PRACTICAL MALWARE ANALYSIS: SHELLCODE ANALYSIS (LAB 19-03)

Tools Used

  1. IDA Pro
  2. Ollydbg
  3. remnux

Sample:

  1. Lab19-03.pdf SHA256: 216a1b095dbcf7099f3b91633d5b663554bda1363d8a6a31323a28d4328c380e

VirusTotal:

  • Detection Rate: 37/56
  • Analyzed on 2016-06-15
  • Compilation Date:2010:06:11 15:30:10
  • View report here

Lab 19-3
Analyze the file Lab19-03.pdf. If you get stuck and can’t find the shellcode,
just skip that part of the lab and analyze file Lab19-03_sc.bin using
shellcode_launcher.exe.

Questions
1. What exploit is used in this PDF?

Lets recce the pdf file first to get more insight. We can see that it contains /JS and /JavaScript elements. Which indicates that this pdf might be using javascript to exploit the pdf…

pdfid
Figure 1. pdfid to recce the pdf file

using pdfextract we can easily extract the javascript contents.

pdfextract
Figure 2. Extract javascript via pdfextract tool

The extracted javascript contains the payload and some pdf version check to filter which pdf reader version can be exploited followed by some standard heapspray and finally the trigger “util.printf“. A google search on this printf exploit surfaced the following article from CORE security. CVE-2008-2992 a Printf buffer overflow exploit.

cve
Figure 3. CVE-2008-2992; Printf buffer overlow

2. How is the shellcode encoded?

Referring to Figure 3, we can easily see that the payload is unicode encoded by the %u symbol. We could convert it using a unicode2raw tool provided in remnux… or you can write your own simple tool to do it.

unicode2raw
Figure4. unicode2raw

3. Which functions does the shellcode manually import?

Before jumping straight into analyze the shellcode, we could use sctest to generate a nice little graph of the piece of shellcode we are analyzing.

graph
Figure 5. sctest and dot
diagram
Figure 6. Flow Graph

Notice the GetFileSize at the bottom left, this indicates that the shellcode is attempting to open a file and is using GetFileSize to find the correct file handler. Perhaps more payload is in the file.

using shellcode_launcher.exe provided by the book, we could launch the shellcode in ollydbg with a open file handle to the pdf file.

shellcode_launcher
Figure 7. shellcode_launcher

On running the malware, the program will break automatically. Manually set the new origin to the next instruction to resume program flow as shown below.

set new origin
Figure 8. Set new origin to jmp instruction

If we look at the handles, we would see that the pdf file is in it as well.

fileHandle
Figure 9. File Handle open

Tracing the shellcode we will soon come into the following codes. The code here is trying to find the function address from kernel32.dll by using a computed checksum.

GetProcAddress
Figure 10. imports

The shellcodes then attempts to Load shell32 library followed by a search for ShellExecuteA as shown in Figure 11 to 13.

loadLibrary
Figure 11. LoadLibraryA on shell32
shellA
Figure 12. finding ShellExecuteA address
ShellExeLoaded
Figure 13. ShellExecuteA added to list of imports

The populated functions are:

  1. LoadLibraryA
  2. CreateProcessA
  3. TerminateProcess
  4. GetCurrentProcess
  5. GetTempPathA
  6. SetCurrentDirectryA
  7. CreateFileA
  8. GetFileSize
  9. SetFilePointer
  10. ReadFile
  11. WriteFile
  12. CloseHandle
  13. GlobalAlloc
  14. GlobalFree
  15. ShellExecuteA

4. What filesystem residue does the shellcode leave?

Set breakpoint @ WriteFile and let the shellcode run. As shown in figure 11 and 12, 2 files are dropped on the victim’s machine. They are foo.exe and bar.pdf. Both are located in the temp folder as defined in the env variables of the victim’s machine.

droppingMZ
Figure 14. MZ dropped in Temp\foo.exe
droppingpdf2
Figure 15. PDF dropped in Temp\bar.pdf

5. What does the shellcode do?

The shellcode attempt to import various functions from kernel32.dll and then using its LoadLibraryA function to load shell32 library to import ShellExecuteA function.

The shellcode then attempts to read the pdf file to extract both the executable payload and a pdf file which are both dropped in the temp folder as foo.exe and bar.pdf respectively.

foo.exe is then executed via CreateProcessA as shwon in Figure 16 and 17.

CreateProcess_foo
Figure 16. CreateProcessA for foo.exe
createProcess
Figure 17. foo.exe

Bar.pdf is then opened via ShellExecuteA. ShellExecuteA uses the victim’s default application to open the pdf file.

shellexePDF
Figure 18. ShellExecuteA for bar.pdf
pdfFile
Figure 19. bar.pdf pops up

 

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PRACTICAL MALWARE ANALYSIS: SHELLCODE ANALYSIS (LAB 19-03)

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