As per the sources, a harmless Microsoft OneNote file has become a popular file format for malware-distributing phishing attacks for hackers to spread malware and breach corporate networks. Initially, threat actors have been abusing macros in Microsoft Word and Excel documents to spread malware on Windows devices. But, after Microsoft disabled macros by default in Word and Excel Office documents in July 2022, threat actors began turning to ISO files and password-protected ZIP archives, as the Windows and 7-Zip bugs allow these file formats to bypass Mark-of-the-Web (MoTW) security warnings. However, after both 7-ZIP and Windows fixed these bugs, threat actors began finding another file format to use in attacks, which led to Microsoft OneNote attachments from mid-December 2022. Microsoft OneNote attachments that use the '.one' file extension did not distribute malware through macros or vulnerabilities, but threat actors create the intricate templates that appear to be a protected document with a message to a 'double-click' button to view the file. Threat actors hide a series of embedded files under a 'double-click' button layer, so when victims click that for viewing a file it will lead to launching the hidden files. While double-clicking an embedded file will always display a security warning, the users commonly ignore the security warnings. Furthermore, to prevent malicious Microsoft OneNote attachments from infecting Windows, it is recommended to block the '.one' file extension at your secure mail gateways or mail servers. But in some cases, if it is not possible, it is recommended to use Microsoft Office group policies to restrict the launching of embedded file attachments in Microsoft OneNote files, by enabling Microsoft OneNote policies named 'Disable embedded files' and 'Embedded Files Blocked Extensions'. The 'Disable embedded files' group policy is the most restrictive as it prevents all embedded OneNote files from being launched, which can be enabled if there is no use case for using embedded OneNote attachments. The 'Embedded Files Blocked' group policy is a less restrictive option, which allows users to input a list of embedded file extensions that will be blocked from opening in a Microsoft OneNote document. Other than the '.one' file extension, it is also recommended to block .js, .exe, .com, .cmd, .scr, .ps1, .vbs, and .lnk file extensions, but this list may be bypassed by other malicious file types as the threat actors discover new file extensions to abuse.
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