Attachment Blocking in Office 365
Office 365 and Microsoft Outlook include a security feature that blocks attachments that might put your computer at risk. Attachments that are known to be unsafe are added to a block list.
When receiving a message in Outlook with an attachment that is on the block list, the attachment will be stripped from the message and the user will see the message: “Outlook blocked access to the following potentially unsafe attachments: <list of file blocked>"
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When sending a message with an attachment type on the block list, Outlook will warn the user that the attachment may not be received by Outlook users.
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When sending or receiving a message in Office 365 with an attachment that is on the block list, a red icon is placed over the attachment and it cannot be previewed or downloaded.
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Blocked Attachments:
| Attachment Type | Description of Extension Use |
|---|---|
| ".py", ".pyc", ".pyo", ".pyw", ".pyz", ".pyzw" | Python scripting language |
| ".ps1", ".ps1xml", ".ps2", ".ps2xml", ".psc1", ".psc2", ".psd1", ".psdm1", “.cdxml”, “.pssc” | PowerShell scripting language |
| “.appref-ms” | Windows ClickOnce |
| “.udl” | Microsoft Data Access Components (MDAC) |
| “.wsb” | Windows Sandbox |
| ".cer", ".crt", ".der" | Digital Certificates |
| ".jar", ".jnlp" | Java programming language |
|
".appcontent-ms", ".settingcontent-ms", ".cnt", ".hpj", ".website", ".webpnp", ".mcf", ".printerexport", ".pl", ".theme", ".vbp", ".xbap", ".xll", ".xnk", ".msu", ".diagcab", ".grp" ".src" |
Various Applications, commonly vulnerable to exploits |
If you need to send an attachment that is on the block list, there are a number of options to get the file to the destination. Some of these include, using file compression (e.g. zip) to change the file type, or sharing the file through a file share or other means. To see the complete list with instructions, review the following Microsoft help document.
Last Updated: October 16, 2019