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Code Mutations via Behaviour Analysis

SPTH
Virus Writing Bulletin [1]
January 2011

1
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1) Introduction

The basic idea is: The file analyses the behaviour of its own code and compares it with the behaviour of a randomly generated code. If the behaviour is the same, the original file-code will be substituted by the new random code.

This article describes how this idea can be realized.

2) Basics - Register Operators only

2.1) Analyse me: Local Behaviour Table

The program's code is splitted into 8byte blocks of code, padded with NOPs). With that knowlegde we can parse the whole file, and analyse the behaviour of each 8byte part.

We only use Register operators for the moment. When we run an unknown code, all that can be changed are the 8 Registers and FLAGS Registers.

We define a table consisting of 8+1 * 4 bytes:

	Local Behaviour Table (LBT):

	Each behaviour Block consists of:
	Registers: 9*4=36 byte
		0x00 dd EAX
		0x04 dd EBX
		0x08 dd ECX
		0x0C dd EDX
		0x10 dd EBP
		0x14 dd ESI
		0x18 dd EDI
		0x1C dd FLAGS
		0x20 dd ESP

Before running the unknown code, we set all registers (except of EPS) to zero. After running the code, we save the change of the registers:

        sub     dword[LBT+0x00], eax
        sub     dword[LBT+0x04], ebx
;       ...
        sub     dword[LBT+0x20], esp
 

(For the flag-register: pushfd/pop Reg32) With that table, we know the principal behaviour of out code. Thats all we need for the moment.

2.2) Create Random Code

We want to create an 8 byte random code. The best way is to use a template system. One template could look like this:

OPAlImm8:
        cmp     ebp, (RandomCodeBufferSize-2)
        jg      EndCRCCycle

        mov     ebx, 9
        xor     ecx, ecx
        call    CreateSpecialRndNumber

        mov     byte[RandomCode+ebp], 0x04      ; add al, Imm8
        cmp     dl, 0
        je      OPAlImm8Cont
;       ...

        mov     byte[RandomCode+ebp], 0x3C      ; cmp al, Imm8
        cmp     dl, 7
        je      OPAlImm8Cont

        jmp     OPAlImm8End

OPAlImm8Cont:
        mov     byte[RandomCode+ebp+1], dl
        add     ebp, 0x2
        jmp     EndCRCCycle
OPAlImm8End:
 

One important restriction is the ESP. The Stack Pointer must stay in the reserved Stack Memory (from [fs:0x04]-[fs:0x08]). Therefore the RCG must not be the source of MOV, ADD, ... . INC and DEC are usually OK if you restore the ESP variable after running the unknown code.

2.3) Compare me: Blackbox-Analyse

We could create a Local Behaviour Table for the file code and the random code now, and compare the tables. Thats all?

No! We have used an initial condition (setting all registers to zero), thus the LBT is dependent on that. For example these codes would have the same LBT:

	mov eax, 1		inc al
	inc bl			inc bl

Or this one:

	mov eax, ebx		xor eax, eax

That means, when we found equivalent LBTs, we have to make further tests using different initial conditions.

We can change the registers and the flags to random values, and create new LBTs with that new conditions. If we run that test with always changing conditions for a big number of times (I used 100 and 1000 for register only and flags tests, respectivly), and every single LBT of the programs code and the random code is equal, we found a code with same behaviour, and can substitute the old one.

2.4) More Freedom: Global Behaviour Table

It is a very strict restriction to the new random code that it has to have exactly the same behaviour. Usually its not required to have have the same behaviour in all registers, most are unused anyway.

We could use this additional freedom, and creating a Global Behaviour Table (GBT) and compile-time, giving the code the information of which registers have to be the same and which can be different.

Its a good idea to use a macro and symbolic variables for that (everything else would be insane):

        ; Restrictions:
        ; A=EAX, B=EBX, C=ECX, D=EDX, P=EPB, S=ESI, I=EDI, F=FLAGS

        rNoRes          EQU 00000000b
        rA              EQU 00000001b
        rB              EQU 00000010b
        rAB             EQU 00000011b
        rC              EQU 00000100b
;       ...
        rABCDPSIF       EQU 11111111b

        GlobalBehaviourTableList equ  

        macro cc restr*, instr*, op1, op2
        {
                ; Macro for padding commands to 8byte each and for
                ; adding the restrictions to the GlobalBehaviourTableList

                local StartCommand, EndCommand
                StartCommand:
                if op2 eq
                        if op1 eq
                                instr
                        else
                                instr op1
                        end if
                else
                        instr op1,op2
                end if
        EndCommand:
                times (8-EndCommand+StartCommand): nop  ; padding

                match any, GlobalBehaviourTableList     ; Add further elements to list
                \{
                        GlobalBehaviourTableList equ GlobalBehaviourTableList,restr
                \}

                match , GlobalBehaviourTableList        ; Add first element to list
                \{
                        GlobalBehaviourTableList equ restr
                \}
        }
 

There is no restiction for ESP, as this register always has to be equivalent in file code and random code.

One simple example:

        cc      rNoRes, nop
        cc      rA, mov, eax, 0x0       ; eax=0
        cc      rA, inc, eax            ; eax=1
        cc      rA, add, eax, 2         ; eax=3
        cc      rA, nop                 ; eax=3
        cc      rC, mov, ecx, eax       ; ecx=3
        cc      rC, nop                 ; ecx=3  
 

will be translated to

       00402000 > $ 90             NOP
       00402001   . 90             NOP
       00402002   . 90             NOP
       00402003   . 90             NOP
       00402004   . 90             NOP
       00402005   . 90             NOP
       00402006   . 90             NOP
       00402007   . 90             NOP
       00402008   . B8 00000000    MOV EAX,0
       0040200D   . 90             NOP
       0040200E   . 90             NOP
       0040200F   . 90             NOP
       00402010   . 40             INC EAX
       00402011   . 90             NOP
       00402012   . 90             NOP
       00402013   . 90             NOP
       00402014   . 90             NOP
       00402015   . 90             NOP
       00402016   . 90             NOP
       00402017   . 90             NOP
       00402018   . 83C0 02        ADD EAX,2
       0040201B   . 90             NOP
       0040201C   . 90             NOP
       0040201D   . 90             NOP
       0040201E   . 90             NOP
       0040201F   . 90             NOP
       00402020   . 90             NOP
       00402021   . 90             NOP
       00402022   . 90             NOP
       00402023   . 90             NOP
       00402024   . 90             NOP
       00402025   . 90             NOP
       00402026   . 90             NOP
       00402027   . 90             NOP
       00402028   . 89C1           MOV ECX,EAX
       0040202A   . 90             NOP
       0040202B   . 90             NOP
       0040202C   . 90             NOP
       0040202D   . 90             NOP
       0040202E   . 90             NOP
       0040202F   . 90             NOP
       00402030   . 90             NOP
       00402031   . 90             NOP
       00402032   . 90             NOP
       00402033   . 90             NOP
       00402034   . 90             NOP
       00402035   . 90             NOP
       00402036   . 90             NOP
       00402037   . 90             NOP

After a few minutes, the new code looks like that:

       00402000 > $ 80D5 39        ADC CH,39
       00402003   . 90             NOP
       00402004   . 92             XCHG EAX,EDX
       00402005   . 4B             DEC EBX
       00402006   . 8BD6           MOV EDX,ESI
       00402008   . 45             INC EBP
       00402009   . 20C8           AND AL,CL
       0040200B   . 33C0           XOR EAX,EAX
       0040200D   . 2BF2           SUB ESI,EDX
       0040200F   . 4F             DEC EDI
       00402010   . 88EE           MOV DH,CH
       00402012   . 39CD           CMP EBP,ECX
       00402014   . 12F5           ADC DH,CH
       00402016   . 40             INC EAX
       00402017   . 4F             DEC EDI
       00402018   . 22FA           AND BH,DL
       0040201A   . 14 02          ADC AL,2
       0040201C   . 13C8           ADC ECX,EAX
       0040201E   . 3C 07          CMP AL,7
       00402020   . 83ED C6        SUB EBP,-3A
       00402023   . 29DE           SUB ESI,EBX
       00402025   . 83DA D9        SBB EDX,-27
       00402028   . 8BD8           MOV EBX,EAX
       0040202A   . 80E2 37        AND DL,37
       0040202D   . 23D4           AND EDX,ESP
       0040202F   . 91             XCHG EAX,ECX
       00402030   . 2C 05          SUB AL,5
       00402032   . 88CE           MOV DH,CL
       00402034   . 21EF           AND EDI,EBP
       00402036   . 4F             DEC EDI
       00402037   . 90             NOP                   ; ecx=3

3) Extentions

3.1) Including the Stack

Including Stack-Commands is quite easy. There are some things to note:

What happens at 'pop Reg32'? Reg32 changes and ESP changes, we already have all of that informations in the LBT, so nothing to change for POP.

But what is with 'push Reg32/Imm32'? ESP changes, but we dont know what is on the stack from the above rules. 'PUSH 0x0' would result in the same LBT as 'PUSH 0x1' does.

Therefore we have to add an additional dword to the LBT, defining the latest value of the stack (if newESP=oldESP-4 changed):

                  Local Behaviour Table (LBT):
                          ...
                     Stack: 1*4=4 byte
                     0x44 STACK change (if push is used)

One restriction to the random code appears, too: You must not use pop/push as trash (push/pop is ok). As an example, you have:

        xor     eax, eax
        ret
 

This could be translated to:

        xor     eax, eax
        pop     eax
        push    0
        ret
 

In principal, a legal substituation from the rules above, but fatal.

One can solve that in two ways: Increase the Local Behaviour Table by 8 dwords, and compare the changing of the stack; or you could just add a byte to note whether the pop occured before the push (this is what I've used in my example virus).

3.2) Including Memory

It is very tricky to use memory instructions for the random code and get the behaviour later.

First thing is, that we have to use an Exception Handler. I tried SEH, but there are problems because the EXCEPTION_REGISTRATION and the callback function handle has to be on the stack, and this value can be changed by the random code (for instance "SBB DWORD PTR SS:[ESP+B],EAX", which actually happened). As it's the worst thing when the Exception Handler will be destroyed, we have to use something else: VEH (Vectored Exception Handler). This is provided by Windows XP+ as AddVectoredExceptionHandler-API.

Second problem: The memory may overwrite any data that is not protected. That means: Its own code (if its in the .data section), any data created with Virtual Alloc (and was no Read/Execution-Only-Flag). We have to use VirtualProtect to any allocated memory.

Third problem: We can not and should not protect the .data section, as we want new code-substituations for that, too. But the random code can overwrite some important data in the .data section. A solution is to mirror the .data memory with READ-ONLY potection (or NOACCESS flag) After the execution and analyse of the random code, we can restore the original .data section.

Fourth problem: We have to save the pointer to the DataMirror somewhere. The register may change, so we have to use the .data section. But this pointer can be overwritten by random code, then we dont know the place of the DataMirror anymore, and can not restore the original .data - big problem! However, we can use some self-repair technique: We store the pointer to the DataMirror three times at totally different positions in the .data section, and after execution use that algorithm:

        mov     eax, hDataMirror1;
        ;if (hDataMirror1!=hDataMirror2) {
                mov     eax, hDataMirror3;
        ;}
 

eax is the pointer to the DataMirror if there is maximum one destroyed pointer. This should be enough for us (we just have 8 byte of random code).

Fifth problem: The instructions may rely on the values of the memory, just the same thing as with initial register values. We have to fill the memory as well as the stack with random memory, to prevent that dependence on the initial values.

Sixth problem: The result of the random code (the local behaviour table) can not be saved in .data (as this one will be restored later) or in the DataMirror (as this one is used for comparing). So one has to allocate additional memory for saving the LBT temporarily, and copy it to the restored .data Section later.

For saving the results I added 2*4 additional dwords to the Local Behaviour Table:

	Memory: 2*4*4=8*4=32 byte:
		0x24 dd MemOffset1
		0x28 dword[MemOffset1]
		0x2C dd MemOffset2
		0x30 dword[MemOffset2]
		0x34 dd MemOffset3
		0x38 dword[MemOffset3]
		0x3C dd MemOffset4
		0x40 dword[MemOffset4]

There is space for saving results of four changes of the random code, each change uses two dwords, one for the addresse that has been changed, and one for the actual data that has been changed.

The chance that a valid change appears is very slow if we use totally random initial values. One could increase the chance by using the following trick:

All registers get a small random value between 0 and ~200, and one variable gets the value of the .data-section:

	eax=0x33	ebp=0x67
	ebx=0x8A	esi=.data + 0x12
	ecx=0x04	edi=0xAA
	edx=0x12	esp=esp

This increases the chance of valid memory changes alot - for instance:

        mov     dword[esi+ecx], eax
 

will be valid, which would have not be if the registers were totally random.

This diagram shows what will be done with the memory blocks while analysing one unknown code:

     Preparation
     ===========

                        ___________ 
            mirroring  |           |
           ----------> | Data      |
          |            |   Mirror  |
     ___________       |           |
    |           |      |           |
    | .data     |      |___________|       ___________ 
    |           |                         |           |
    |           |                         | Random    |
    |           |  -------------------->  |   Data    |
    |___________|    important values     |           |
                                          |           |
                                          |___________|


                        ___________ 
                       |           |
                       | Data      |
                       |   Mirror  |
     ___________       |           |
    |           |      |           |
    | .data     |      |___________|       ___________ 
    |           |                         |           |
    |           |                         | Random    |
    |           |  <--------------------  |   Data    |
    |___________|     randomize .data     |           |
                                          |           |
                                          |___________|




                    E X E C U T I O N



     Comparison
     ==========
                        ___________ 
                       |           |
                       | Data      |
                       |   Mirror  |
     ___________       |           |
    |           |      |           |
    | .data     |      |___________|       ___________ 
    |           |                         |           |
    |           |                         | Random    |
    |           |  <<------------------>> |   Data    |
    |___________|       compare data      |           |
                             |            |           |
                             |            |___________|
                             |             ___________
                             |            |           |
                             -----------> | temp. LBT |
                               results    |___________|


     Restoration 
     ===========


                        ___________ 
            restore    |           |
           ----------  | Data      |
          |            |   Mirror  |
          V            |           |
     ___________       |           |
    |           |      |___________|
    | .data     |      
    |           |  
    |           |                        ___________
    |           |        save LBT       |           |
    |___________|  <------------------- | temp. LBT |
                                        |___________|  

4) Possible extentions

These are some ideas that increase the power of this technique and are easy to implement - everything without the need of deep code analyse or full disassembling.

4.1) Hide me: Global Behaviour Table at Runtime

The technique above uses a behaviour-table hardcoded in the file, which could be a source for detection by lazy avers. There is no need for that, one could create the GBT at runtime, and write it to the memory. It's just a little bit tricky as the GBT is defined implicit for all commands - but some fake-code or hand-adjusted GBT can solve that problem.

4.2) Permutation - easy as it can be

As the whole code consists of 8byte command blocks, its very easy to use permutation - there is no need of disassembling as in usual permutation engines. One can simply copy the different 8byte blocks to a random place in the file and connect them with Jmp-Operations.

4.3) Jump somewhere

One weakness of this technique may be the handling of jmp/ret/call commands. One must not execute them due to unexpected behaviour, therefore I have defined a further restriction in the GBT: rNoEmul. If this command appears, the corresponding 8byte block will not be analysed, thus no equivalent codes will be found. As the code knows where a jump will be (because of that rNoEmul byte in the GBT), it can include trash or change the instruction (call -> pop Reg32 + jmp Reg32). When trash is included, it should restrict the trash to not contain significant operation opcodes (0xC3 = ret, 0xE8 = call, 0xEB = jmp, 0x7x = jxxx).

4.4) Behaviour Flow

Imagine you have a code like this:

	cc rA,	mov, eax, 0
	cc rAB,	mov, ebx, eax
	cc rB,	inc, ebx
	cc rBC,	xor, ecx, ecx

Command 3 and 4 are independent and could be exchanged. But how can the code itself find that out?

We could analyse both commands (16 bytes) together in a black-box test and compare output. If the behaviour of the changed 16 bytes is the same as the original one, we could use the new one. We can see immediatly that line 2 and 3 can not be exchanged - exactly the same result we would get in the blackbox test.

The only thing we have to think of is to adjust the restrictions, which is straight forward.

In the end, we get some kind of behaviour flow, a very beautiful result:

cc rA,   mov, eax, 0        cc rA,   mov, eax, 0          cc rA,   mov, eax, 0
cc rAB,  mov, ebx, eax      cc rAB,  mov, ebx, eax        cc rAC,  xor, ecx, ecx
cc rB,   inc, ebx      ->   cc rBC,  xor, ecx, ecx   ->   cc rABC, mov, ebx, eax
cc rBC,  xor, ecx, ecx      cc rBC,  inc ebx              cc rBC,  inc ebx

4.5) Automatic shrinking/extention/trash

Shrinking and extention is usually a very hard buisness - requires full code disassembling to some meta-languages. Here it is much easier, the only thing we have to do is adjusting the jump addresses.

For shrinking, we just use take a 16byte block (instead of the usual 8 byte block) for analysing, and compare it with a 8 byte block of random data - very simple!

The extention is the inverse process - we take a 8byte code from the file code and compare it with a 16byte block of random code. The random 16byte block should consist of 2 independent 8 byte blocks, to prevent problems when this code will be analysed alone. This is very trivial too.

The trash is a similar technique: We get an 8 byte code and analyse its behaviour with respect to the restrictions of the direct-above block. Again - very simple.

5) Conclusion

Analysing the behaviour of code and substituate with random instructions is powerful as well as beautiful.

We should use more the power of unpredictable randomness!

Dr. Theodore Spankman
December 2010

PS1: The initial idea comes from an article released in August 2004

PS2: Special thanks goes to hh86 for motivation, help and useful ideas in connection with this research... Merci! :)

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