We identified a potential campaign in preparation where the victim would receive a zip file containing a Malicious Excel file embedding Excel 4.0 Macros — requiring user interaction to infect the victim.
Malicious Excel File
The chain works as the following:
- Victim receives a compressed archive (.xls.zip) file.
- Once opened, the .xls file asks the user to enable macros to allow the document to connect to a remote server to send a web request that returns back the malicious macros to be executed. This is quiet ingenious as it allows some degree of flexibility to the attacker — but also to evade traditional detection since the malicious macros would not be inside the file.
- Malicious macro downloads a dll which gets executed with
- Weirdly enough, the dll that gets downloaded is a 32-bits dll which WinExec() Windows’s Calculator application.
We found that the distributing domains are hosted on Alibaba Cloud. Details are provided at the end of the blog-post. Domains were registered on Feb 7, 2020 and Feb 10, 2020.
Web Query Dynamically Retrieving the DLL
Some you see it, sometimes you don’t.
It is unclear at this point if the attackers are just doing some scoping & testing on an upcoming campaign.
Malicious DLL executing WinExec(‘calc’)
Original DLL name appears to be w32-dll-run-shellcode.dll
This could be that attackers are in training and learning how to spam and infect victims, or also that those servers will be rotated with more malicious contents. It is also unclear if this campaign is connected to Dudear.
Detection on VirusTotal is still pretty low at the time of writing the article.
- Do not enable macros on files from unknown senders
- Always be suspicious of legacy office files such as .XLS, .DOC or .RTF.
- Make sure to have memory analysis as part of your incident response strategy to detect and assess potential infections on hosts. We can help you with our automated platform and utilities.
- Consider using Application Guard for Microsoft Office.
- Follow us on Twitter/ LinkedIn to stay informed about emerging campaigns and techniques.
Indicator of compromise (IoC):
Excel File Hashes:
Domain Names & Servers:
Domains are sharing a common IP address, and to are hosted in Alibaba Cloud.
veqejzkb.xyz (Registered On 2020-02-10)
merystol.xyz (Registered On 2020-02-07)