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Sıra DOSYA ADI Format Bağlantı
01. Encoding Functions Execute Instruction ppt Sunumu İndir

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Advanced Buffer Overflow TechniqueGreg Hoglund
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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
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Exploits• A “BUG” in Software• New bugs reported every day• automated testing tools – USSR Labs• “Exploit” is code that takes advantage of a bug in order to cause an effect
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What can happen?• Machine Crash• kernel exception• VIP process• Application Crash (most common)• Recoverable Exception• Mobile Code (deadly)• File Access (read or write)• Denial of Service
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Exploits can be grouped• Some bugs are all the same• Some bugs keep coming back– improper filtering– bounds checking– bad authentication– impersonation• In other words, need better testing
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Entry -vs- Effect• The attack payload is not the same as the entry point• Missle -vs- Warhead analogy• The Delivery Mechanism can be decoupled from the Payload
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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.
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Injection Vector• Target Dependant• OS Dependant• Application Version Dependant• Protocol Dependant• Encoding Dependant
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Payload• Independent of Injection Vector• Still Depends on Machine, Processor, etc. • With some exceptions• Mobile Code, Just like a Virus• Once established, can spread by any means– trust– scanning for more bugs
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Payload• Denial of Service– use as launching point (arp spoofing)• Remote Shell (common)– covert channel or ‘netcat’ like• Worm/Virus – extremely dangerous• Rootkit (common - stealth)
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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
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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.
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Types of Injection• Trust Based– Boot virus/ Floppy/ CD (parasite process)– MACRO virus– Email Attachments (Melissa, etc)– Web Browsing (exploit user’s trust, etc)• click thru
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Governments write 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
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Mobile code in Global 2000?• 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
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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
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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
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Buffer Overflow Injection• Overflow the Stack• Overflow the Heap• Goal: Must control the value of the instruction pointer (processor specific)• Goal: Get the Instruction Pointer to point to a user-controlled buffer.
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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, BAD characters– use encoding and stack tricks
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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
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Address Housekeeping•A•B•C•D•code•heap•IP•DI•SI•FLAG•SP•BP•stack•IP
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Stack Overflow00 40 20 0800 40 20 0C00 40 20 1000 40 20 1400 40 20 1800 40 20 1C
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The Problem with NULL•STOPS00 40 20 0800 40 20 0C00 40 20 1000 40 20 1400 40 20 1800 40 20 1C
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NULL must be PAST housekeeping data•OK00 40 20 0800 40 20 0C00 40 20 1000 40 20 1400 40 20 1800 40 20 1C
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Little and Big Endian• On Intel x86 (Little Endian), Values are stored ‘backwards’ - least significant byte goes first:• 00 40 10 FF is stored as:FF 10 40 00
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We store address in housekeeping data00 40 21 0400 40 21 0000 40 20 0C00 40 20 0800 40 20 0400 40 20 00CD 68 45 7FOriginal Address0C 20 40 00New Address
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Injection is Complete• We control the instruction pointer04 21 40 00New Address
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Where to put the payload00 40 21 0400 40 21 0000 40 20 0C00 40 20 0800 40 20 0400 40 20 0004 21 40 00New Address
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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
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Using more stack for payload•OK77 40 20 0877 40 20 0C77 40 20 1077 40 20 1477 40 20 1877 40 20 1C0D 45 68 77NO NULL in Address
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Much Larger Payload
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When does the address contain a NULL character • Lowland Address - starts with 00– stack is in lowland on Windows NT• usually 00 40 XX XX– limits size of payload• Highland Address - no zeros in address– stack is in highland under Linux– unlimited payload size
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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
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A register points to the stack•A•B•C•D•code•heap•IP•DI•SI•FLAG•SP•BP•stack
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Call thru a Register• Call eax, call ebx, etc– FF D0 = call eax– FF D3 = call ebx– FF D1 = call ecx– etc, etc
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Push a register then return• Push register– push eax = 50– push ebx = 53– etc• Then RET– RET = C3
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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”
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NOP Sled•End up at payload
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Inject the Payload into the HEAP• When the stack is limited in size• Store part on the payload on stack, the other on the heap• Protocol Headers– HTTP headers• Recent Transactions• Open Files
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Execute code on the heap•A•B•C•D•code•heap•IP•DI•SI•FLAG•SP•BP•stack
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Trespassing the HEAP• Two C++ objects near one another• Any buffer that can overwrite a pointer– function pointer– string pointer (alter behavior w/o mobile code)
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Overwrite the VTABLE• C++ objects have a virtual function table•Vtable pointer•Member variables grow away from vtable pointer (NT)
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Overwrite VTABLE• Must have 2 C++ Objects (on heap)•Overwrite vtable ptr
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Where do I make the VTABLE point?
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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
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Injection is complete• Kernel level overflows all over in NT• Off by one errors causing frame pointer overwrite• Multi-stage attacks where you must first get the target into a state before attempting overflow• The effects of URL or MIME encoding
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Now for the Payload• Using Loaded Functions• Encoding our own data• Loading new functions & DLL’s• Making a shell
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The Payload•Real Code•DATA•NOP Sled
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Getting Bearings– Call RELOC:– RELOC: pop edi• EB 00 00 00 00– edi now has our code address– we can use this as an offset to our data
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Reverse Short Call• NO NULL Bytes– RELOC: jmp RELOC2– Call RELOC:– RELOC2: pop edi• EB FF FF FF FE
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XOR Protection• Cannot have NULL’s in data portion•XOR every BYTE
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XOR again to decode•Begin decode
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Hardcoded Function Calls•code
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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
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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
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Load Function by Name•Function name stored here•getprocaddress
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Build a jumptable•getprocaddress
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Use Jumptable
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HASH Loading (el8)• 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!
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PE Header•PE OFFSET•Optional Header•ASCII NAME•Address
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Check CRC’s•CRC
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Limited Character Set means Limited Instruction Set• Payload is filtered– MIME– URL• alphanumeric only (email headers)– short jumps (difficult to maintain) – pop/push– subtract
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The Bridge•Avoids jump instruction•size must be calculated exactly
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Load New DLL
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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
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WS2_32.DLL• Socket• bind• listen• send • recv• accept
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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)
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Remote Command Shell• Spawn a process– CreateProcessA (kernel32 function)– INT 80 (linux) (execve syscall)• Pipe the output thru socket– Named pipes (~5 functions) – Connect in or out over any TCP socket
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Covert Channel• If exploited process is root or SYSTEM– TDI or NDIS hook– session over ACK packets or ICMP • IIS– Patch any point where URL requests are handled– no kernel required
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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?
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Lysine Deficiency• Worm will die if certain condition is not met• Existance of File• Existance of Network Entity• Floppy in floppy drive (testing lab)
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RECAP• Injection is not the same as payload• Payloads can perform– Denial of Service– WORM– Remote Shell– Rootkit
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RECAP• Injection has many challenges– NULL characters– Stack size– Highland/Lowland address– Calling thru CPU registers
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RECAP• Filters limit what we can use in a payload• Limited OP-CODE sets can still be used to build fully functional programs
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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
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Thank YouYour mind is your primary weaponhttp://www.rootkit.comhoglund@ieway.com