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Microsoft OneNote Starts Blocking Dangerous File Extensions

Microsoft is boosting the security of OneNote users by blocking embedded files with extensions that are considered dangerous.

Microsoft has announced improved protections for OneNote users with automatic blocking of embedded files with extensions that are considered dangerous.

OneNote is an Office suite component typically used by enterprise users for note taking and task management, which also provides multi-user collaboration capabilities, among others.

Just as other Office applications, OneNote has been abused for malware delivery, especially since OneNote documents allow attackers to attach files that would be executed with few warnings to the user.

Specifically, users were informed that opening a OneNote attachment could be harmful, but were provided with the option to dismiss the warning and open the embedded file immediately.

After security researchers warned last year that the Mark-of-the-Web (MOTW) protection was not applied to OneNote documents and their attachments, the abuse of OneNote in malicious campaigns surged.

In response to this threat, Microsoft is making it more difficult for attackers to abuse OneNote for malware delivery, by blocking the opening of embedded files with a dangerous extension.

To open an attachment marked as potentially dangerous, OneNote users would need to save the file to their device, and open it from there, allowing security applications running on the device to detect any malicious code in the attachment.

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According to Microsoft, OneNote will block by default the same extensions that Word, Excel, Outlook, and PowerPoint block. However, Microsoft 365 administrators can set policies to block additional file types or to allow specific file types to be opened.

“Malicious scripts and executables can cause harm if clicked by the user. If extensions are added to this allow list, they can make OneNote and other applications, such as Word and Excel, less secure,” Microsoft warns.

Only impacting OneNote for Microsoft 365 on devices running Windows, the change will begin to roll out in April, and should become visible for all users by January 2024. OneNote in retail versions of Office 2021, Office 2019, and Office 2016 will also be affected.

Related: Microsoft Ups Office Protections With Improved Blocking of Macros

Related: Microsoft Office to Block XLL Add-ins From Internet

Related: Microsoft Publishes Office Symbols to Improve Bug Hunting

Written By

Ionut Arghire is an international correspondent for SecurityWeek.

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