CVE-2021-34501

Code execution via CONTINUEFRT12 heap buffer overflow

Published Jul 14, 2021 · Updated Aug 10, 2026

Heap-based buffer overflow in Microsoft Excel allows attackers to execute code when a user opens a crafted XLS file. The CONTINUEFRT12 record parser copies user-supplied data into a heap buffer without validating its length. Exploitation requires the target to visit a malicious page or open a malicious file, and successful code runs in the current process.

CVSS severity7.8
High
EPSS probability51.39%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap-based buffer overflow in Microsoft Excel allows attackers to execute code when a user opens a crafted XLS file. The CONTINUEFRT12 record parser copies user-supplied data into a heap buffer without validating its length. Exploitation requires the target to visit a malicious page or open a malicious file, and successful code runs in the current process.

The record

CVE
CVE-2021-34501
Published
Jul 14, 2021
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Microsoft Office 2019
Classifications
T1203
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 14, 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 <16.0.5188.100
Microsoft · Microsoft Excel 2013version=15.0.0.0 <15.0.5363.1000
Microsoft · Microsoft Office 2019 for Macversion=16.0.0 <16.51.210711.01
Microsoft · Microsoft Office Online Serverversion=16.0.1 <16.0.10376.20001
Microsoft · Microsoft 365 Apps for enterpriseversion=16.0.1 <https://aka.ms/OfficeSecurityReleases

Published CVSS scores

8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
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

T1203Exploitation for Client Execution

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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