CVE-2026-55898 Microsoft CVE debrief

Who should care

Defenders responsible for Microsoft Office and Excel deployments should assess exposure and prioritize patching vulnerable versions to prevent potential information disclosure.

Why it matters

CVE-2026-55898 is a MEDIUM-severity vulnerability allowing local information disclosure through an out-of-bounds read in Microsoft Office Excel. Defenders should prioritize patching vulnerable versions to prevent potential information disclosure.

• Defenders must verify and apply updates for multiple Microsoft Office products to prevent potential information disclosure.
• Inventory checks are necessary to identify affected Microsoft Excel and Office versions.
• Monitoring for potential information disclosure attempts is recommended.
• Remediation priority follows from the need to patch vulnerable versions to prevent exploitation.

Technical summary

The vulnerability is caused by an out-of-bounds read in Microsoft Office Excel, allowing an unauthorized attacker to disclose information locally. Affected products include Microsoft 365 Apps for Enterprise, Microsoft Excel 2016, Microsoft Office 2019, and others.

Defensive priority

Defenders should prioritize patching vulnerable Microsoft Excel and Office versions to prevent potential information disclosure.

Recommended defensive actions

• Patch vulnerable Microsoft Excel and Office versions
• Verify and apply updates for Microsoft 365 Apps for Enterprise, Microsoft Excel 2016, Microsoft Office 2019, and other affected products
• Monitor for potential information disclosure attempts

Evidence notes

The CVE record and NVD vulnerability detail provide information on the vulnerability, but do not specify which versions are patched or provide details on exploitation.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.