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Upgrade to ProDetect Data Exfiltration via Anonymous SharePoint/OneDrive Sharing Links in Microsoft Sentinel
Rather than uploading data to an external cloud storage account (T1537) or a third-party tool like rclone or Mega, an adversary — or a malicious/negligent insider — can exfiltrate data using Microsoft 365's own file-sharing feature: generating an 'Anyone' (anonymous) or externally-shared link for a SharePoint or OneDrive file or folder, then downloading it from an unmanaged device or forwarding the link outside the organization. Because the traffic never leaves Microsoft's infrastructure and no suspicious process or unusual outbound network connection is generated, this technique evades every process- and network-telemetry-based exfiltration detection (EDR process monitoring, DeviceNetworkEvents, proxy logs). Detection instead depends entirely on the Microsoft 365 Unified Audit Log (Office 365 Management Activity API), which records SharePoint/OneDrive sharing operations such as AnonymousLinkCreated, SharingInvitationCreated, AddedToSecureLink, and the corresponding *LinkUsed events when the link is subsequently accessed.
MITRE ATT&CK
- Tactic
- Exfiltration
KQL Detection Query
// THREAT: Anonymous Sharing Link Exfiltration (Microsoft 365 Unified Audit Log)
// Requires the Office 365 data connector (OfficeActivity table) in Microsoft Sentinel
let SensitiveExt = dynamic([".xlsx", ".docx", ".pdf", ".csv", ".pst", ".zip", ".sql", ".pptx"]);
// Signal 1: burst of anonymous/external link creation by a single user
let LinkCreationBurst = OfficeActivity
| where TimeGenerated > ago(24h)
| where Workload =~ "SharePoint"
| where Operation in ("AnonymousLinkCreated", "SharingInvitationCreated", "AddedToSecureLink")
| summarize LinkCount=count(), Files=make_set(OfficeObjectId, 20), FirstLink=min(TimeGenerated), LastLink=max(TimeGenerated)
by UserId, ClientIP
| where LinkCount >= 10
| extend Signal = "BulkAnonymousLinkCreation";
// Signal 2: anonymous link created for a sensitive file type
let SensitiveFileShare = OfficeActivity
| where TimeGenerated > ago(24h)
| where Workload =~ "SharePoint"
| where Operation =~ "AnonymousLinkCreated"
| where OfficeObjectId has_any (SensitiveExt)
| project TimeGenerated, UserId, ClientIP, OfficeObjectId
| extend Signal = "AnonymousLinkOnSensitiveFile";
// Signal 3: anonymous link accessed/used from an external or unrecognized IP shortly after creation
let LinkUsedExternally = OfficeActivity
| where TimeGenerated > ago(24h)
| where Workload =~ "SharePoint"
| where Operation =~ "AnonymousLinkUsed"
| project TimeGenerated, UserId, ClientIP, OfficeObjectId
| extend Signal = "AnonymousLinkAccessed";
LinkCreationBurst
| project TimeGenerated=LastLink, UserId, ClientIP, LinkCount, Files, Signal
| union (SensitiveFileShare | project TimeGenerated, UserId, ClientIP, LinkCount=1, Files=pack_array(OfficeObjectId), Signal)
| union (LinkUsedExternally | project TimeGenerated, UserId, ClientIP, LinkCount=1, Files=pack_array(OfficeObjectId), Signal)
| sort by TimeGenerated descDetects potential data exfiltration via Microsoft 365 anonymous sharing links using the OfficeActivity table populated by the Office 365 Management Activity API. Signal 1 identifies a single user creating 10 or more anonymous/external sharing links within 24 hours (bulk sharing is atypical for normal collaboration). Signal 2 flags any anonymous link created directly on a sensitive file type (spreadsheets, PST exports, SQL dumps, archives). Signal 3 flags subsequent use of an anonymous link, which confirms the shared content was actually retrieved. All three signals are audit-log based — there is no corresponding process or network telemetry because the traffic stays within Microsoft's SaaS boundary.
Data Sources
Required Tables
False Positives & Tuning
- External collaboration teams (sales, partnerships, legal) that routinely share proposals or contracts via anonymous links as part of normal business process
- Marketing or communications staff distributing public-facing assets (brochures, press kits) via SharePoint anonymous links
- Automated workflows (Power Automate flows) that generate sharing links as part of an approved business process
Other platforms for THREAT-Exfiltration-M365AnonymousShareLinks
Testing Methodology
Validate this detection against 2 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Create Anonymous Sharing Link via PnP PowerShell
Expected signal: Office 365 Unified Audit Log: Operation=AnonymousLinkCreated, Workload=SharePoint, RecordType=SharePointSharingOperation, with the UserId and ObjectId of the test file.
- Test 2Bulk Anonymous Link Creation Simulation
Expected signal: 12 AnonymousLinkCreated audit events for the same UserId within a short time window.
References (4)
- https://attack.mitre.org/techniques/T1567/002/
- https://learn.microsoft.com/en-us/microsoft-365/compliance/detailed-properties-in-the-office-365-audit-log
- https://learn.microsoft.com/en-us/sharepoint/turn-external-sharing-on-or-off
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1567.002/T1567.002.md
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Related Detections
Tactic Hub
Detection Variants (1)
Different telemetry and tradecraft for the same technique — pick the one that matches the data you collect.