Users may be unable to download or open email attachments in Outlook on the web

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may have been unable to download or open email attachments in Outlook on the web. Final status: We've restarted the affected portion of infrastructure and confirmed through monitoring internal service health telemetry that the impact has been remediated. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web may have been impacted. Start time: Thursday, April 9, 2026, at 4:00 AM UTC End time: Friday, April 17, 2026, at 5:00 PM UTC Root cause: An authentication issue on a portion of Outlook on the web infrastructure responsible for downloading and opening attachments was resulting in impact. Next steps: - We're reviewing the affected portion of Outlook on the web infrastructure to determine how the authentication issue was introduced, so that we can prevent similar instances of impact from occurring in the future. This is the final update for the event.

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may be unable to download or open email attachments in Outlook on the web. Current status: We're continuing our efforts of restarting the affected portion of infrastructure, which we anticipate will be complete by our next scheduled update. Afterwards, we'll monitor internal telemetry to validate if the impact is remediated or if additional actions are needed to resolve the underlying issue. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web may be impacted. This information may be updated as our investigation continues. Start time: Thursday, April 9, 2026, at 4:00 AM UTC Root cause: An authentication issue on a portion of Outlook on the web infrastructure responsible for downloading and opening attachments is resulting in impact. Next update by: Friday, April 17, 2026, at 6:00 PM UTC

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may be unable to download or open email attachments in Outlook on the web. Current status: We've identified an authentication issue on a portion of Outlook on the web infrastructure responsible for downloading and opening attachments is resulting in impact. We're restarting the affected portion of infrastructure to remediate impact, and we're monitoring telemetry to ensure the issue is resolved, or to identify if any additional remediation actions may be needed. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web may be impacted. This information may be updated as our investigation continues. Start time: Thursday, April 9, 2026, at 4:00 AM UTC Root cause: An authentication issue on a portion of Outlook on the web infrastructure responsible for downloading and opening attachments is resulting in impact. Next update by: Friday, April 17, 2026, at 5:00 AM UTC

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may be unable to download or open email attachments in Outlook on the web. Current status: While we're continuing our analysis of potential authentication issues, the restarted machines have seen significant improvement. We're in the process of initiating safe restarts on all the affected infrastructure to remediate impact. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web may be impacted. This information may be updated as our investigation continues. Start time: Thursday, April 9, 2026, at 4:00 AM UTC Next update by: Friday, April 17, 2026, at 2:30 AM UTC

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may be unable to download or open email attachments in Outlook on the web. Current status: While we were in the process of analyzing a potential token failure, system monitoring indicated that the service had started returning to normal health. Per service health telemetry, we see that this impact has started to recover gradually for impacted users, however we haven't reached full remediation for this event. We're reviewing the timeline of this impact start and recovery to isolate whether any specific change caused this event and isolate a means to expedite a full mitigation for the remaining impacted users. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web may be impacted. Start time: Thursday, April 9, 2026, at 4:00 AM UTC Next update by: Friday, April 17, 2026, at 12:00 AM UTC

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may be unable to download or open email attachments in Outlook on the web. Current status: Our analysis of the isolated token failure is ongoing as we work to confirm this suspected root cause and begin developing potential solutions for remediating impact. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web may be impacted. Next update by: Thursday, April 16, 2026, at 6:30 PM UTC

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may be unable to download or open email attachments in Outlook on the web. Current status: We're continuing to analyze the isolated token failure as we seek to confirm this suspected root cause and appropriately determine our next steps in mitigating impact. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web are impacted. This information may be updated as our investigation continues. Next update by: Wednesday, April 15, 2026, at 10:30 PM UTC

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may be unable to download or open email attachments in Outlook on the web. Current status: Our HAR trace analysis has focused our investigation to a token failure that we believe could be preventing the opening of email attachments for users in Outlook on the web. We're further analyzing the isolated failure to confirm our findings and so our next troubleshooting steps for remediating the impact can be determined. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web are impacted. This information may be updated as our investigation continues. Next update by: Wednesday, April 15, 2026, at 8:30 PM UTC

Title: Users may be unable to download or open email attachments in Outlook on the web User impact: Users may be unable to download or open email attachments in Outlook on the web. Current status: We've collected and are analyzing HAR traces from affected users to help us determine our next troubleshooting steps for remediating the impact. Scope of impact: Some users attempting to download or open email attachments in Outlook on the web are impacted. This information may be updated as our investigation continues. Next update by: Tuesday, April 14, 2026, at 8:30 PM UTC

Title: Users are unable to download or open email attachments in Outlook on the web User impact: Users are unable to download or open email attachments in Outlook on the web. Current status: We're requesting additional HTTP Archive Format (HAR) traces from some affected users to further our understanding of the underlying issue and determine our next troubleshooting steps. Scope of impact: Your organization is affected by this event, and some users attempting to download or open email attachments in Outlook on the web are impacted. Next update by: Monday, April 13, 2026, at 8:30 PM UTC

Title: Users are unable to download or open email attachments in Outlook on the web User impact: Users are unable to download or open email attachments in Outlook on the web. Current status: We're continuing to gather network trace logs and additional details to help us identify the underlying cause of impact and implement the appropriate mitigation steps. Scope of impact: Your organization is affected by this event, and some users attempting to download or open email attachments in Outlook on the web are impacted. Next update by: Friday, April 10, 2026, at 8:00 PM UTC

Title: Users are unable to download or open email attachments in Outlook on the web User impact: Users are unable to download or open email attachments in Outlook on the web. Current status: We're continuing to gather network trace logs and additional details to help us identify the underlying cause of impact and implement the appropriate mitigation steps. Scope of impact: Your organization is affected by this event, and some users attempting to download or open email attachments in Outlook on the web are impacted. Next update by: Friday, April 10, 2026, at 12:00 PM UTC

Title: Users are unable to download or open email attachments in Outlook on the web User impact: Users are unable to download or open email attachments in Outlook on the web. Current status: Our analysis of service health telemetry diagnostics and data from impacted users is taking an extended period of time. We’re in the process of reaching out to affected users to gather network trace logs and additional supplementary details, which we anticipate will expedite our understanding of the underlying cause of impact. Scope of impact: Your organization is affected by this event, and some users attempting to download or open email attachments in Outlook on the web are impacted. Next update by: Friday, April 10, 2026, at 10:00 AM UTC

Title: Users are unable to download or open email attachments in Outlook on the web User impact: Users are unable to download or open email attachments in Outlook on the web. Current status: Our analysis of service health telemetry diagnostics and data from impacted users is ongoing as we seek to isolate this issue's root cause and develop potential strategies for remediating impact. Scope of impact: Your organization is affected by this event, and some users attempting to download or open email attachments in Outlook on the web are impacted. Next update by: Friday, April 10, 2026, at 12:00 AM UTC

Title: Users are unable to download or open email attachments in Outlook on the web User impact: Users are unable to download or open email attachments in Outlook on the web. Current status: We're continuing to analyze diagnostic data and details from service health telemetry in order to isolate a potential root cause for this issue and determine our next troubleshooting steps. Scope of impact: Your organization is affected by this event, and some users attempting to download or open email attachments in Outlook on the web are impacted. Next update by: Thursday, April 9, 2026, at 6:00 PM UTC

Title: Users are unable to download or open email attachments in Outlook on the Web User impact: Users are unable to download or open email attachments in Outlook on the Web. Current status: We're continuing to review diagnostic data from our service telemetry to isolate the source of the issue and determine appropriate mitigation steps. Scope of impact: Your organization is affected by this event, and some users attempting to download or open email attachments in Outlook on the Web are impacted. Next update by: Thursday, April 9, 2026, at 4:00 PM UTC

Title: Users are unable to download or open email attachments in Outlook on the Web User impact: Users are unable to download or open email attachments in Outlook on the Web. Current status: We're reviewing service monitoring telemetry to isolate the source of the issue and establish a fix. Scope of impact: Your organization is affected by this event, and some users attempting to download or open email attachments in Outlook on the Web are impacted. Next update by: Thursday, April 9, 2026, at 2:00 PM UTC