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Did you know that the default “copy link” option in O365 personal accounts generates a public shared link with edit permissions? In this edition, we will cover how link sharing in O365 can lead to the accidental internal and public exposure of sensitive data.
Netskope Threat Labs have shared a series of posts highlighting the exposure concerns in Google Calendar, Google Groups, Google link sharing, and Zendesk. The exposure concerns we highlight in this post about Office 365 are similar to those in our Google link sharing post, as both of these apps offer similar sharing settings to the user and security settings to the administrator. This blog post details how accidental data exposure happens in Office 365 and details the steps you can take to limit or disable external sharing.
O365 link sharing
Similar to Google, Microsoft also provides link sharing for a wide range of O365 apps, including OneDrive, Word, Excel, PowerPoint, OneNote, Calendar, and Sway. In each app, users can choose between view or edit permissions, and grant those permissions to “Anyone with the link,” “Specific people,” or “Only people in your organization” (only for users in Business accounts) as shown in Figure 1.
Unlike Google, O365 users are provided with an option to set a password for the link sharing as shown in Figure 2.
In O365 Business accounts, users are provided with an additional set of link sharing settings as shown in Figure 3.
The link sharing settings have the following options:
- Anyone with the link → for sharing a link that allows everyone to access.
- People in [your organization] with the link → for sharing a link that everyone within your organization can access.
- People with existing access → for sharing with users who already have access granted explicitly to them
- Specific people → for sharing with specific authenticated users either inside or outside your organization
Accidental exposure
Link sharing can lead to accidental exposure when a user sets inappropriately broad permissions when sharing a document. For example, an O365 personal account user might use the “Copy link” option to share a document with a friend. In this case, it would have been more secure to share the file explicitly with only that friend. If the link gets forwarded to someone else or shared publicly, anyone with the link can edit the document. The workflow that has the default permission allowing anyone with the link to edit, as shown in Figure 4.