Anatomy of a Buffer Overflow Attack Rob Gillen @argodev
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Don’t Be StupidThe following presentation describesreal attacks on real systems. Pleasenote that most of the attacksdescribed would be considered ILLEGALif attempted on machines that you donot have explicit permission to testand attack. I assume no responsibilityfor any actions you perform based onthe content of this presentation orsubsequent conversations. Pleaseremember this basic guideline: Withknowledge comes responsibility.
DisclaimerThe content of this presentationrepresents my personal views andthoughts at the present time. Thiscontent is not endorsed by, orrepresentative in any way of myemployer nor is it intended to be aview into my work or a reflection onthe type of work that I or my groupperforms. It is simply a hobby andpersonal interest and should beconsidered as such.
CreditsThe vulnerability that we’ll be discussingwas initially discovered by C4SS!0 G0M3S(firstname.lastname@example.org) and was publishedon June 17, 2011.http://www.exploit-db.com/exploits/17539/James Fitts created a MetaSploit modulethat I also reviewed while building thismodulehttp://www.exploit-db.com/exploits/17540/
Example Overview• Scenario – Machine 1: BackTrack 5 SR1 – Machine 2: • Windows 7 Professional x64, SP1, fully patched • Freefloat FTP Server v1.0• Tasks – Discover a vulnerability exists – Craft & test an exploit• Goal: Obtain reverse shell
Attack Process• Identify target of interest• Identify software/versions being used• Setup local Instance• Fuzz to identify vulnerability• Design/Develop Exploit• Test• Package/Weaponize
CPU Registers (8086)• EAX – Accumulator Register• EBX – Base Register• ECX – Counter Register• EDX – Data Register• ESI – Source Index• EDI – Destination Index• EBP – Base Pointer• ESP – Stack Pointer Content from: http://www.swansontec.com/sregisters.html
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: http://en.wikipedia.org/wiki/Instruction_pointer
Buffer Overflow• Software accepts input, but doesn’t ensure that it is only as long as supported.• In this case, software accepts a value into the variable A, but the user sends an overly-long string (“excessive”) and overflows the space allocated to A and overwrites the integer previously stored in B Content from: http://en.wikipedia.org/wiki/Buffer_overflow
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)
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: http://en.wikipedia.org/wiki/Shellcode
[BITS 32]mov ebx, 0x00424F52 ; Loads a null-terminated string “ROB” to ebxpush ebx ; pushes ebx to the stackmov esi, esp ; saves null-terminated string “ROB” in esixor 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 eaxcall eax ; Call the MessageBoxA function with all parameters supplied.
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.
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
Bind Shell Code executes ontarget and exposes a listener on a Attacker connects specific port (Binds) to client (i.e. 4444) ip:4444 Attacker Target sends shell to attacker Target
Reverse Shell Attacker exposes a listener on a specific port (i.e. 4444) Code executes on target and connects to the attacker ip:4444 Attacker Target sends shellTarget to attacker
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
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)
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