If you provide security awareness training to the workforce, you will no doubt have highlighted the risk of opening Microsoft Office email attachments, especially when sent from unknown individuals. Microsoft Office files can include macros, which if allowed to run, can silently deliver malicious payloads. Comma-separated values (CSV) files are often not viewed as malicious, as they are simple text files, but a campaign has been identified by security researcher Chris Campbell that uses CSV files to deliver BazarBackdoor malware.

BazarBackdoor is a fileless malware that is believed to have been created by the threat actors behind the TrickBot banking Trojan. BazarBackdoor is used as the first stage of an attack that provides threat actors with remote access to an infected device, which can be leveraged to conduct more extensive compromises and deliver other malicious payloads. BazarBackdoor is fileless malware, which makes it difficult to detect. It resides in the memory, does not touch the hard drive, and does not leave a footprint.

Throughout the pandemic, BazarBackdoor has been delivered using COVID-19-themed and business-related lures via embedded hyperlinks in emails. The links direct users to a web page where they are tricked into downloading and running an executable file. The landing pages often claim to be web-hosted PDF, Word, or Excel files. When the file is downloaded and executed, it delivers BazarBackdoor malware. The latest campaign is a departure from the typical method of malware delivery and is one that could easily fool users as CSV files are often viewed as benign.

CSV files are often used to transfer data between different applications, such as databases and spreadsheets. A CSV file contains text separated by commas, with each comma denoting a new column and each line denoting a new row. Since a CSV file is a text file, it cannot contain any macros and cannot, by itself, execute any malicious code; however, that does not mean CSV files are entirely benign, as this latest campaign demonstrates.

The issue is not the CSV file itself, but a feature of Microsoft Excel that allows CSV files to be used in a malicious way. Excel supports Dynamic Data Exchange (DDE), which is a message-based protocol for sharing data between applications running under Windows systems. DDE can be used to execute commands that have their output inputted into an open spreadsheet, including CSV files.

The CSV files used in this campaign are like any other, with data separated by commas; however, the file includes a WMIC call that launches a PowerShell command. If the CSV file is opened using Excel – on most devices CSV files are associated with Excel – DDE uses WMIC to create a PowerShell process, which opens a remote URL that uses PowerShell to download a .jpg file, which is saved as a DLL file and executed using rundll32.exe. The DLL file installs BazarLoader, which in turn downloads and executes BazarBackdoor. If the CSV file is opened in Excel, two warnings will be generated, but users may ignore those warnings, and it would appear many have done so.

Since BazarBackdoor and other fileless malware are difficult to detect, the key to protecting against campaigns such as this is to block the threat before the malware can be delivered, which requires a combination of technical measures and end user training.

The lures and techniques used to deliver malware via phishing emails are diverse and new methods are constantly being developed to fool end users and email security solutions. While the use of Office files for delivering malware is common, other files can also be used so it is important to teach employees to be wary of any email file attachment and to never ignore any security warnings. An advanced email security solution is required to identify malicious email attachments, but antivirus engines alone will not block threats such as this. Email security solutions that include sandboxing are important. Suspicious email attachments are opened in the sandbox and are subjected to in-depth analysis. It is also recommended to also use a web filter to block access to malicious websites and control the files that can be downloaded to users’ devices.

If you want to improve your defenses against email- and web-based cyber threats, give the TitanHQ team a call. TitanHQ has developed advanced, effective, and easy-to-use cloud-based cybersecurity solutions for SMBs, enterprises, and managed service providers to protect against all email- and web-delivered threats. You may be surprised to discover how little it costs to implement these solutions and ensure malware and phishing threats never trouble your business.