CVE-2026-55949: Microsoft Excel use of uninitialized resource code execution

CVE-2026-55949 · Severity: high · CVSS 7.8 · Published 2026-07-14

A security vulnerability exists in Microsoft Excel, a widely used spreadsheet application. An attacker could exploit this flaw by tricking a user into opening a specially crafted file, potentially allowing the attacker to take control of the user's computer. This could lead to the theft of sensitive data, unauthorized changes to files, or a complete system compromise.

Technologies: Microsoft 365 Apps for Enterprise, Microsoft Excel 2016, Microsoft Office 2019, 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, Microsoft Office Online Server. Vendors: Microsoft.

Quick answers

Is CVE-2026-55949 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-55949 being exploited in the wild.
How severe is CVE-2026-55949?
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 Excel 2016 16.0.0.0 to 16.0.5561.1001; Microsoft Office 2019 19.0.0 to latest security release; Microsoft Office 365 for Mac 1.0.0 to 16.111.26071215; and 5 more.
Is there a fix for CVE-2026-55949?
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-2026-55949 attacked?
It is attacked with local access, in a network appliance.
Am I affected by CVE-2026-55949?
Check whether you run Microsoft 365 Apps for Enterprise, Microsoft Excel 2016, Microsoft Office 2019 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 vulnerability classified as 'Use of Uninitialized Resource' (CWE-908) exists in multiple versions of Microsoft Excel and Office suites. The flaw is triggered when the application attempts to use a memory resource that has not been properly initialized. An attacker can exploit this by convincing a target user to open a malicious Excel file. Successful exploitation allows for arbitrary code execution in the context of the current user. The attack vector is local, but requires user interaction (UI:R). Microsoft has released security updates to address this issue across affected platforms including Windows and macOS.

Affected products

  • Microsoft Microsoft 365 Apps for Enterprise 16.0.1 to latest security release
  • Microsoft Microsoft Excel 2016 16.0.0.0 to 16.0.5561.1001
  • Microsoft Microsoft Office 2019 19.0.0 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
  • Microsoft Office Online Server 16.0.0.0 to 16.0.10417.20175

Timeline

  • 2026-07-14: disclosed: Initial publication of the CVE record
  • 2026-07-14: advisory: Microsoft released the security update guide for this vulnerability

References

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