CVE-2026-123380: Microsoft Outlook heap-based buffer overflow

CVE-2026-123380 · Severity: high · CVSS 8.8 · Published 2026-11-10

Microsoft Outlook, which manages email and calendar communications for millions of business users, contains a heap-based buffer overflow vulnerability that allows attackers on a network to execute arbitrary code without authentication. An attacker could exploit this flaw to gain unauthorized access to sensitive emails, calendar data, and potentially spread malware across an organization's network.

Technologies: Microsoft Outlook. Vendors: Microsoft.

Quick answers

Is CVE-2026-123380 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-123380 being exploited in the wild.
How severe is CVE-2026-123380?
High severity, with a CVSS score of 8.8, published 10 November 2026.
Which products and versions are affected?
Microsoft Outlook.
Is there a fix for CVE-2026-123380?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-123380 attacked?
It is attacked over the network, in a web application.
Am I affected by CVE-2026-123380?
Check whether you run Microsoft Outlook. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A heap-based buffer overflow in Outlook allows remote code execution over the network without requiring user authentication or interaction. The vulnerability permits an attacker to write beyond allocated memory boundaries, corrupting heap metadata and achieving arbitrary code execution with the privileges of the user running Outlook. No patch information is available in the advisory at this time.

Affected products

  • Microsoft Outlook

Timeline

  • 2026-11-10: disclosed

References

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