External file sharing through Microsoft 365 — via anonymous links or guest user invitations — is a routine and legitimate feature. It is also one of the most common ways sensitive data leaves an organisation without clear audit trails, oversight, or the ability to revoke access after the fact.

Overe detects when files are shared externally using anonymous links or guest user access. Anonymous links in particular present a high risk because they require no authentication — anyone with the link can access the file, and the link may be forwarded, shared, or indexed without the original sharer's knowledge or consent.

The risk is not that external sharing happened, but that it is often done without considering the sensitivity of the content, the intended audience, or how long the link will remain active.

• Sensitive internal document shared via an anonymous link forwarded by the recipient to unauthorised parties, including competitors or the press
• Anonymous link to a financial report, client proposal, pricing schedule, or technical document indexed by a search engine after being embedded in a public-facing page
• Attacker who has compromised a user account uses external sharing to rapidly exfiltrate large volumes of files before the account is locked, bypassing DLP controls that only monitor downloads to unmanaged devices
• Insider shares an entire SharePoint document library or Teams channel files externally to a personal storage service before departing, taking client data, business plans, or source code
• Guest user access to a SharePoint folder remains active long after the original sharing reason is resolved, and the guest's home account is later compromised
• Bulk anonymous link creation used to package and exfiltrate files without triggering volume-based download alerts
• Anonymous link URL embedded in a phishing email to make the link appear legitimate, masking the actual source of the document theft

External sharing is expected and legitimate for client deliverables, vendor collaboration, and public documentation. What makes it acceptable is proportionality — the right files shared with the right people for a defined period.

Anonymous link sharing carries higher risk than authenticated guest sharing because it requires no identity verification. For sensitive files, authenticated sharing with a defined expiry date is preferable. Anonymous links are more acceptable for genuinely public-facing content where the audience is intentionally unrestricted.

Before acting on an external sharing alert:

• Identify what was shared, with whom (anonymous link or specific guest), and when
• Confirm whether the file contains sensitive content — financial data, PII, client information, or technical IP
• Check whether the sharing was intentional and whether the user can identify the intended recipient
• Review whether the link has an expiry date or whether it is set to persist indefinitely
• Check whether the user account shows any signs of compromise around the time of the sharing event
• Review Overe for other alerts tied to the same user — forwarding rules, deletion events, or unusual sign-ins

1. Use Overe to review external sharing events across the tenant, filtered by anonymous links and sensitive content locations
2. For anonymous links to sensitive files, revoke the link immediately if the sharing was not intentional or the content is sensitive
3. For guest user sharing, confirm whether the guest still requires access and set an expiry if not already configured
4. For sharing events from potentially compromised accounts, revoke sessions and investigate the account
5. Review organisational sharing settings — consider disabling anonymous link creation for sensitive document libraries
6. Overe can automatically revoke sessions when a File Shared Externally alert fires — configure auto-response in Org Config > Auto-Response

Microsoft: Manage sharing settings in SharePoint - https://learn.microsoft.com/en-us/sharepoint/turn-external-sharing-on-or-off

Microsoft: Best practices for sharing files with unauthenticated users - https://learn.microsoft.com/en-us/microsoft-365/solutions/best-practices-anonymous-sharing

Microsoft: Manage OneDrive sharing settings - https://learn.microsoft.com/en-us/onedrive/manage-sharing

After investigating an external sharing alert, review these related risk areas:

• Guest Users with Risky Access — anonymous link sharing often transitions to persistent guest access; check whether the sharing event created or expanded guest permissions beyond the original intent
• Inactive Guest Users — recipients who were granted guest access alongside a shared link may have become inactive guests with residual permissions; review and clean up dormant guest identities
• PST Export & eDiscovery Abuse — a user sharing high volumes of files externally may also be using content search or eDiscovery tools; check for export activity from the same account
• Anomalous Delete Sequence — bulk external sharing followed by deletion is a recognised insider threat pattern; check whether files were deleted shortly after being shared
• Power Automate Flow Created — automated flows can be used to share files externally on a schedule; check for flows created by the same account
• Users with Risky MFA Settings — an account actively sharing files externally and using weak or bypassable MFA represents a compound risk if the sharing is attacker-driven