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#1
12-31-2011, 22:06
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Removing Obfuscation
You are probably familiar with the type of obfuscation which looks like this in IDA :
Code:
0000008:1005F233 loc_1005F233: ; CODE XREF: _0000008:1005F22Ej _0000008:1005F233 ; _0000008:1005F230j _0000008:1005F233 8B 15 64 6E 04 10 mov edx, ds:dword_10046E64 _0000008:1005F239 B8 2C 00 00 00 mov eax, 2Ch _0000008:1005F23E 2B D0 sub edx, eax _0000008:1005F240 89 15 64 6E 04 10 mov ds:dword_10046E64, edx _0000008:1005F246 7E 03 jle short near ptr loc_1005F24A+1 _0000008:1005F248 7F 01 jg short near ptr loc_1005F24A+1 _0000008:1005F24A _0000008:1005F24A loc_1005F24A: ; CODE XREF: _0000008:1005F246j _0000008:1005F24A ; _0000008:1005F248j _0000008:1005F24A 25 01 05 68 6E and eax, 6E680501h _0000008:1005F24F 04 10 add al, 10h _0000008:1005F251 7E 03 jle short near ptr loc_1005F255+1 _0000008:1005F253 7F 01 jg short near ptr loc_1005F255+1 _0000008:1005F255 _0000008:1005F255 loc_1005F255: ; CODE XREF: _0000008:1005F251j _0000008:1005F255 ; _0000008:1005F253j _0000008:1005F255 E9 8B 15 68 6E jmp near ptr 7E6E07E5h _0000008:1005F255 ; --------------------------------------------------------------------------- _0000008:1005F25A 04 db 4 _0000008:1005F25B 10 db 10h Code:
_0000008:1005F233 loc_1005F233: ; CODE XREF: _0000008:1005F22Ej _0000008:1005F233 ; _0000008:1005F230j _0000008:1005F233 8B 15 64 6E 04 10 mov edx, ds:dword_10046E64 _0000008:1005F239 B8 2C 00 00 00 mov eax, 2Ch _0000008:1005F23E 2B D0 sub edx, eax _0000008:1005F240 89 15 64 6E 04 10 mov ds:dword_10046E64, edx _0000008:1005F246 7E 03 jle short loc_1005F24B _0000008:1005F248 7F 01 jg short loc_1005F24B _0000008:1005F248 ; --------------------------------------------------------------------------- _0000008:1005F24A 25 db 25h ; % _0000008:1005F24B ; --------------------------------------------------------------------------- _0000008:1005F24B _0000008:1005F24B loc_1005F24B: ; CODE XREF: _0000008:1005F246j _0000008:1005F24B ; _0000008:1005F248j _0000008:1005F24B 01 05 68 6E 04 10 add ds:dword_10046E68, eax _0000008:1005F251 7E 03 jle short near ptr loc_1005F255+1 _0000008:1005F253 7F 01 jg short near ptr loc_1005F255+1 _0000008:1005F255 _0000008:1005F255 loc_1005F255: ; CODE XREF: _0000008:1005F251j _0000008:1005F255 ; _0000008:1005F253j _0000008:1005F255 E9 8B 15 68 6E jmp near ptr 7E6E07E5h _0000008:1005F25A ; --------------------------------------------------------------------------- _0000008:1005F25A 04 10 add al, 10h The obfuscation usually appears in blocks of 5 bytes that do nothing, like jnz lab jz lab <random byte> lab: ... Sometimes you also get a push/pop pair or an add/sub pair. These can be NOP'd out to finally give : Code:
_0000008:1005F233 8B 15 64 6E 04 10 mov edx, ds:dword_10046E64 _0000008:1005F239 B8 2C 00 00 00 mov eax, 2Ch _0000008:1005F23E 2B D0 sub edx, eax _0000008:1005F240 89 15 64 6E 04 10 mov ds:dword_10046E64, edx _0000008:1005F246 90 nop _0000008:1005F247 90 nop _0000008:1005F248 90 nop _0000008:1005F249 90 nop _0000008:1005F24A 90 nop _0000008:1005F24B 01 05 68 6E 04 10 add ds:dword_10046E68, eax _0000008:1005F251 90 nop _0000008:1005F252 90 nop _0000008:1005F253 90 nop _0000008:1005F254 90 nop _0000008:1005F255 90 nop _0000008:1005F256 8B 15 68 6E 04 10 mov edx, ds:dword_10046E68 _0000008:1005F25C 89 15 40 6E 04 10 mov ds:dword_10046E40, edx _0000008:1005F262 81 7C 24 28 75 03 74+ cmp dword ptr [esp+28h], 1740375h You can now turn the full block into a Procedure if relevant and the code is readable and assemblable. If you've got this far I have 2 questions. Firstly, what is this obfuscation called? (ie, name of the program that obfuscates it) and secondly, is there a more automated way of removing it?. I wrote a script which I use to turn a selected block into NOPs which helps, but it's still quite a trudge to do it by hand. If you read this far, thanks! Git 
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#2
12-31-2011, 23:16
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I don't think this is a specific kind of obfuscation.
most of the time they are based on dissassembly way and an added junk byte see PHP Code:
also, I remember the plugin CodeDoctor remove obfuscation but I didn't try it. 
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#4
01-03-2012, 21:57
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I ended up using a script to use by hand. Put cursor at first of the 2 bad jumps and hit alt-F9 to run the script. It nops the 5 bad positions, makes a block of code Unknown and then makes it code from the first address. :
Code:
#include <idc.idc>
static main()
{
auto i,j,from,size, addr1;
addr1 = ScreenEA();
if(addr1==BADADDR)
{
Message("Bad address");
Exit();
}
for ( i=addr1; i<addr1+5; i++ )
{
PatchByte(i, 0x90);
}
MakeUnknown(addr1, 10, DOUNK_DELNAMES);
MakeCode(addr1);
Message("\n" + "OK\n");
}
Code:
#include <idc.idc>
static main()
{
auto i,j,from,size, addr1, addr2;
addr1 = SelStart();
addr2 = SelEnd();
if(addr1==BADADDR || addr2==BADADDR)
{
Warning("No area selected");
Exit();
}
for ( i=addr1; i<addr2; i++ )
{
PatchByte(i, 0x90);
}
if(Name(addr2+1) != "")
MakeNameEx(addr2+1, "", SN_PUBLIC);
MakeUnknown(addr1, addr2-addr1+6, DOUNK_DELNAMES);
MakeCode(addr1);
Message("\n" + "OK\n");
}
Git
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#5
01-04-2012, 19:35
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I agree it`s probably the most common anti-disassembler trick. Olly handles it quiet well, if the code is within the code section & analyzed.
ASProtect uses this quiet heavily, and back in the day i also wrote a script to combat this.
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