CVE-2026-21258

Home/CVE/Improper input validation in Microsoft Office Excel allows an unauthorized attacker to disclose information locally.

CVE

Improper input validation in Microsoft Office Excel allows an unauthorized attacker to disclose information locally.

MEDIUM · CVSS 5.5 EPSS 0.00063

Monitor

No active-exploitation, high-EPSS, or public-exploit signals - routine patching cadence

Sigma rules13 YARA rules0

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Weakness Classification

CWE-125Out-of-bounds Read

CWE-20Improper Input Validation

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Affected Products & Versions

microsoft 365 appsall versions
microsoft excelall versions
microsoft officeall versions
microsoft office long term servicing channelall versions
microsoft office online server< 16.0.10417.20097

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Sigma Hunt Rules

Exact rules name this CVE ID. Product rules name an affected product in their title. Related rules cover techniques used by actors who exploited this CVE. Showing the most relevant matches; the complete related set is on the full drill-down.

producthighPython Function Execution Security Warning Disabled In Excel

producthighPotential Excel.EXE DCOM Lateral Movement Via ActivateMicrosoftApp

producthighPotential Persistence Via Excel Add-in - Registry

producthighPython Function Execution Security Warning Disabled In Excel - Registry

productmediumMicrosoft Excel Add-In Loaded From Uncommon Location

producthighSuspicious Microsoft Office Child Process - MacOS

Show all 13 top matchesproducthighCode Executed Via Office Add-in XLL File
producthighPotential Persistence Via Microsoft Office Add-In
producthighPotential Persistence Via Microsoft Office Startup Folder
producthighFile With Uncommon Extension Created By An Office Application
producthighOffice Macro File Creation From Suspicious Process
producthighUncommon File Created In Office Startup Folder
producthighPotentially Suspicious Office Document Executed From Trusted Location

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Scoring & Timeline

5.5

MEDIUM · CVSS v3.1 · secure@microsoft.com

View on NVD

Attack Vector

Network Adjacent Local Physical

Attack Complexity

Low High

Privileges Required

None Low High

User Interaction

None Required

Scope

Unchanged Changed

Confidentiality

None Low High

Integrity

None Low High

Availability

None Low High

Published to NVD10 Feb 2026 · 06:16 PM

CVSS VectorCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

SSVC triage · cisa-vulnrichment

Exploitation

none

Automatable

Technical impact

partial

SSVC asks the questions that actually drive patch urgency: is it being exploited, can attacks be automated, and how total is the impact.

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Vendor Advisories

msrcCVE-2026-21258Information Disclosure

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CVETechnique ActorTool Family

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External lookups - second-class, for what we don’t hold ourselves

NVD CVE.org CISA Vulners Exploit-DB GitHub PoC VulnCheck KEV GreyNoise

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Detection & defense
Sigma rules
YARA rules
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Compliance
NIST 800-53
ISO 27001:2022
SOC 2 TSC
PCI-DSS v4.0
CIS Controls v8.1

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