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The recent WannaCry ransomware attack exploited a vulnerability in older Windows SMB code. The presumably reverse-engineered function that has been cited 1 as vulnerable is as follows:

unsigned int __fastcall SrvOs2FeaToNt(int a1, int a2)
# {
#   int v4; // edi@1
#   _BYTE *v5; // edi@1
#   unsigned int result; // eax@1
#
#   v4 = a1 + 8;
#   *(_BYTE *)(a1 + 4) = *(_BYTE *)a2;
#   *(_BYTE *)(a1 + 5) = *(_BYTE *)(a2 + 1);
#   *(_WORD *)(a1 + 6) = *(_WORD *)(a2 + 2);
#   _memmove((void *)(a1 + 8), (const void *)(a2 + 4), *(_BYTE *)(a2 + 1));
#   v5 = (_BYTE *)(*(_BYTE *)(a1 + 5) + v4);
#   *v5++ = 0;
#   _memmove(v5, (const void *)(a2 + 5 + *(_BYTE *)(a1 + 5)), *(_WORD*)(a1 + 6));
#   result = (unsigned int)&v5[*(_WORD *)(a1 + 6) + 3] & 0xFFFFFFFC;
#   *(_DWORD *)a1 = result - a1;
#   return result;
# }

While it looks like this function copies data from one place to another and we're told the application fails to perform adequate boundary-checks on user-supplied input can someone explain in detail what this function is supposed to do line by line and where it is flawed?

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1 Answer 1

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Please check these links,might help....

https://gist.github.com/worawit/bd04bad3cd231474763b873df081c09a https://zhuanlan.zhihu.com/p/26592431

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  • 2
    answers formed solely from links are likely to be deleted. Please include in your answer why you think that the information from these links would be helpful.
    – peter ferrie
    May 19, 2017 at 16:15
  • I am not able to form a clear overall picture right now,but however these links helped me a bit...I am also in the process to find out.once i get it,i will for sure
    – Warm_up
    May 19, 2017 at 16:52
  • Thanks for those links. The second one references blogs.360.cn/360safe/2017/04/17/nsa-eternalblue-smb which I think is more useful. Though the translation from Chinese is pretty garbled I think the problem is in code for a different function (SrvOs2FeaListSizeToNt) that gets called before the quoted function in my original question. Here a DWORD calculation is cast to a WORD. While that sounds unsafe I am still not sure how the EternalBlue exploit takes advantage of this.
    – julian
    May 23, 2017 at 23:06

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