i am reversing a malware sample .but getting stucked in a loop below.the line written in the bold letter generated random letter like "/2","/lm32" etc to EDX.The thing is that ECX does not contains any contents memory ,so only register address will be added

004011F7              >  8BD4              MOV EDX,ESP
004011F9              .  03E1              ADD ESP,ECX
004011FB              .  4C                DEC ESP
004011FC              .  66:8B3C24         MOV DI,WORD PTR [ESP]
00401200              .  8BE2              MOV ESP,EDX
00401202              .  8BD6              MOV EDX,ESI
**00401204              .  03D1              ADD EDX,ECX**
00401206              .  50                PUSH EAX
00401207              .  03C2              ADD EAX,EDX
00401209              .  2D 01000000       SUB EAX,1
0040120E              .  81E7 FFFF0000     AND EDI,0FFFF
00401214              .  52                PUSH EDX
00401215              .  8BD7              MOV EDX,EDI
00401217              .  8810              MOV BYTE PTR [EAX],DL
00401219              .  5A                POP EDX
0040121A              .  58                POP EAX
0040121B              .  49                DEC ECX
0040121C              .  83F9 00           CMP ECX,0
0040121F              .  0F84 02000000     JE test.00401227
00401225              .^ EB D0             JMP SHORT test.004011F7

i am adding the below screenshots before stepin and after stepout. before stepin After stepin please let me know what is happening there. Any help will be great. Thanks

  • this is a string copy routine, though written horribly and hard to read. ecx is the index into the string to copy. Jun 26, 2016 at 15:57

1 Answer 1


My guess that ECX is a counter holds the index in C:\WINDOWS\system32\ so the "m32\" you are talking about is part of the string that EDX+ECX points to, a wild guess is it might be some kind of sprintf with the system directory and catsrv.dll.

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