Microsoft Is Now Blocking 120 Extensions by Default in Office 365 Products

Microsoft is finally dealing with a major security issue, and has published a list of attachment extensions that will be blocked when sending or receiving certain types of files.

One of the biggest security problems in the online world is caused by people opening emails attachment they shouldn’t. One way to deal with this is to train people to do the right thing, but that doesn’t always produce the right outcome.

So, the next solution would be to force the measure via software. And that’s how Microsoft has been cracking down on these security issues. For example, the company is now blocking macros downloaded from untrusted sources. Blocking attachments in Outlook and other Office 365 products such as OneNote will likely increase security because it takes the decision out of users’ hands.

“One of the most common ways of transmitting computer viruses is through file attachments,” explains Microsoft. “To help protect you and your recipients against computer viruses, Outlook blocks the sending and receiving of certain types of files (such as .exe and certain database files) as attachments.”

A total of 120 file extensions are now blocked by Microsoft. Some of them are easily recognizable, like .EXE or .CMD. But there’s also .ISO, which is often used by attackers to hide the real attachment from security solutions. Of course, .VB is also among the blocked types of files, but that’s to be expected for any Visual Basic source. Malware campaigns using Microsoft OneNote Files are a key reason for this push.

Users who need to send these types of files will have to resort to a couple of alternatives, as instructed by Microsoft. They can archive the attachment, upload it to the cloud and attach the only the link, or simply rename the extension to something that’s not blocked by default.

Organizations that run their own Microsoft Exchange Server will be able to customize these security settings. According to a BleepingComputer report, the updates will arrive starting with the 2304 version of OneNote and Office 2021, Office 2019, and Office 2016 by the end of May 2023.