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see that it points somewhere into NTDLL s header (the specific value is likely to change with each new update of the operating system). Taking a quick look at the NTDLL headers using DUMPBIN shows you that the address in EAX is the beginning of NTDLL s export directory. Going to the structure definition for IMAGE_EXPORT_DIRECTORY, you will find that offset +18 is the Number OfFunctions member. Here s the final preparation of the block size:
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00403649 0040364F 00403652 MOV EAX,DWORD PTR [EBP-88] MOV ECX,DWORD PTR [EBP-78] LEA EAX,DWORD PTR [ECX+EAX*8+8]
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The total block size is calculated according to the following formula: BlockSize = NTDLLCodeSize + (TotalExports + 1) * 8. You re still not sure what Defender is doing here, but you know that it has something to do with NTDLL s code section and with its export directory. The function proceeds into another iteration of the NTDLL export list, again computing that strange checksum for each function name. In this loop there are two interesting lines that write into the newly allocated memory block:
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0040380F MOV DWORD PTR DS:[ECX+EAX*8],EDX
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MOV DWORD PTR DS:[EDX+ECX*8+4],EAX
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The preceding lines are executed for each exported function in NTDLL. They treat the allocated memory block as an array. The first writes the current function s checksum, and the second writes the exported function s RVA (Relative Virtual Address) into the same memory address plus 4. This indicates that the newly allocated memory block contains an array of data structures, each 8 bytes long. Offset +0 contains a function name s checksum, and offset +4 contains its RVA. The following is the next code sequence that seems to be of interest:
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004038FD 00403903 00403906 00403909 0040390F 00403912 00403916 00403918 0040391B 0040391D 0040391F 00403922 MOV MOV ADD MOV MOV LEA MOV SHR REP MOV AND REP EAX,DWORD PTR [EBP-C8] ESI,DWORD PTR [EBP+8] ESI,DWORD PTR [EAX+2C] EAX,DWORD PTR [EBP-D8] EDX,DWORD PTR [EBP-C] EDI,DWORD PTR [EDX+EAX*8+8] EAX,ECX ECX,2 MOVS DWORD PTR ES:[EDI],DWORD PTR [ESI] ECX,EAX ECX,3 MOVS BYTE PTR ES:[EDI],BYTE PTR [ESI]
This sequence performs a memory copy, and is a commonly seen sentence in assembly language. The REP MOVS instruction repeatedly copies DWORDs
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from the address at ESI to the address at EDI until ECX is zero. For each DWORD that is copied ECX is decremented once, and ESI and EDI are both incremented by four (the sequence is copying 32 bits at a time). The second REP MOVS performs a byte-by-byte copying of the last 3 bytes if needed. This is needed only for blocks whose size isn t 32-bit-aligned. Let s see what is being copied in this sequence. ESI is loaded with [EBP+8] which is NTDLL s base address, and is incremented by the value at [EAX+2C]. Going back a bit you can see that EAX contains that same PE header address you were looking at earlier. If you go back to the PE headers you dumped earlier from WinDbg, you can see that Offset +2c is BaseOf Code. EDI is loaded with an address within your newly allocated memory block, at the point right after the table you ve just filed. Essentially, this sequence is copying all the code in NTDLL into this memory buffer. So here s what you have so far. You have a memory block that is allocated in runtime, with a specific effort being made to put it at a random address. This code contains a table of checksums of the names of all exported functions from NTDLL alongside their RVAs. Right after this table (in the same block) you have a copy of the entire NTDLL code section. Figure 11.15 provides a graphic visualization of this interesting and highly unusual data structure. Now, if I saw this kind of code in an average application I would probably think that I was witnessing the work of a mad scientist. In a serious copy protection this makes a lot of sense. This is a mechanism that allocates a memory block at a random virtual address and creates what is essentially an obfuscated interface into the operating system module. You ll soon see just how effective this interface is at interfering with reversing efforts (which one can only assume is the only reason for its existence). The huge function proceeds into calling another function, at 4030E5. This function starts out with two interesting loops, one of which is:
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00403108 0040310E 00403110 00403113 00403115 00403117 CMP ESI,190BC2 JE SHORT Defender.0040311E ADD ECX,8 MOV ESI,DWORD PTR [ECX] CMP ESI,EBX JNZ SHORT Defender.00403108
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This loop goes through the export table and compares each string checksum with 190BC2. It is fairly easy to see what is happening here. The code is looking for a specific API in NTDLL. Because it s not searching by strings but by this checksum you have no idea which API the code is looking for the API s name is just not available. Here s what happens when the entry is found:
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