Microsoft Blacklists Dozens of New File Extensions in Outlook

Microsoft Outlook now blocks 142 file extensions that it deems as at risk or that are typically sent as malicious attachments in emails. 40 of these extensions have just been added.

Microsoft is banning almost 40 new types of file extensions on its Outlook email platform. The aim is to protect email users from what it deems “at-risk” file attachments, which are typically sent with malicious scripts or executables.

The newly blocked file types are rarely used, so most organizations will not be affected by the change. However, if your users are sending and receiving affected attachments, they will report that they are no longer able to download them.

If you want a particular file type to be allowed, you can add that file type to the AllowedFileTypes property of your users’ OwaMailboxPolicy objects.

What file extensions will be added to the BlockedFileTypes list with this change?

The following extensions are used by the Python scripting language:

“.py”, “.pyc”, “.pyo”, “.pyw”, “.pyz”, “.pyzw”

The following extensions are used by the PowerShell scripting language:

“.ps1”, “.ps1xml”, “.ps2”, “.ps2xml”, “.psc1”, “.psc2”, “.psd1”, “.psdm1”, “.cdxml”, “.pssc”

The following extension is used by Windows ClickOnce

“.appref-ms”

The following extension is used by Microsoft Data Access Components (MDAC)

“.udl”

The following extension is used by the Windows sandbox

“.wsb”

The following extensions are used for digital certificates:

“.cer”, “.crt”, “.der”

The following extensions are used by the Java programming language:

“.jar”, “.jnlp”

The following extensions are used by various applications. While the associated vulnerabilities have been patched (for years, in most cases), they are being blocked for the benefit of organizations that might still have older versions of the application software in use:

“.appcontent-ms”, “.settingcontent-ms”, “.cnt”, “.hpj”, “.website”, “.webpnp”, “.mcf”, “.printerexport”, “.pl”, “.theme”, “.vbp”, “.xbap”, “.xll”, “.xnk”, “.msu”, “.diagcab”, “.grp”

Duncan Newell

Duncan is a technology professional with over 20 years experience of working in various IT roles. He has a interest in cyber security, and has a wide range of other skills in radio, electronics and telecommunications.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: