CVE-2021-38655

Code execution via XLS parser use-after-free

Published Sep 15, 2021 · Updated Aug 10, 2026

Use-after-free in Microsoft Excel allows remote attackers to execute arbitrary code when a crafted XLS file is opened. The XLS parser performs operations on an object without first validating that the object still exists, producing a use-after-free condition. Exploitation requires the target to open the malicious file or visit a malicious page, and code runs in the current process context.

CVSS severity7.8
High
EPSS probability6.33%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Microsoft Excel allows remote attackers to execute arbitrary code when a crafted XLS file is opened. The XLS parser performs operations on an object without first validating that the object still exists, producing a use-after-free condition. Exploitation requires the target to open the malicious file or visit a malicious page, and code runs in the current process context.

The record

CVE
CVE-2021-38655
Published
Sep 15, 2021
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Microsoft Office 2019
Classifications
CWE-416, T1203, T1204.002
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 15, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 10, 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.

  1. Affected versionversion=19.0.0 <https://aka.ms/OfficeSecurityReleases

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Office 2019version=19.0.0 <https://aka.ms/OfficeSecurityReleases
Microsoft · Microsoft Excel 2016version=16.0.0.0 <5215.1000
Microsoft · Microsoft Excel 2013version=15.0.0.0 <5381.1000
Microsoft · Microsoft Office 2019 for Macversion=16.0.0 <16.53.21091200
Microsoft · Microsoft Office Online Serverversion=16.0.1 <10378.20000
Microsoft · Microsoft Office Web Apps Server 2013version=15.0.1 <5381.1000
Microsoft · Microsoft 365 Apps for enterpriseversion=16.0.1 <https://aka.ms/OfficeSecurityReleases

Published CVSS scores

6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
7.8Microsoft CorporationCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-416Use After Free
T1203Exploitation for Client Execution
T1204.002Malicious File

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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