CVE‑2026‑45494: Remote Code Execution in Microsoft Outlook

Impact

• Severity: CVSS v3.1 score 9.8 (Critical)
• Risk: Attackers can run arbitrary code on the victim’s machine with the user’s privileges.
• Affected users: All Outlook 2026 clients on Windows, macOS, and mobile platforms.
• Potential damage: Data theft, ransomware deployment, lateral movement.

Technical Details

• Root cause: A buffer overflow in the MIME parser when processing malformed attachment headers.
• Trigger: Receipt of a specially crafted email containing an oversized header field.
• Exploit path: The overflow corrupts the return address on the stack, redirecting execution to attacker‑controlled shellcode.
• Detection: Outlook logs an EXCEPTION_ACCESS_VIOLATION error; network traffic shows anomalous outbound connections from the client.

Mitigation Steps

1. Apply the latest security update immediately. Download from the Microsoft Update Catalog.
2. Disable automatic attachment download in Outlook settings until the patch is installed.
3. Enable attachment scanning via Microsoft Defender for Office 365.
4. Educate users to verify email senders and avoid opening unknown attachments.
5. Monitor logs for EXCEPTION_ACCESS_VIOLATION events and block suspicious IPs.

Timeline

• 2026‑04‑10: CVE disclosed by Microsoft Security Response Center (MSRC).
• 2026‑04‑12: Advisory released with preliminary patch notes.
• 2026‑04‑15: Patch KB5001234 published for Outlook 2026.
• 2026‑04‑20: Advisory updated with full mitigation guidance.
• 2026‑05‑01: End‑of‑support for unpatched Outlook 2026 clients.

Additional Resources

• Microsoft Security Update Guide – CVE‑2026‑45494
• Outlook Security Best Practices
• Microsoft Defender for Office 365 Overview

Act now. Install the patch, enforce attachment controls, and audit your environment for signs of exploitation. Failure to do so exposes your organization to severe compromise.