Hello!
I am looking at the capabilities of Nim, coming from a python and bash background. I have been reading different tutorials and the great documentation. I saw nim is able to output assembly with the --asm flag, so I thought of trying to create small code to launch for example calc.exe. Does anyone have an idea how to create calc.exe shell code with nim?
This:
runsc(): void =
asm """
.byte 0xfc,0x48,0x83,0xe4,0xf0,0xe8,0xc0,0x00,0x00,0x00,0x41,0x51,0x41,0x50,0x52,0x51,0x56,0x48,0x31,0xd2,0x65,0x48,0x8b,0x52,0x60,0x48,0x8b,0x52,0x18,0x48,0x8b,0x52,0x20,0x48,0x8b,0x72,0x50,0x48,0x0f,0xb7,0x4a,0x4a,0x4d,0x31,0xc9,0x48,0x31,0xc0,0xac,0x3c,0x61,0x7c,0x02,0x2c,0x20,0x41,0xc1,0xc9,0x0d,0x41,0x01,0xc1,0xe2,0xed,0x52,0x41,0x51,0x48,0x8b,0x52,0x20,0x8b,0x42,0x3c,0x48,0x01,0xd0,0x8b,0x80,0x88,0x00,0x00,0x00,0x48,0x85,0xc0,0x74,0x67,0x48,0x01,0xd0,0x50,0x8b,0x48,0x18,0x44,0x8b,0x40,0x20,0x49,0x01,0xd0,0xe3,0x56,0x48,0xff,0xc9,0x41,0x8b,0x34,0x88,0x48,0x01,0xd6,0x4d,0x31,0xc9,0x48,0x31,0xc0,0xac,0x41,0xc1,0xc9,0x0d,0x41,0x01,0xc1,0x38,0xe0,0x75,0xf1,0x4c,0x03,0x4c,0x24,0x08,0x45,0x39,0xd1,0x75,0xd8,0x58,0x44,0x8b,0x40,0x24,0x49,0x01,0xd0,0x66,0x41,0x8b,0x0c,0x48,0x44,0x8b,0x40,0x1c,0x49,0x01,0xd0,0x41,0x8b,0x04,0x88,0x48,0x01,0xd0,0x41,0x58,0x41,0x58,0x5e,0x59,0x5a,0x41,0x58,0x41,0x59,0x41,0x5a,0x48,0x83,0xec,0x20,0x41,0x52,0xff,0xe0,0x58,0x41,0x59,0x5a,0x48,0x8b,0x12,0xe9,0x57,0xff,0xff,0xff,0x5d,0x48,0xba,0x01,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x48,0x8d,0x8d,0x01,0x01,0x00,0x00,0x41,0xba,0x31,0x8b,0x6f,0x87,0xff,0xd5,0xbb,0xaa,0xc5,0xe2,0x5d,0x41,0xba,0xa6,0x95,0xbd,0x9d,0xff,0xd5,0x48,0x83,0xc4,0x28,0x3c,0x06,0x7c,0x0a,0x80,0xfb,0xe0,0x75,0x05,0xbb,0x47,0x13,0x72,0x6f,0x6a,0x00,0x59,0x41,0x89,0xda,0xff,0xd5,0x63,0x61,0x6c,0x63,0x2e,0x65,0x78,0x65,0x00
ret
"""
runsc()
But I can't think of a way to create that byte code with pure Nim. Ideas?
I have no idea how the calc.exe byte code is supposed to be generated but you can do everything in macros.
Here is an x86-64 assembler I have written in Nim macros:
And here is how to use it, for example to generate a conditional copy kernel using cmov
And the initial experiments with very small macros i got started with: https://github.com/mratsim/finite-fields/blob/97e4006/macro_add_carry_v2.nim#L18-L66
Otherwise check out the OffensiveNim repo: https://github.com/byt3bl33d3r/OffensiveNim
Wow, thanks for the answer! I have to admit, I have the links you provided but I am not sure whether I am a step further to the final goal. I have been looking at offensivenim as well, I don't think that is of use all these project seem to be using shellcode as input.
What would you approach be to, for example spawn a messagebox with a simple "hello, world!" by using nim to create that in shellcode? Maybe, calc.exe is a bit too far fetched at this point.
Have no clue about shellcodes etc but I assume:
Find out how to actually call shellcode from nim.
What would you approach be to, for example spawn a messagebox with a simple "hello, world!" by using nim to create that in shellcode? Maybe, calc.exe is a bit too far fetched at this point.
There are 2 ways:
I would start with implementing that in a nim proc with inline assembly, ensure that it does what I want. Then create a shellcode assembler in Nim macros. This is what I did with https://github.com/mratsim/finite-fields/blob/97e4006/macro_add_carry_v2.nim#L18-L66 and then productionizing the assembler in https://github.com/mratsim/constantine/blob/928f515/constantine/platforms/isa/macro_assembler_x86.nim
But whatever you choose, you need to understand shellcode. I know x86 assembly, I don't know anything about shellcode so ¯\_(ツ)_/¯