Advanced Buffer Overflow Technique Greg Hoglund

Attack Theory Formalize the Attack Method Re-Use of Attack Code Separate the Deployment from the Payload Payloads can be chosen for desired effect Details and Restraints of both Payload and Deployment code

Exploits A “BUG” in Software New bugs reported every day “Exploit” is code that takes advantage of a bug in order to cause an effect

What can happen? Machine Crash Application Crash (most common) Recoverable Exception Mobile Code (deadly) File Access Denial of Service

Exploits can be grouped Some bugs are all the same Some bugs keep coming back –improper filtering –bounds checking –bad authentication –impersonation

Entry -vs- Effect The attack payload is not the same as the entry point Missle -vs- Warhead analogy sometimes called “Egg -vs- Shell”

Exploits come in 2 parts Injection Vector (deployment) –the actual entry-point, usually tied explicity with the bug itself Payload (deployed) –usually not tied to bug at all - limited only by imagination. Some restraints.

Injection Vector Target Dependant OS Dependant Application Version Dependant Protocol Dependant Encoding Dependant

Payload Independent of Injection Vector Still Depends on Machine, Processor, etc. Like a Virus Once established, can spread by any means

Payload Denial of Service Remote Shell (common) Worm/Virus Rootkit (common)

Injector/Payload Pairs One injector works on ‘n qualified hosts’ Example - IIS Injector works on ~20% of Web Hosts. Payload –Remote Shell for control –Shutdown Machine –Shutdown ALL Machines on subnet

Types of Injection Content Based –characters inserted into a data stream that result in the remote process doing something it shouldn’t. Process is still in control. Buffer Overflow –poor programming practice subverts architecture of code execution. Process loses control.

Types of Injection Trust Based –Boot virus/ Floppy/ CD –MACRO virus – Attachments –Web Browsing

Who writes Injector Code? 1995 US Defense Intelligence Agency Report –Cuban Military targets US w/ custom virii University of Havana, team of less than 20 computer experts –Russian KGB prior to 1991 coup attempt, KGB has virii intended to shut down US computers in times of war

How hard can it hit? 1995 E&Y report –67% of companies hit bit virus 1996 E&Y report –63% of companies hit by virus 1996 UK Information Security Breaches Survey –51% of companies hit by virus

How hard can it hit? NCSA 1997 report –33% of all machines infected with virus –average cost of recovery ~$8000 US dollars November 1988 Morris Worm –strikes ~6,000 computers (10% of Internet at time) within hours –spreads via Buffer Overflow in fingerd –spreads via Sendmail exploit

How hard can it hit? 1989, “WANK” Worm –Hits NASA Goddard Space Center –spreads to US DOE High Energy Physics network (HEPNET) –2 weeks to clean all systems 1998 ADM-W0RM –buffer overflow in Linux DNS server

Buffer Overflow Injection Overflow the Stack Overflow the Heap Must control the value of the instruction pointer (processor specific) Goal: Get the Instruction Pointer to point to a user-controlled buffer.

Challenges Injector/Payload size restrictions –tight coding requirements Injector and Payload in same buffer –cannot step on each other Guessing Address Values –sometimes called ‘offsets’ NULL characters –use encoding and stack tricks

Stack Injection Stack is used for execution housekeeping as well as buffer storage. Stack-based buffer must be filled in direction of housekeeping data. Must overwrite the housekeeping data

Address Housekeeping A B C D code heap IP DI SI FLAG SP BP stack IP

Stack Overflow C C

The Problem with NULL STOPS C C

NULL must be PAST housekeeping data OK C C

Little and Big Endian On Intel x86 (Little Endian), Values are stored ‘backwards’ - least significant byte goes first: FF is stored as: FF

We store address in housekeeping data C CD F Original Address 0C New Address

Injection is Complete We control the instruction pointer New Address

Where to put the payload C New Address

Confined Payload Byte Compression Use only preloaded functions –Payload doesn’t need to build jumptables –Useable functions must be loaded Use Hardcoded addresses –Payload designed for a specific process with predictable features Data portion of payload needs to be small

Using more stack for payload OK C C 0D NO NULL in Address

Much Larger Payload

When does the address contain a NULL character Lowland Address - starts with 00 –stack is in lowland on Windows NT usually XX XX –limits size of payload Highland Address - no zeros in address –stack is in highland under Linux –unlimited payload size

Large payload, Lowland address We cannot use a lowland address directly, because it limits our payload We can use a CPU register We can use stack values that remain undamaged

A register points to the stack A B C D code heap IP DI SI FLAG SP BP stack IP

Call thru a Register Call eax, call ebx, etc –FF D0 = call eax –FF D3 = call ebx –FF D1 = call ecx –etc, etc

Push a register then return Push register –push eax = 50 –push ebx = 53 –etc Then RET –RET = C3

Guessing where to go We jump to the wrong address –crashes software –payload doesn’t execute Use NOP (no-op) - a single byte instruction –NOP = 90 Fill buffer with NOP’s –“NOP Sled”

NOP Sled End up at payload

Inject the Payload into the HEAP Environment Variables –HTTP headers Protocol Headers Recent Transactions Open Files

Use the CPU A B C D code heap IP DI SI FLAG SP BP stack

Execute code on the heap A B C D code heap IP DI SI FLAG SP BP stack IP

Trespassing the HEAP Two C++ objects near one another Environment Variable Any buffer that can overwrite a pointer –function pointer –string pointer (alter behavior w/o mobile code)

Overwrite the VTABLE C++ objects have a virtual function table Vtable pointer Member variables grow away from vtable pointer (NT)

Overwrite VTABLE Must have 2 C++ Objects (on heap) Overwrite vtable ptr

Where do I make the VTABLE point?

Your own VTABLE The VTABLE has addresses for all virtual functions in the class. This usually includes a destructor - which will be called when the object is destroyed (deallocated from memory) Overwrite any function that works

Overwrite Exception Handler C Ex-handler New Handler Return addr

The Payload Using Loaded Functions Encoding our own data Loading new functions & DLL’s Making a shell

The Payload Real Code DATA NOP Sled

Getting Bearings –Call RELOC: –RELOC: pop edi –edi now has our code address –we can use this as an offset to our data

Getting Bearings Call RELOC trick has NULL’s –E –5F

Getting Bearings w/o using NULL BACK:popebp jmpOVER START:callBACK OVER: –5D –EB 05 –E8 F8 FF FF FF

Avoiding NULLS moveax,0x401AD0FF shreax,8 –EAX results in 00401AD0 mov eax, h mov edx, h xor eax, edx –EAX results in 000F003F

XOR Protection Cannot have NULL’s in data portion XOR every BYTE

XOR again to decode Begin decode

Encode/Decode payload –MOVEAX, EBP (start of data payload) –ADDEAX, some value make eax point past the decoder the decoder itself is not encoded –XORECX, ECX –MOVECX, size of payload the 16 bit version of call avoids NULL character 66 B9 XX XX -vs- B9 XX

Encode/Decode LOOP LABEL:XOR[EAX], 0xAA INCEAX LOOP LABEL –80 30 AA –40 –E2 FA –NO NULL CHARACTERS

Hardcoded Function Calls code

Pros/Cons to hard coding PRO: makes code smaller CON: what if function isn’t always in same place? –Dynamically loaded DLL’s PRO: some DLL’s are *usually* always in the same place –KERNEL32.DLL

Dynamic Function Loading Use LoadLibrary() and GetProcAddress() –usually always in same place –hard coding usually works Load New DLL’s Find any function by ASCII name –handy

Load Function by Name Function name stored here getprocaddress

Build a jumptable getprocaddress

Use Jumptable

HASH Loading Process already has ASCII names of all loaded functions stored in process-header We can locate any loaded function by checking the CRC of each loaded ASCII name We do not need to store function names in our DATA section - only CRC’s –makes payload smaller!

PE Header PE OFFSET Optional Header ASCII NAME Address

Check CRC’s CRC

Nybble Compression Store each byte as a single nybble Doubles capacity of payload Only works for a confined set of 16 values –push / pop / add / sub –pop into register, add/sub until desired value –push back onto stack

Limited Character Set Payload is filtered alphanumeric only ( headers) –short jumps (difficult to maintain) –pop/push –subtract

The Bridge Avoids jump instruction size must be calculated exactly

Load New DLL

WININET.DLL Use DLL functions –InternetOpenURL() –InternetReadFile() Does all the hard work Makes payload smaller Download and Execute any file, anywhere File stored anonymously - hard to trace

WS2_32.DLL Socket bind listen send recv accept

Interrupt Calls Don’t require addresses Small Easy to use –Load register with call number –Load register with argument pointer –interrupt (2 bytes long) –CD 2E (interrupt 2E) –CD 80 (interrupt 80)

Remote Command Shell Spawn a process –CreateProcessA (kernel32 function) –INT 80 (linux) (execve syscall) Pipe the output thru socket –Named pipes (~5 functions) –Winsock / sockets

WORMS Payload searches for new hosts to attack Trust Exploitation –sniff passwords on wire –SMB sessions to other NT hosts –NT Registry Alteration –NFS/Drive Sharing Consider survivability of Payload –what % of hosts are eligible?

Lysine Deficiency Worm will die if certain condition is not met Existance of File Existance of Network Entity Floppy in floppy drive (testing lab)

RECAP Injection is not the same as payload Payloads can perform –Denial of Service –WORM –Remote Shell –Rootkit

RECAP Injection has many challenges –NULL characters –Stack size –Highland/Lowland address –Calling thru CPU registers

RECAP Filters limit what we can use in a payload Limited OP-CODE sets can still be used to build fully functional programs

RECAP Our payload is encoded We can build jumptables We can load new DLL’s and Functions We can hard-code addresses or load them dynamically We can use Lysine Deficiency to keep Worms from spreading uncontrolled

Thank You Your mind is your primary weapon