An actively exploited zero-day flaw that prevents Windows from properly identifying potentially malicious documents by flagging them as downloaded from the web has been given an unofficial patch. The security issue was first brought to Microsoft’s attention by a security researcher 3 month ago, however no official update to correct this has been released. Security patching company 0patch have released a free unofficial patch for this zero-day flaw, which can be used to mitigate the risk of this vulnerability until an official Windows patch becomes available.
The flaw affects a Windows security feature that is designed to flag documents and executables downloaded from untrusted sources on the web. Windows Mark of the Web (MotW) label is used to trigger the display of warnings about the file’s source to the user, and is used by Windows, Microsoft Office, web browsers, and other apps to warn them to treat the file with caution. It is used by Smart App Control to identify potentially malicious executables and warn users before they proceed with opening untrusted apps, and a Microsoft Office file with a MotW flag also has its macros disable by default.
Windows MotW should flag all files, including extracted ZIP archive files, using an alternative data stream Zone.Id, which is used to provide extra information about the file without modifying the contents of the file. However, due to the zero-day flaw that is currently unpatched, MotW is not being applied automatically to any files extracted from a ZIP archive. This means that malicious files extracted from a ZIP archive are not blocked by Smart App Control, and do not have their macros disabled. Attackers are known to be actively exploiting this vulnerability to deliver malware via ZIP archived files.
The unofficial patch works to correct the labelling of these files extracted from ZIP archives so that Windows correctly identifies them with MotW. As a result, security software and mechanisms that rely on searching for the MotW label can correctly warn and protect users about these documents. The micropatch is available for Windows 10 v1803 and later, Windows 7 with or without ESU, Windows Server 2012, 2012 R2, 2016, 2019, and 2022, and Windows Server 2008 R2 with or without ESU. An 0patch account is needed in order to download and apply this free patch, which is the best mitigation of an exploit for this flaw until an official Windows patch is released.
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