette Content from:
  • 14. Shellcode Example [BITS 32] mov ebx, 0x00424F52 push ebx mov esi, esp xor eax, eax push eax push esi push esi push eax mov eax, 0x7E45058A call eax
  • 15. [BITS 32] mov ebx, 0x00424F52 ; Loads a null-terminated string “ROB” to ebx push ebx ; pushes ebx to the stack mov esi, esp ; saves null-terminated string “ROB” in esi xor eax, eax ; Zero our eax (eax=0) push eax ; Push the fourth parameter (uType) to the stack (value 0) push esi ; Push the third parameter (lpCaption) to the stack (value ROB00) push esi ; Push the second parameter (lpText) to the stack (value ROB00) push eax ; Push the first parameter (hWnd) to the stack (value 0) mov eax, 0x7E45058A ; Move the MessageBoxA address in to eax call eax ; Call the MessageBoxA function with all parameters supplied.
  • 16. Shellcode Example BB 52 4F 42 00 53 89 E6 31 C0 50 56 56 50 B8 8A 05 45 7E FF D0
  • 17. Encoding • There are often restrictions as to what data can be sent via the exploit (NULLs, etc.) • Self-extracting (smaller shellcode) • Self-decrypting (avoid IDS signatures) • Tools such as msfencode offer many options.
  • 18. Encoded Shellcode xbex13xafx49x81xdaxc7 xd9x74x24xf4x58x31xc9 xb1x06x83xe8xfcx31x70 x0fx03x70x1cx4dxbcx3a x70xdex7dx3dx27x69x67 x0cx07x39x3ex39xd7x02 x34xc0x92x0cxb6x1b
  • 19. Bind Shell/Reverse Shell • Bind Shell – Target exposes a shell on a given port – Attacker connects to that port and executes commands – Remote Administration • Reverse Shell – Attacker listens for connections on a given port – Shell code on target connects to attacker and sends a shell – NAT-safe!
  • 20. Bind Shell Code executes on target and exposes a listener on a specific port (i.e. 4444) Attacker connects (Binds) to client ip:4444 Attacker Target Target sends shell to attacker
  • 21. Reverse Shell Code executes on target and connects to the attacker ip:4444 Attacker exposes a listener on a specific port (i.e. 4444) Attacker Target Target sends shell to attacker
  • 22. Fuzzing Pseudo-Code • Build array of increasing length strings (“A”) • Build array of valid commands • For each command in arrayOfCommands – For each string in arrayOfStrings • Establish FTP connection • Submit command + string • Watch for application hang/crash • Inspect register values/pointers
  • 23. Demonstration FUZZING THE SERVICE
  • 24. Design The Exploit • Iterate with various malicious buffer sizes to see how much space is available • Locate where within the evil buffer we actually overwrite EIP • Locate where within the evil buffer we can locate our shellcode (pointed to by other register)
  • 25. Design The Exploit • Select / configure / encode shellcode • Integrate into exploit script (NOP slide, breakpoints, etc) • Identify reusable jump address to consistently move to shellcode • Test with breakpoints • Test in “real world” scenario
  • 26. Demonstration DESIGNING THE EXPLOIT
  • 27. Solutions? • • • • This was an “easy” scenario Bounds checking is critical! Fuzz your own applications Address Space Layout Randomization (ASLR) makes life harder • Operating System Support – Data Execution Prevention
  • 28. Questions/Contact Rob Gillen @argodev