Microsoft Purview Information Protection began rolling out AES256-CBC encryption in late August 2023, and it became the default for Microsoft 365 Apps documents and email by October 2023. Whether an organization must act depends chiefly on whether it uses Exchange Online or Exchange Server/hybrid: Microsoft 365 Apps with Exchange Server or hybrid requires remediation because Exchange Server cannot decrypt content encrypted with AES256-CBC.
What changed in Microsoft Purview Information Protection?
Purview Information Protection moved to AES256-CBC: Advanced Encryption Standard with a 256-bit key in Cipher Block Chaining mode. The rollout began in late August 2023, and AES256-CBC became the default encryption mode for Microsoft 365 Apps documents and email by October 2023. Microsoft listed the 256-bit key length in its 2023 documentation. Microsoft Purview encryption update
Microsoft Office release notes confirmed the feature for Excel, Outlook, PowerPoint, and Word in Version 2309 in the November 14 release notes. Microsoft 365 Apps release notes
This is an encryption-mode compatibility change, not a published performance comparison: Microsoft’s reviewed material provides no benchmark, incident count, or adoption percentage comparing AES256-CBC with AES128-ECB.
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Which deployments need administrator action?
Microsoft’s action guidance distinguishes Exchange Online from Exchange Server or hybrid deployments, and also accounts for the Office client and SDK configuration.
| Client or integration | Exchange Online / SharePoint Online | Exchange Server or hybrid | SharePoint Server |
|---|---|---|---|
| Microsoft 365 Apps | No action required | Action required | No action required |
| Office 2013, 2016, 2019, or 2021 | Optional: review CBC configuration | Action required | No action required |
| Microsoft 365 Apps with MIP SDK | Optional: review SDK support | Action depends on Exchange deployment; Exchange Server compatibility applies | No action required |
These are Microsoft’s stated categories; confirm the actual mail and content-protection paths in a mixed or hybrid estate before applying a tenant-wide policy. Microsoft deployment guidance
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Why Exchange Server and hybrid environments must plan a change
Microsoft’s compatibility statement is explicit: “Exchange Server doesn’t support decrypting content that uses AES256-CBC.” An on-premises Exchange Server or hybrid environment therefore cannot simply rely on the new default when protected content may need to be decrypted there. Microsoft Exchange compatibility guidance
For Exchange Server or hybrid estates, Microsoft’s remediation path includes installing the Exchange hotfix and, where the Azure Rights Management Connector is used, running GenConnectorConfig.ps1. Administrators then open a Microsoft support case to enable AES256-CBC publishing. Microsoft identifies forcing AES128-ECB through the same IRM policy as a temporary fallback while remediation is completed; it is not the target configuration for a completed migration. Microsoft remediation steps
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How to set the IRM encryption mode with Group Policy or Cloud Policy
The policy to configure is Encryption mode for Information Rights Management (IRM). Its location is User Configuration/Administrative Templates/Microsoft Office 2016/Security Settings. Microsoft says that starting with Microsoft 365 Apps version 16.0.16227, CBC is used by default. The documented CBC policy value is [1, Cipher Block Chaining (CBC)]. Microsoft IRM policy documentation
- Open the applicable Group Policy management tool or Microsoft 365 Cloud Policy configuration for the Microsoft 365 Apps users in scope.
- Navigate to
User Configuration/Administrative Templates/Microsoft Office 2016/Security Settings. - Set Encryption mode for Information Rights Management (IRM) to the required mode. For CBC, use the documented value
[1, Cipher Block Chaining (CBC)]. - For Exchange Server or hybrid environments, use AES128-ECB only as a temporary fallback while completing Microsoft’s hotfix, connector, and service-enablement steps; coordinate the final CBC change with the support case.
Microsoft’s supplied guidance identifies the AES256-CBC value, but does not provide an exact policy value for AES128-ECB in the cited details; do not infer or substitute a value without using the corresponding current Microsoft policy documentation.
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What MIP SDK developers need to update
Applications integrating the Microsoft Information Protection SDK should update to MIP SDK version 1.13 or later. Microsoft says SDK 1.13 requires a setting to force AES256-CBC, while later SDK versions protect Microsoft 365 files and email with AES256-CBC by default. Review the application’s SDK version and configuration rather than assuming the desktop Office default also governs an SDK integration. MIP SDK release information
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Practical decision path
- Exchange Online with Microsoft 365 Apps: Microsoft lists no action required for the encryption change.
- Exchange Online with Office 2013, 2016, 2019, or 2021: review the CBC policy; action is optional under Microsoft’s guidance.
- Exchange Server or hybrid: treat this as an administrator remediation. Exchange Server cannot decrypt AES256-CBC content; follow the hotfix and service-enablement path, using AES128-ECB only as a temporary fallback if needed.
- MIP SDK integration: verify SDK version 1.13 or later and configure CBC as required by that version.
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