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Office CVE-2026-20957

HIGH
Heap-based Buffer Overflow (CWE-122)
2026-01-13 secure@microsoft.com
Integer Overflow Microsoft 365 Apps Excel Office Office Long Term Servicing Channel Office Online Server
7.8
CVSS 3.1 · NVD
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    User opens malicious Excel file
  2. Exploit
    Integer underflow in cell calculation
  3. Execution
    Memory buffer overflow
  4. Impact
    Arbitrary code execution with user privileges

Vulnerability Assessment

Weakness
CWE
CWE-122
OWASP Top 10:2025
A06:2021 - Vulnerable and Outdated Components
ASVS 5.0
V15 - Secure Coding and Architecture Heap buffer bounds checking
MITRE ATT&CK

1 technique across 1 tactic: Execution

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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Exploitation Conditions

User must open a specially crafted Excel file (.xls, .xlsx, or related format) containing malicious data designed to trigger integer underflow in Excel's calculation engine. No authentication required; exploitation occurs during file parsing.
Impact

CVSS 7.8 (HIGH). Attack vector: Local. Complexity: Low. Privileges required: None. User interaction: Required. Exploitation no authentication required. Exploitable via local access.
Exploit ScenarioDerived

An attacker without authentication could exploit this vulnerability to compromise the affected system.
This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
Patch available 0.1% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 12, 2026

Arbitrary code execution in Microsoft Office Excel results from an integer underflow vulnerability in the Long Term Servicing Channel and Online Server editions, exploitable by local attackers with user interaction. This HIGH severity flaw (CVSS 7.8) grants full system compromise capabilities including code execution, data theft, and service disruption with no available patch.

Earlier versions of this analysis (1)
v1 Mar 12, 2026 - 21:54 CVSS 7.8

Integer underflow (wrap or wraparound) in Microsoft Office Excel allows an unauthorized attacker to execute code locally. [CVSS 7.8 HIGH]

RemediationAI

Monitor vendor advisories for a patch.

Affected Packages

microsoft 365_apps_for_enterprise_https_aka_ms_officesecurityreleases fix: Microsoft 365 Apps for Enterprise for 32-bit Systems https://aka.ms/OfficeSecurityReleases, Microsoft 365 Apps for Enterprise for 64-bit Systems https://aka.ms/OfficeSecurityReleases
microsoft excel_2016_32_bit_edition_16_0_5535_1000 fix: Microsoft Excel 2016 (32-bit edition) 16.0.5535.1000
microsoft excel_2016_64_bit_edition_16_0_5535_1000 fix: Microsoft Excel 2016 (64-bit edition) 16.0.5535.1000
microsoft office_2019_editions_https_aka_ms_officesecurityreleases fix: Microsoft Office 2019 for 32-bit editions https://aka.ms/OfficeSecurityReleases, Microsoft Office 2019 for 64-bit editions https://aka.ms/OfficeSecurityReleases
microsoft office_ltsc_2021_editions_https_aka_ms_officesecurityreleases fix: Microsoft Office LTSC 2021 for 32-bit editions https://aka.ms/OfficeSecurityReleases, Microsoft Office LTSC 2021 for 64-bit editions https://aka.ms/OfficeSecurityReleases
microsoft office_ltsc_2024_editions_https_aka_ms_officesecurityreleases fix: Microsoft Office LTSC 2024 for 32-bit editions https://aka.ms/OfficeSecurityReleases, Microsoft Office LTSC 2024 for 64-bit editions https://aka.ms/OfficeSecurityReleases
microsoft office_ltsc_for_mac_2021_16_105_26011018 fix: Microsoft Office LTSC for Mac 2021 16.105.26011018
microsoft office_ltsc_for_mac_2024_16_105_26011018 fix: Microsoft Office LTSC for Mac 2024 16.105.26011018
microsoft office_online_server_16_0_10417_20083 fix: Office Online Server 16.0.10417.20083

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 9 packages in microsoft

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Technical ContextAI · show details

Classified as CWE-122 (Heap-based Buffer Overflow). Affects 365 Apps. Integer underflow (wrap or wraparound) in Microsoft Office Excel allows an unauthorized attacker to execute code locally.

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  • Exploit and PoC references, threat intelligence from 22 sources
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Severity by source

NVD PRIMARY 7.8 HIGH
AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
ENISA EUVD HIGH

Primary rating from NVD.

CVSS VectorNVD

Attack Vector
Local
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

DescriptionCVE.org

Integer underflow (wrap or wraparound) in Microsoft Office Excel allows an unauthorized attacker to execute code locally.

Executive ViewAI

Why It Matters

CVE-2026-20957 presents notable security risk, a heap-based buffer overflow vulnerability rated CVSS 7.8. Exploitation could allow attackers to corrupt memory, crash the application, or potentially execute arbitrary code. Organizations should apply vendor updates when available and consider compensating controls in the interim.

Business Impact: High
Compensating Controls

Apply vendor patch. If immediate patching is not feasible: (1) Enable ASLR and DEP where available; (2) Implement network segmentation; (3) Monitor for exploitation indicators.

Threat Intelligence Overview

EPSS Score
0.1%
Top 77% exploitable
Threat Intel
1
1 source
Data Quality
4 sources
Confidence: Medium
NVD CVE.org EPSS NCSC-NL

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.1%
Patch available Yes
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Scheduled
Exploitation· derived
None
System Exposure· derived
Small
Automatable· derived
No
Human Impact
High

Priority Score

44
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +39
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: +5

Attack Technique

Integer Overflow MEDIUM

Integer overflow occurs when an arithmetic operation produces a result that exceeds the maximum value a given integer type can store.

Affected Products (CPE)CVE.org

cpe:2.3:a:microsoft:365_apps:-:*:*:*:enterprise:*:x64:*
cpe:2.3:a:microsoft:365_apps:-:*:*:*:enterprise:*:x86:*
cpe:2.3:a:microsoft:excel:2016:*:*:*:*:*:*:*
cpe:2.3:a:microsoft:office:2019:*:*:*:*:*:*:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:-:x64:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:-:x86:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:macos:*:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:-:x64:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:-:x86:*
cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:macos:*:*
cpe:2.3:a:microsoft:office_online_server:*:*:*:*:*:*:*:*

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References

Advisories & patches
ADVISORY https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20957

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