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

The campaign, called MirrorBlast, was discovered in early September by security firm ET Labs. Another security firm, Morphisec, analyzed the malware and says the malicious Excel files are able to bypass malware detection systems because they contain “extremely lightweight” embedded macros, making them “particularly dangerous” for organizations that rely on detection-based security and sandboxing.

Macros have been used by government-sponsored hackers because they often work. Microsoft recently expanded its Antimalware Scan Interface (AMSI) for antivirus to counter the rise of macro malware, and a new trend by attackers to use legacy Excel 4.0 XLM macros (instead of newer VBA macros) to bypass anti-malware systems.

According to Morphisec, the chain of attack in MirrorBlast reflects the techniques of an established, financially motivated Russian cybercrime gang, known by researchers as TA505, which has been active since 2014 and known for variations in the tools they use.

Morphisec said that for compatibility with ActiveX objects, the macro code can only be executed on a 32-bit version of Office.