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When sharing a document the sharing box look like this:
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The user is able to change the way the document is shared:
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When sharing in this manner, the inheritance will break.
You can prevent users with members permissions to break inheritance by doing the following:
Go to Site Settings à -> Site Permissions à -> Access Request Settings
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By removing the checkmark for “Allow members to share the site and individual files and folders” ordinary users will be prevented from breaking inheritance anywhere on the site.
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The sharing box will look like this:
If the user tries to manually add an email address of an external user the following error page appears:
Changing default settings for sharing
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In SharePoint Admin Center chose Sites à -> Active sites à -> The site to change à -> Settings à More sharing settings
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Default sharing link type is usually set to “Anyone with the link”.
Remove the checkmark and choose “People with existing access” and Save.
With this setting the inheritance will not break as the sharing will be with users that already have access and no change in permissions is necessary.