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Ransomware can reach SharePoint Online and OneDrive when malware changes files on a user’s computer and those changes are carried into the cloud through synchronization or a mapped library connection. Microsoft documents this path, but it does not mean every Microsoft 365 ransomware incident begins or spreads this way. If you suspect it is happening, immediately stop OneDrive sync or disconnect the mapped SharePoint library drive.
How ransomware reaches SharePoint and OneDrive
In the documented scenario, the malware runs on a user’s computer. It changes files that the user can access through a mapped SharePoint library or a OneDrive connection. The sync client or WebDAV methods can then carry those changes to the online service. The cloud copy can therefore be affected by activity that began on an endpoint; SharePoint Online is not the malware’s execution environment. Microsoft’s SharePoint Online ransomware guidance describes this mechanism.
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Changes may include encrypting files, appending extensions to filenames, deleting content, or adding files with ransom instructions. Synchronization can propagate harmful edits and deletions, which is why interrupting the connection is the immediate response for this pattern—not simply waiting for sync to finish.
Warning signs in a SharePoint library
Microsoft lists these possible indicators:
- Many files show the same Modified By timestamp.
- Files will not open or appear corrupted.
- Ransom-note files appear in directories.
- Files have been renamed or have unexpected extensions appended.
These signs justify prompt investigation, but none alone proves ransomware, identifies the source, or establishes how much content is affected. Determine the scope before choosing a restoration point.
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What to do first
- Interrupt propagation. Immediately stop OneDrive sync or disconnect the mapped drive to the SharePoint library, as Microsoft instructs in its incident guidance. If you administer the tenant, coordinate with affected users so they do not resume synchronization while the incident is being assessed.
- Assess affected content and accounts. Identify which users, libraries, and files show changes, and distinguish encrypted or overwritten files from deleted items. The appropriate recovery path depends on the event and the restoration scope needed.
- Choose a recovery method and point. Check which versions, recycle-bin items, restore points, or backup capabilities are actually available in your tenant before restoring. Follow Microsoft’s current product instructions for the affected workload.
Which Microsoft 365 recovery option fits?
These options address different failure modes and scopes; availability depends on tenant configuration, retention, and, for some services, licensing and current service terms.
| Option | Best suited to | Scope and recovery point | Important limits |
|---|---|---|---|
| Version history | An individual file changed in place, including an encrypted version. | View, compare, and restore an earlier version of a file. Microsoft explicitly names ransomware as a use case. See About version history and Microsoft’s cloud data security measures. | Available versions depend on retention and configuration; it does not by itself restore an entire library to a coordinated earlier state. |
| Recycle bins | Deleted files or folders. | Recover deleted content from SharePoint recycle bins. Microsoft Service Assurance describes 93 days of retention across the recycle-bin flow; the cited page does not state a publication year. See SharePoint and OneDrive data resiliency in Microsoft 365. | Applies to deleted content, not simply an encrypted file that remains in place. Confirm current tenant behavior and service details. |
| Files Restore | Broad unwanted changes, such as a large number of altered or deleted files. | Microsoft describes restoring OneDrive or SharePoint content to an earlier point in time. The Service Assurance page describes SharePoint restoration to any second during the previous 30 days; the cited page does not state a publication year. | It relies on file versions, so reducing version retention can reduce restore effectiveness. Verify current product scope and limits for your tenant. |
| Microsoft 365 Backup | Bulk recovery after ransomware or accidental or malicious overwrite or deletion. | Microsoft describes self-service bulk recovery as a service option. See Deploy ransomware protection for your Microsoft 365 tenant. | The cited documentation is a previous-versions resource. Check current configuration, licensing, availability, and service terms before relying on it. |
| Microsoft support | Some cases where content has been removed from the site collection recycle bin. | Microsoft’s ransomware guidance, updated July 14, 2025, describes contacting support within 14 days after the site collection recycle-bin deletion window in the circumstances it covers. | This is not a guaranteed recovery path or a substitute for customer-controlled backups. |
How to choose a restore point
First identify when the unwanted changes began and which files were affected. Restoring too far back can discard legitimate work; restoring too close to the incident may leave harmful changes in place. For individual in-place edits, inspect version history. For deletions, check the recycle bins. For broad changes across content, assess Files Restore or an available backup service with an administrator. Verify that synchronization remains stopped until the affected endpoint and restore plan are addressed.
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