CVE-2026-56156: Microsoft Office Excel heap buffer overflow

CVE-2026-56156 · Severity: high · CVSS 7.8 · Published 2026-07-14

Microsoft Excel, a widely used spreadsheet application, is vulnerable to a security flaw that could allow an attacker to take control of a user's computer. To exploit this, an attacker would typically need to trick a user into opening a specially crafted malicious file. If successful, the attacker could gain the same permissions as the user, potentially leading to data theft, unauthorized software installation, or disruption of business operations.

Technologies: Microsoft 365 Apps for Enterprise, Microsoft Office 365 for Mac, Microsoft Office LTSC 2021, Microsoft Office LTSC 2024, Microsoft Office LTSC for Mac 2021, Microsoft Office LTSC for Mac 2024. Vendors: Microsoft.

Quick answers

Is CVE-2026-56156 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-56156 being exploited in the wild.
How severe is CVE-2026-56156?
High severity, with a CVSS score of 7.8, published 14 July 2026.
Which products and versions are affected?
Microsoft 365 Apps for Enterprise 16.0.1 to latest security release; Microsoft Office 365 for Mac 1.0.0 to 16.111.26071215; Microsoft Office LTSC 2021 16.0.1 to latest security release; Microsoft Office LTSC 2024 16.0.0 to latest security release; and 2 more.
Is there a fix for CVE-2026-56156?
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-2026-56156 attacked?
It is attacked with local access, in a network appliance.
Am I affected by CVE-2026-56156?
Check whether you run Microsoft 365 Apps for Enterprise, Microsoft Office 365 for Mac, Microsoft Office LTSC 2021 or one of the others listed at an affected version. 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 (CWE-122) exists in Microsoft Office Excel across multiple versions, including Microsoft 365 Apps and LTSC editions for both Windows and Mac. The vulnerability is triggered when the application processes a specially crafted file, leading to memory corruption. While the attack vector is classified as local, it requires user interaction, typically involving the victim opening a malicious spreadsheet. Successful exploitation allows for arbitrary code execution in the context of the current user. Microsoft has released security updates to address this issue; users should refer to the Microsoft Security Update Guide for specific patch versions.

Affected products

  • Microsoft Microsoft 365 Apps for Enterprise 16.0.1 to latest security release
  • Microsoft Microsoft Office 365 for Mac 1.0.0 to 16.111.26071215
  • Microsoft Microsoft Office LTSC 2021 16.0.1 to latest security release
  • Microsoft Microsoft Office LTSC 2024 16.0.0 to latest security release
  • Microsoft Microsoft Office LTSC for Mac 2021 16.0.1 to 16.111.26071215
  • Microsoft Microsoft Office LTSC for Mac 2024 16.0.0 to 16.111.26071215

Timeline

  • 2026-07-14: disclosed: Initial publication by Microsoft and NVD
  • 2026-07-14: advisory: Microsoft Security Update Guide published

References

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