A new phishing campaign is targeting financial sector employees by using links to download a ‘weaponized’ Excel document.

MirrorBlast, a phishing effort, was discovered in early September by security firm ET Labs. Morphisec, a fellow security firm, has now studied the malware and warns that the malicious Excel files might escape malware-detection systems due to "extremely lightweight" embedded macros, making it especially risky for businesses that rely on detection-based protection and sandboxing. 

Macros, or scripts for automating activities, have grown in popularity among cybercriminals. Despite the fact that macros are disabled by default in Excel, attackers employ social engineering to deceive potential victims into allowing macros. Despite appearing to be a simple approach, macros have been employed by state-sponsored hackers because they frequently work. 

Microsoft earlier this year extended its Antimalware Scan Interface (AMSI) for antivirus to combat the rise in macro malware and a recent phenomenon by attackers to utilise outdated Excel 4.0 XLM macros (rather than newer VBA macros) to circumvent anti-malware systems. 

As per Morphisec, the MirrorBlast attack chain is similar to tactics used by TA505, a well-established, financially focused Russia-based cybercriminal group. The group has been active since at least 2014 and is well-known for its usage of a wide range of tools. 

Morphisec researcher Arnold Osipov stated in a blog post, "TA505 is most known for frequently changing the malware they use as well as driving global trends in malware distribution." 

While the MirrorBlast attack begins with a document attached to an email, it afterwards uses a Google feed proxy URL with a SharePoint and OneDrive trap that masquerades as a file-sharing request. When the user clicks the URL, they are sent to a hacked SharePoint site or a bogus OneDrive site. Both versions will take to the malicious Excel document. 

The sample MirrorBlast email demonstrates how the attackers are capitalising on company-issued data on COVID-related modifications to working conditions. Morphisec points out that due to compatibility issues with ActiveX components, the macro code can only be run on a 32-bit version of Office. The macro itself runs a JavaScript script meant to avoid sandboxing by determining if the computer is in administrator mode. The msiexec.exe process is then launched, which downloads and instals an MSI package. 

Morphisec discovered two MIS installation versions that employed legal scripting tools named KiXtart and REBOL. The KiXtart script transmits information about the victim's workstation to the attacker's command and control server, including the domain, computer name, user name, and process list. It then answers with a number indicating whether the Rebol version should be used. Morphisec states that the Rebol script leads to a remote access tool called FlawedGrace, which the group has previously utilised. 

Osipov added, "TA505 is one of many financially motivated threat groups currently active in the marketplace. They are also one of the most creative, as they have a tendency to constantly shift the attacks they leverage to achieve their goals."