If you're seeing this message it means the remote server that hosts the file you're trying to open is using an unsecure sign-in method known as "Basic Authentication". Basic Authentication is one of the primary ways that criminals try to break into servers through password attacks.
Microsoft encourages companies to move to more secure authentication methods and will soon block basic Authentication by default for your safety.
Notes:
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Depending on the version of Office you are using, you might see a warning about potentially unsafe website rather than blocking the site altogether.
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If you're an IT Pro looking for more information on this see Basic authentication sign-in prompts are blocked by default in Microsoft 365 apps.
What can you do to access the file?
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If you manage the server that hosts this file, you may be able to upgrade the server from Basic Authentication to Modern Authentication.
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If you're not the server manager, you'll need to contact the person who owns or manages the server and request they upgrade the server to Modern Authentication.
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If you own the file you're trying to access you should be able to move it to a more secure location, such as OneDrive or SharePoint. If you don't own the file you may be able to ask the file owner to do so.
Note: If none of those steps restore your access to the file, please contact your IT support.
