Microsoft Will Block Excel XLL Add-ins from the Internet

Starting in March, Microsoft will block XLL files coming from the Internet in Office Excel. In its Microsoft 365 roadmap, Microsoft writes that it is making this change “to combat the increasing number of malware attacks in recent months.”

Note

• Nice to see Microsoft continuing its quest to block common malware delivery methods. XLL files are not seen as frequently as some of the other malware delivery vehicles, but they are gaining more prominence after “classic” macros became less effective.
• XLL files are dynamic link libraries (add-ins) that extend the functionality of Excel. As such these should only come from trusted locations. While users are warned before an XLL is activated, too often they just click Enable and move on. Another topic to cover in your user training, even after the block is rolled out later this year.
• In the physical world, there are many additives that have some advantages (red dye #2 in food, lead in gasoline, DDT in pesticides) that provide some clear usage advantages but overall are found to be too dangerous to use. We are going through that process in software, but software makes it easier for more new dangerous “additives” to be put in use. Ideally, software is also easier to test for danger – let’s hope Microsoft, Google, etc. will not just remove old bad additives, but also release many fewer dangerous ones in the future.
• Microsoft Excel is such a powerful toolset. The fact that it can load its sort of DLL payload is unsurprising when you search for videos on Excel Programming. It is, however, interesting to see that as Microsoft Harden Excel, the attackers will move into other Office products (such as OneNote), Teams, and maybe even non-office products like PowerTools.
• And so continues the ‘cat and mouse’ game between attacker and defender. Last year, MSFT began blocking VBA macros by default. With that change the attacker shifted their tactics, tools, and procedures moving to an increase in the use of XLL files for payload delivery. By blocking these macros and extensions by default, MSFT is doing two things: 1) reducing the attack surface; and, 2) forcing a cost on the adversary to attack organizations.

Read more in

• Microsoft 365 roadmap > Excel: Blocking XLL add-ins from the internet.
• Extend custom functions with XLL user-defined functions
• Threat Spotlight: XLLing in Excel – threat actors using malicious add-ins