C3

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有一個 such_evil 的檔案
看 hex 發現是 MZ
用 DIE 看發現是 PE32 用 C 寫的 exe 檔案
編譯器是Tiny C
這題為了練習 IDA python 而主要用靜態分析

分析

start 裡面有一段 Code = sub_401000(v4, v3, v2);
sub_401000 因為 有一段 call eax 而不能反編譯
於是用 x64dbg 跑起來發現eax為0x19FD33

dump

在 call eax之前
進到 Memory Map
找到位置在 19B000 長度為 5000 (也就是包含 eax 這段 shellcode)
Dump memory to file
然後就把.bin 丟進 IDA
offset 記得設在 19B000
然後就可以慢慢拆

script

import ida_bytes
start_address = 0x19fd54
key = 0x66
length = 0x1df
while length > 0:
    origin_byte = ida_bytes.get_byte(start_address)
    origin_byte^=key
    ida_bytes.patch_byte(start_address,origin_byte)
    start_address+=1
    length-=1
print("Patch Done")

import ida_bytes
start_address = 0x19fda7
end_address = 0x19ff33+1
key = b"nopasaurus"
point = 0
for i in range(start_address,end_address):
    original_byte = ida_bytes.get_byte(i)
    original_byte^=key[point]
    ida_bytes.patch_byte(i,original_byte)
    point = ((point+1)%0xa)
    start_address+=1
print("Patch Done")

import ida_bytes
import idc
 
base_addr = 0x19fddd + 0x1e
length = 0x138
key = 0x476C4F62
while length > 0:
    patch_byte = ida_bytes.get_wide_dword(base_addr)
    patch_byte^=key
    ida_bytes.patch_dword(base_addr,patch_byte)
    base_addr+=4
    length-=4
 
print("Patch Done")

import ida_bytes
key = b"omg is it almost over?!?"
point = 0
start_address = 0x19fe5d
end_address = 0x19ff33
for i in range(start_address,end_address):
    origin_byte = ida_bytes.get_byte(i)
    origin_byte^=key[point]
    ida_bytes.patch_byte(i,origin_byte)
    point = (point+1)%24
    start_address+=1
print("Patch Done")

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