Dyre IE Hooks

I recently wrapped up my analysis of Dyre. A PDF document can be found in my papers repo. Most of the document focuses on the different stages that  Dyre interacts with the operating system. There are still some areas that I'd like to dig deeper into. For now it should be a good resource for anyone trying to identify a machine infected with Dyre or wanting to know more about the family of malware.

During the reversing process I found one part of Dyre functionality worthy of a post. As with most banking trojans Dyre contains functionality to hook APIs to log browser traffic. Typically to get the addresses of the APIs the sample will call GetProcAddress or manually traverse the portable executable file format to resolve symbols. If you are unfamiliar with the later technique I'd highly recommend reading section 3.3 of "Understanding Windows Shellcode" by Skape [1]. Dyre attempts to hook APIs in firefox.exe, chrome.exe and iexplorer.exe. It uses the standard GetProcAddress approach for resolving symbols in firefox.exe, is unsuccessful in chrome.exe and uses the GetProcAddress approach for the APIs LoadLibraryExW and CreateProcessInternalW in iexplorer.exe. Dyre hooks two APIs in WinInet.dll but it does it in a unique way. Dyre will read the image header timedatestamp [2] from WinInet. This value contains the time and date from when Wininet was created by the linker during compiling.  It will then compare the timedatestamp to a list of timedatestamps stored by Dyre.  The list contains presumably every time stamp for WinInet.dll since '2004-08-04 01:53:22' to '2014-07-25 04:04:59'.  Below is an example of the values that can be found in the list.

seg000:00A0C05F           db    0
seg000:00A0C060 TimeStampList dd 4110941Bh              ; DATA XREF: TimeStamp:_loopr
seg000:00A0C064 dword_A0C064 dd 0                       ; DATA XREF: TimeStamp+1Cr
seg000:00A0C064                                         ; TimeStamp:loc_A07A0Dr ...
seg000:00A0C068           dd 411095F2h <- Time stamp
seg000:00A0C06C           dd 0         <- WinInet index
seg000:00A0C070           dd 4110963Fh
seg000:00A0C074           dd 0
seg000:00A0C078           dd 4110967Dh
seg000:00A0C07C           dd 0
seg000:00A0C080           dd 411096D4h
seg000:00A0C084           dd 0
seg000:00A0C088           dd 411096DDh
seg000:00A0C08C           dd 0
seg000:00A0C090           dd 41252C1Bh
seg000:00A0C094           dd 0
seg000:00A0C0AC           dd 1
seg000:00A0C0B0           dd 435862A0h
seg000:00A0C0B4           dd 2
seg000:00A0C0B8           dd 43C2A6A9h
seg000:00A0C0BC           dd 3
seg000:00A0D230           dd 4CE7BA3Fh
seg000:00A0D234           dd 78h
seg000:00A0D238           dd 53860FB3h
seg000:00A0D23C           dd 79h
seg000:00A0D240           dd 53D22BCBh
seg000:00A0D244           dd 7Ah

Values converted to time

>>> datetime.datetime.fromtimestamp(0x411095F2).strftime('%Y-%m-%d %H:%M:%S')
'2004-08-04 01:53:22'

>>> datetime.datetime.fromtimestamp(0x53D22BCB).strftime('%Y-%m-%d %H:%M:%S')
'2014-07-25 04:04:59'    

If the timedatestamp is not present or an error occurs Dyre will send the hash of WinInet to the attackers server. If the hash is not found it will send WinInet back to the attackers. Below are some of the strings responsible for displaying errors for the command and control.

"Check wininet.dll on server failed"
"Send wininet.dll failed"

If the timedatestamp is found in the list the next value is used as an index into another list. For example if the timedatestamp was 4802A13Ah it would be found at the 49th entry and the next value would be 0x15 or 21.

seg000:00A0C1E8           dd 4802A13Ah  <- '2008-04-13 18:11:38'
seg000:00A0C1EC           dd 15h  <- 21 index

Assembly to read index value

seg000:00A07A0D           movsx   edx, word ptr ds:TimeStampIndex[eax*8] ; edx = 21
seg000:00A07A15           lea     edx, [edx+edx*2] ; edx  = 63
seg000:00A07A18           mov     edx, ds:offset[edx*4]
seg000:00A07A1F           mov     [ecx], edx            ; save off value

Python: calculate offset
Python>hex(0x0A0D3E0 + (21+21* 2) * 4)

seg000:00A0D4DC           dw 0F3Ch  0x0f3C offset to inline hook in wininet

The value 0xF3C + the base address of WinInet is the function prologue for ICSecureSocket::Send_Fsm. Dyre uses this to know the address to place it's hooks.

ICSecureSocket::Send_Fsm(CFsm_SecureSend *)
77200F37    90              NOP
77200F38    90              NOP
77200F39    90              NOP
77200F3A    90              NOP
77200F3B    90              NOP
77200F3C  - E9 C7F0398A     JMP 015A0008   <- Inline hook
015A0008    68 4077A000     PUSH 0A07740
015A000D    C3              RETN

00A07740    55              PUSH EBP
00A07741    8BEC            MOV EBP,ESP
00A07743    83EC 08         SUB ESP,8
00A07746    894D FC         MOV DWORD PTR SS:[EBP-4],ECX
00A07749    68 2077A000     PUSH 0A07720
00A0774E    FF75 08         PUSH DWORD PTR SS:[EBP+8]
00A07751    FF75 FC         PUSH DWORD PTR SS:[EBP-4]
00A07754    FF15 94DEA000   CALL DWORD PTR DS:[A0DE94]
00A0775A    8945 F8         MOV DWORD PTR SS:[EBP-8],EAX

It will also hooks ICSecureSocket::Receive_Fsm in the same fashion.

Rather than calling GetProcAddress (the hooked APIs are not exportable) Dyre stores the timedatestamp and patch offset of every known version of WinInet to avoid triggering heuristic based scanners. Seems like an arduous approach but still kind of cool. Another interesting fact is Dyre has the ability to patch Trusteer's RapportGP.dll if found in the browser memory. Dyre is actually a family of malware worthy of a deep dive. At first glance I ignored it because everything looked pretty cut & paste. I'd recommend others to check it out. If you find anything useful please shoot me an email. Cheers.

Hash Analyzed 099c36d73cad5f13ec1a89d5958486060977930b8e4d541e4a2f7d92e104cd21
  1. http://www.nologin.org/Downloads/Papers/win32-shellcode.pdf
  2. http://msdn.microsoft.com/en-us/library/ms680313.aspx

1 comment:

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