Mark new files as sensitive by default
When new files are added to SharePoint or OneDrive in Microsoft 365, it takes a while for them to be crawled and indexed. It takes additional time for the Office Data Loss Prevention (DLP) policy to scan the content and apply rules to help protect sensitive content. If external sharing is turned on, sensitive content could be shared and accessed by guests before the Office DLP rule finishes processing.
Instead of turning off external sharing entirely, you can address this issue by using a PowerShell cmdlet to block external access to new content unless it's explicitly authorized in a DLP rule and it has been verified that there's no sensitive content that goes against the policy rules. The setting enabled by this cmdlet prevents external users from accessing newly added files until at least one Office DLP policy scans the content and determines that the document doesn't contain any sensitive information that's against the rules defined in the policy. If the file has been indexed and scanned and it has no sensitive content that's against the rules in the DLP policy, then guests can access the file. If the policy identifies sensitive content in the document, or if there's no DLP rule explicitly authorizing access to the file, then guests will not be able to access the file, and they will receive the following access denied error message: "This file is being scanned right now. Please try again in a few minutes. If you still don't have access, contact the file owner."
This cmdlet applies to newly added files in all SharePoint sites and OneDrive accounts. It doesn't block sharing if an existing file is changed.
Since once this setting is enabled any content not explicitly checked in a DLP policy will be blocked from being externally accessed, you must ensure any content that needs to be shared externally is covered by at least one DLP policy. If you prefer to require explicit authorization in a DLP policy for a site to be shareable, no further action is needed after this setting is enabled. If not all locations with content that needs to be shared externally are already covered by an existing DLP policy, you must add them to at least one policy. The easiest way to do this is to create a DLP policy that includes all locations, that has any “content contains” condition selected, and that specifies no actions, no alerts, no notifications and no reports. Also, make sure the rule doesn’t use the option to stop processing additional DLP rules. Learn how to create and turn on a DLP policy
Unless you want to block access to all locations by default and only allow locations authorized in a DLP polcy, the DLP policy must include all SharePoint sites and OneDrive accounts and exclude none.
If you installed a previous version of the SharePoint Online Management Shell, go to Add or remove programs and uninstall "SharePoint Online Management Shell."
Connect to SharePoint as a global admin or SharePoint admin in Microsoft 365. To learn how, see Getting started with SharePoint Online Management Shell.
Run the following command:
Set-SPOTenant -MarkNewFilesSensitiveByDefault BlockExternalSharing
To disable this feature, run the following command:
Set-SPOTenant -MarkNewFilesSensitiveByDefault AllowExternalSharing