💡 Our Technical Review in summary
Summary
Group owners were unable to modify group memberships through the Global Address Book (GAB) when using the classic Outlook desktop client. The issue was triggered by a recent feature update intended to enable a new API for managing distribution group modifications, which inadvertently introduced a bug in the legacy interface.
Impact
This issue primarily affected Security Groups; Distribution Groups remained largely functional. Affected group owners were unable to add or remove members via the classic desktop interface. Users were still able to perform these actions using the “New Outlook” desktop client or Outlook on the web during the incident period.
Status
Service Restored. Microsoft has reverted the problematic feature change and confirmed that group management functionality is fully remediated. The incident lasted approximately 17 hours on February 3, 2026.
Microsoft Official Issue Details
User Impact
Group owners were unable to modify group memberships in the Global Address Book in the classic Outlook desktop client.
Status: serviceRestored
Service: Exchange Online
Classification: advisory
Origin: microsoft
Updates
- 2026-02-03 18:29
Title: Group owners may be unable to modify group memberships in the Global Address Book in the classic Outlook desktop clientUser impact: Group owners were unable to modify group memberships in the Global Address Book in the classic Outlook desktop client.
More info: Distribution Groups may have been able to be modified, but Security Groups may have been affected.
While we worked towards mitigation, affected users may have been able to utilize the new Outlook desktop client or Outlook on the web to perform Distribution Group modifications.
Final status: We’ve identified that a recent feature change intended to enable the use of a new API for managing distribution group modifications introduced an issue resulting in impact. We’ve reverted the affecting change and confirmed that impact was successfully remediated.
Scope of impact: This issue may have affected all group owners modifying certain group memberships through the Outlook Address Book in the classis Outlook desktop client.
Start time: Tuesday, February 3, 2026, at 12:15 AM UTC
End time: Tuesday, February 3, 2026, at 5:29 PM UTC
Root cause: A recent feature change intended to enable the use of a new API for managing distribution group modifications introduced an issue resulting in impact.
Next steps:
– We’re reviewing the affecting change to understand why it resulted in impact to better prevent similar future occurrences.This is the final update for the event.
- 2026-02-03 17:00
Title: Group owners may be unable to modify group memberships in the Global Address Book in the classic Outlook desktop clientUser impact: Group owners are unable to modify group memberships in the Global Address Book in the classic Outlook desktop client.
More info: Distribution Groups may be modified, but Security Groups may be affected.
While we work towards mitigation, affected users may be able to utilize the new Outlook desktop client or Outlook on the web to perform Distribution Group modifications.
Current status: We’re reviewing Problem Steps Recorder (PSR) logs in parallel with Fiddler trace logs collected from an affected user to identify and isolate any errors which may be contributing to impact for further investigation.
Scope of impact: This issue may affect all group owners modifying certain group memberships through the Outlook Address Book in the classis Outlook desktop client.
Next update by: Tuesday, February 3, 2026, at 6:30 PM UTC
- 2026-02-03 16:38
Title: Group owners may be unable to modify group memberships in the Global Address Book in the classic Outlook desktop clientUser impact: Group owners are unable to modify group memberships in the Global Address Book in the classic Outlook desktop client.
More info: Distribution Groups may be modified, but Security Groups may be affected.
While we work towards mitigation, affected users may be able to utilize the new Outlook desktop client or Outlook on the web to perform Distribution Group modifications.
Current status: We’re investigating a potential issue with Exchange Online and checking for impact to your organization. We’ll provide an update within 30 minutes.
