CVE-2026-123378: Microsoft Office Outlook integer overflow remote code execution

CVE-2026-123378 · Severity: high · CVSS 7.5 · Published 2026-11-10

Microsoft Office Outlook, an email and calendar client used across enterprises, contains an integer overflow vulnerability that allows a network attacker to execute arbitrary code. An attacker can exploit this flaw by sending a specially crafted message, potentially leading to complete compromise of the affected system including data theft and ransomware installation.

Technologies: Microsoft Office Outlook. Vendors: Microsoft.

Quick answers

Is CVE-2026-123378 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-123378 being exploited in the wild.
How severe is CVE-2026-123378?
High severity, with a CVSS score of 7.5, published 10 November 2026.
Which products and versions are affected?
Microsoft Office Outlook.
Is there a fix for CVE-2026-123378?
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-2026-123378 attacked?
It is attacked over the network, in a web application.
Am I affected by CVE-2026-123378?
Check whether you run Microsoft Office 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

An integer overflow or wraparound vulnerability in Microsoft Office Outlook allows remote code execution via network attack. The vulnerability requires no authentication and can be triggered by sending a malicious message, resulting in arbitrary code execution in the context of the Outlook process.

Affected products

  • Microsoft Office Outlook

Timeline

  • 2026-11-10: disclosed

References

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