Microsoft says new Outlook can't open some Excel attachments

Microsoft Outlook (New Client) Fails to Open Specific Excel Attachments

TL;DR: A known bug in the new Microsoft Outlook client prevents some users from opening certain Excel email attachments, impeding critical business functions.

Technical Analysis

• Affected Components: Microsoft Outlook (New Client) when processing specific Excel email attachments.
• Observed Behavior: Users are unable to open certain Excel files received as email attachments within the new Outlook client interface.
• Current Status: Microsoft has acknowledged the issue and is actively working on a resolution.
• MITRE TTPs/IOCs: Not applicable. This is an operational software bug, not a direct security vulnerability, threat actor activity, or malware incident.

Actionable Insight

• For CISOs: Be aware of potential business continuity impacts and user productivity losses. Advise affected users to utilize Outlook web access or revert to the classic Outlook client as a temporary workaround for opening critical Excel attachments.
• For SOC Analysts/Detection Engineers: While this is an operational bug, not a direct security threat, remain vigilant for potential social engineering campaigns that may attempt to leverage this known issue. Watch for phishing lures impersonating Microsoft or IT support, offering "fixes" for the attachment issue, which could lead to credential theft or malware delivery. Update user awareness training to highlight this specific risk.

Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-says-new-outlook-cant-open-some-excel-attachments/