Microsoft announced the expansion of blocked attachments list in Outlook Web and new Outlook for Windows, adding .library-ms and .search-ms files to the potentially dangerous file types list starting July 2025.
G. Ostrov
Microsoft is implementing new measures to enhance email security by expanding the list of blocked attachments in Outlook Web and new Outlook for Windows. Starting July 2025, the services will automatically block .library-ms and .search-ms file types.
Reasons for New Restrictions
The company's decision is based on analysis of modern cybersecurity threats. Windows library files (.library-ms), which define virtual collections of folders and files in the Windows file system, have been actively exploited by cybercriminals in phishing attacks targeting government agencies and commercial organizations.
Particularly dangerous was the exploitation of Windows vulnerability CVE-2025-24054, which allowed disclosure of NTLM hashes through malicious .library-ms files. The .search-ms URI protocol handler also became a cybercriminal tool since June 2022, enabling automatic launching of Windows search windows and tricking users into running malware.
Impact on Users
Microsoft states that the new blocked file types are rarely used in daily work, so most organizations will not be affected by this change. The update will be automatically applied to all OWA mailbox policies in organizations.
Corporate users who need to work with these file types can contact Exchange Server administrators to configure security settings. Alternative solutions remain the use of archives, changing file extensions, or transferring via OneDrive and SharePoint.
Development of Outlook Security System
This update is part of Microsoft's comprehensive user protection strategy. Since 2018, the company has expanded Antimalware Scan Interface (AMSI) support for Office 365 client applications, started blocking VBA Office macros by default, and disabled Excel 4.0 macros.
In April 2025, Microsoft also began disabling ActiveX controls in Microsoft 365 and Office 2024 versions for Windows, including Word, Excel, PowerPoint, and Visio.
Recommendations for Users
Administrators are recommended to review the complete list of blocked attachments in Microsoft's official documentation and make necessary changes to security policies before the July 2025 update deployment.
More detailed information can be found on Microsoft's official support website.
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