Code execution via undisclosed integer underflow
Published Jan 13, 2026 · Updated Jan 13, 2026
Integer underflow in Microsoft Office Excel allows attackers to execute code when a user opens a crafted workbook on an affected system. Microsoft has not disclosed the affected parser or the arithmetic operation that causes the underflow and resulting heap-based buffer overflow. Exploitation requires user interaction and runs code in the user's local security context, with confidentiality, integrity, and availability impacts.
Summary
What happened
Integer underflow in Microsoft Office Excel allows attackers to execute code when a user opens a crafted workbook on an affected system. Microsoft has not disclosed the affected parser or the arithmetic operation that causes the underflow and resulting heap-based buffer overflow. Exploitation requires user interaction and runs code in the user's local security context, with confidentiality, integrity, and availability impacts.
The record
- CVE
- CVE-2026-20957
- Published
- Jan 13, 2026
- Updated
- Jan 13, 2026
- Vendor
- Microsoft
- Product
- Microsoft Office LTSC for Mac 2024
- Classifications
- CWE-191, CWE-122, T1203, T1204.002
- Attack vector
- local
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jan 13, 2026CVE publishedPublication date reported by the CVE source.
- Jan 13, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=16.0.0 <16.105.26011018
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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