Microsoft has confirmed a critical zero-day vulnerability (CVE-2025-9921, CVSS 9.8) in Microsoft Outlook is being actively exploited. The flaw allows for “zero-click” Remote Code Execution (RCE) simply by viewing a malicious email in the Preview Pane—no open or click required.
Business Impact
This is the highest possible severity for enterprise communication tools. Every employee with Outlook open is a potential entry point. Successful exploitation grants the attacker the same privileges as the user, allowing for immediate ransomware deployment or data exfiltration across the corporate network.
Why It Happened
The vulnerability exists in the way Outlook parses specially crafted TNEF (Transport Neutral Encapsulation Format) files. The parsing engine fails to validate memory bounds, leading to a heap overflow that attackers can reliably trigger.
Recommended Executive Action
IMMEDIATE ACTION: Direct IT to deploy the Microsoft emergency patch released today. If patching cannot be completed within 4 hours, disable the “Preview Pane” in Outlook globally via Group Policy as a temporary mitigation.
Hashtags: #Outlook #ZeroDay #RCE #Microsoft #EnterpriseSecurity #PatchNow #ZeroClick #InfoSec