Cyber Resilience

CVE-2026-20955

Microsoft Office Long Term Servicing Channel 2021 … 2024

Published
13 January 2026
Modified
14 January 2026
Patch / advisory
View advisory
CVSS Score v3.1 7.8
Click a component to see what it means
Raw vectorCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
EPSS Score 0.0061 46th percentile
Risk Priority 58 floored blend · peak EPSS

Summary

CVE-2026-20955 is a high-severity Untrusted Pointer Dereference (CWE-822) vulnerability in Microsoft Office Long Term Servicing Channel. Its CVSS base score is 7.8 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 46th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

CVE-2026-20955 is an untrusted pointer dereference vulnerability (CWE-822) in Microsoft Office Excel. It enables an unauthorized attacker to execute code locally on affected systems. The vulnerability carries a CVSS v3.1 base score of 7.8 (AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H) and was published on 2026-01-13T18:16:23.307.

Exploitation requires local access to the target system, low attack complexity, no user privileges, and user interaction, such as opening a malicious Excel file. A successful attack allows the unauthorized attacker to achieve high-impact effects on confidentiality, integrity, and availability, resulting in arbitrary code execution within the context of the affected application.

The Microsoft Security Response Center advisory provides details on mitigation and patches at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20955.

EU & UK References

Vulnerability Data

Untrusted pointer dereference in Microsoft Office Excel allows an unauthorized attacker to execute code locally.

CWE(s)
CWE-822

Related Threats

MITRE ATT&CK Enterprise Techniques

T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Access
Adversaries may exploit software vulnerabilities in an attempt to collect credentials.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

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CVE-2025-62561Same product: Microsoft 365 Apps
CVE-2025-24083Same product: Microsoft 365 Apps
CVE-2023-36045Same product: Microsoft 365 Apps
CVE-2026-26113Same product: Microsoft 365 Apps
CVE-2026-40367Same product: Microsoft 365 Apps

Affected Assets

microsoft
365 apps
all versions
microsoft
office
2019
microsoft
office long term servicing channel
2021, 2024
microsoft
office online server
≤ 16.0.10417.20083

Mitigating Controls

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation can discover instances where untrusted data is turned into a pointer and dereferenced.

SI-10 Information Input Validation good match

Validating all information inputs stops untrusted values from being accepted and converted into dereferenceable pointers.

SI-16 Memory Protection partial match

Memory-protection mechanisms limit the damage from an invalid pointer dereference without stopping the root coding flaw.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly prevent introduction of untrusted pointer handling during development.

DE.CM-09 partial match
prevents

Runtime monitoring of software and data can detect adverse events resulting from exploitation of the weakness.

ID.RA-01 partial match
prevents

Vulnerability identification processes can discover instances of this weakness via code review or scanning.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect pointer-dereference flaws before release.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle includes pointer-safety practices that reduce untrusted pointer dereference risk.

A.8.26 Application security requirements partial match
prevents

Application security requirements can mandate validation of pointers obtained from untrusted sources.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles discourage direct use of untrusted values as pointers.

A.8.28 Secure coding partial match
prevents

Secure coding standards explicitly forbid dereferencing pointers derived from untrusted input.

References