Microsoft’s announcement that it would block macros in Microsoft Office apps by default didn’t stop threat actors—they have simply resorted to new tricks.

“Threat actors across the landscape responded by shifting away from macro-based threats,” Proofpoint researchers noted in a blog post. In fact, an analysis of campaign data, “which include threats manually analyzed and contextualized,” showed the use of VBA and XL4 Macros ticked down 66% or so between October 2021 and June 2022.

“While Proofpoint observed a notable increase in other attachment types, macro-enabled documents are still used across the threat landscape,” the researchers wrote, explaining that the tactics, techniques and procedures (TTPs) have changed, with miscreants turning to use of container files—like ISO and RAR—and Windows Shortcut files to pass malware along, according to Proofpoint research.

Threat actors have long used VBA macros “to automatically run malicious content when a user has actively enabled macros in Office applications. XL4 macros are specific to the Excel application, but can also be weaponized by threat actors,” researchers pointed out. “Typically, threat actors distributing macro-enabled documents rely on social engineering to convince a recipient the content is important, and enabling macros is necessary to view it.”

Microsoft took steps to block VBA macros by keying on a Mark of the Web (MOTW) attribute called the Zone.Identifier that shows whether a file comes from the internet and is added by Microsoft apps to some documents downloaded from the web. But bad actors can bypass MOTW by using container file formats.

By using container file formats like ISO (.iso), RAR (.rar), ZIP (.zip) and IMG (.img) files to send macro-enabled documents, “ … the ISO, RAR, etc. files will have the MOTW attribute because they were downloaded from the internet, but the document inside, such as a macro-enabled spreadsheet, will not,” researchers noted. “When the document is extracted, the user will still have to enable macros for the malicious code to automatically execute, but the file system will not identify the document as coming from the web.”

They also can distribute payloads directly using container files so that when they’re opened they can contain “additional content such as LNKs, DLLs or executable (.exe) files that lead to the installation of a malicious payload.”

“The change to block macros by default is a very good thing; has been suggested for years and it’s good Microsoft is finally doing it,” said Rob Jenks, SVP strategy and business at Tanium. He explained that “as with all security techniques, it’s not a silver bullet and attackers inevitably move on to the next attack pathway(s)—so the findings aren’t surprising.”

But “regarding the new attacks, there are other restrictions on not trusting zip content, so these other mechanisms throw more consent dialogs into the user’s face, potentially making a phishing attack less reliable,” Jenks said.

Proofpoint researchers have not only noted a two-thirds decrease in macro-enabled documents leveraged as attachments in email-based threats, but they observed “the number of campaigns leveraging container files including ISO and RAR, and Windows Shortcut (LNK) attachments increased nearly 175%,” researchers said.

“They attribute the increase in part to the uptick in use of ISO and LNK files in campaigns. Cybercriminal threat actors are increasingly adopting these as initial access mechanisms, such as actors distributing Bumblebee malware,” they said. “The use of ISO files increased over 150% between October 2021 and June 2022. More than half of the 15 tracked threat actors that used ISO files in this time began using them in campaigns after January 2022.”

Most notably, LNK files have emerged as a go-to for threat actors—at least 10 of them have begun using LNK files since February.  In fact, the number of campaigns containing LNK files exploded an incredible 1,675% since October 2021.

While fewer campaigns are using XL4 macros, Proofpoint did see a spike in macro use in March 2022, which researchers attributed to an uptick in campaigns with higher volumes of messages conducted by the TA542 actor delivering Emotet. “Typically, TA542 uses Microsoft Excel or Word documents containing VBA or XL4 macros,” the researcher wrote. “Emotet activity subsequently dropped off in April and it began using additional delivery methods including Excel Add-In (XLL) files and zipped LNK attachments in subsequent campaigns.”

The adoption of ISO and other container file formats is driving the pivot away from macro-enabled documents to different file types that can bypass the macro-blocking protections offered by Microsoft. “Such filetypes can bypass Microsoft’s macro blocking protections, as well as facilitate the distribution of executables that can lead to follow-on malware, data reconnaissance and theft and ransomware,” said Proofpoint researchers, who called the change “one of the largest email threat landscape shifts in recent history.”

Proofpoint has also observed a slight increase in threat actors using HTML attachments to deliver malware. The number of malware campaigns using HTML attachments more than doubled from October 2021 to June 2022, but the overall number remains low. Proofpoint researchers also observed threat actors increasingly adopt HTML smuggling, a technique used to “smuggle” an encoded malicious file within a specially crafted HTML attachment or web page.

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