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Blog — Jun 27, 2019

New Dridex Variant Evading Traditional Antivirus

Speak With A Security Expert Now

Malware researcher Brad Duncan recently reported analysis on a new variant of Dridex he observed on June 17th[1] (edit: Additional information reveals it may have been originally discovered by @James_inthe_box on June 13.) The malware utilizes an Application Whitelisting technique to bypass mitigation via disabling or blocking of Windows Script Host. The technique takes advantage of WMI command-line (WMIC) utility's weak execution policy around xls scripts.

On June 26, 2019, eSentire Threat Intelligence discovered new infrastructure pointing to a similar Dridex variant (see IOCs below). At the time of discovery, using data from VirusTotal, only six antivirus solutions of about 60 detected suspicious behavior [2]. About 12 hours later, on the morning of June 27, 16 antivirus solutions could identify the behavior.

Evolving Malware
Similar to Emotet, Dridex has undergone numerous transformations as it has evolved over the last decade [3]. Dridex first appeared as Cridex in 2011 boasting features such as dynamic configuration, web injections, and infecting connected USB devices. The malware targets banking information on the victim system. Over the last decade, Dridex underwent a series of feature augmentation, including a transition to XML scripts, hashing algorithms, peer-to-peer encryption, and peer-to-command-and-control encryption. Like Emotet, each new version of Dridex traces a further step in the global arms race as the security community responds with new detection and mitigations.

Trend Micro concluded last year that similarities between Emotet, Ursnif, and Dridex indicate some established social structure might exist between the associated threat actors, but the nature of that social structure is not known [4]. Once black markets reach sufficient size and complexity, analogs of partnerships, insider trading, employee transfer of knowledge, and IP theft are likely to take place between malware groups.

Expect More Infrastructure and other Indicator changes

Two observations indicate this campaign isn’t done shifting identifiers. Given the same-day deployment and implementation of the ssl-pert[.]com domain on June 26thand a tendency to utilize randomly generated variables and URL directories, it is probable the actors behind this variant of Dridex will continue to change up indicators throughout the current campaign.

[image src="/assets/DridexMessage2.png" id="2271" width="400" height="309" class="center ss-htmleditorfield-file image" title="DridexMessage2"]

Further, Duncan [1] also reported polymorphism in the supporting library: “the Dridex DLL files are 64-bit DLLs using file names that are loaded by legitimate Microsoft Windows system EXEs. These file paths, file names, and associated SHA256 hashes change every time the victim logs onto the infected Windows host.”

Initial Access

The malware was observed arriving through email in the form of a malicious document with embedded macros. Depending on the environment, the macros can be triggered by varying levels of employee interaction. The message in the email appears as below with specific numbers redacted (as they could potentially be used by a threat actor to track campaign victim metrics).

[image src="/assets/DridexMessage.png" id="2270" width="500" height="187" class="center ss-htmleditorfield-file image" title="DridexMessage"]


Given email as the initial access point, employees are the first line of defense against this threat. Expect financial departments to be targeted by unsolicited invoices carrying malicious macros within. Some antivirus engines were able to detect (but not specify) the suspicious behavior. Given the rapid turnover of infrastructure and indicators, signature-based antivirus solutions will continue to have gaps throughout the Dridex campaign.

Technical Details

If the macros are successfully executed, they hail the ssl-pert[.]com domain to download servern.exe (the Dridex installer). The macro script utilizes an application whitelisting BYPASS technique first described in April of 2018 by Casey Smith (@subTee on Twitter) [5]. In samples observed by both Duncan [1] and eSentire, JavaScript code was embedded in an XSL template capable of execution by wmic with no integrity checks. The XSL script removes itself, then downloads and executes the Dridex installer

Abuse of MWIC was observed by Symantec in August of 2018 [6]. In that case, however, wmic was used to download the XSL file which was then executed by mshta.

Indicators of Compromise (IOCs):

domain: ssl-pert[.]com

executablefilename: servern.exe

executablemd5: 0a5c6944c3622a303803a058f85304b0

maliciousdocumentmd5: 047e1d697768831a05f2c833b7fd3e25

[1] https://isc.sans.edu/forums/diary/Malspam+with+passwordprotected+Word+docs+pushing+Dridex/25042

[2] https://www.virustotal.com/gui/file/84516414732def8426078d4a0e5ceed058a415ada9511df98e5b2544235ea9fe/detection

[3] https://securelist.com/dridex-a-history-of-evolution/78531/

[4] https://blog.trendmicro.com/trendlabs-security-intelligence/ursnif-emotet-dridex-and-bitpaymer-gangs-linked-by-a-similar-loader/

[5] http://subt0x11.blogspot.com/2018/04/wmicexe-whitelisting-bypass-hacking.html

[6] https://www.symantec.com/blogs/threat-intelligence/wmic-download-malware

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Akash Malhotra
Akash Malhotra Technical Developer/Writer

With diverse knowledge of computer security, threat intelligence and front-end web development, Akash has worked with developing and documenting API libraries with Cymon.io, our open source threat intelligence aggregator.