CVE-2021-33391Public exploitNetwork attack vector

Code execution via freed CleanNode dereference

Published Feb 17, 2023 · Updated Mar 18, 2025

Use-after-free in HTML Tidy 5.7.28 allows attackers to execute code when crafted HTML is processed with the -g option. CleanNode in src/gdoc.c frees an element through prvTidyDiscardElement and then dereferences the freed node during Google Docs cleanup. The trigger requires attacker-controlled HTML and the Google Docs cleanup option; a process abort is demonstrated, while arbitrary code execution is reported.

CVSS severity9.8
Critical
EPSS probability1.13%
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 HTML Tidy 5.7.28 allows attackers to execute code when crafted HTML is processed with the -g option. CleanNode in src/gdoc.c frees an element through prvTidyDiscardElement and then dereferences the freed node during Google Docs cleanup. The trigger requires attacker-controlled HTML and the Google Docs cleanup option; a process abort is demonstrated, while arbitrary code execution is reported.

The record

CVE
CVE-2021-33391
Published
Feb 17, 2023
Updated
Mar 18, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-416, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 17, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 18, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/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.

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No observations available

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

Weakness, pattern, technique

CWE-416Use After Free
T1203Exploitation for Client Execution

Public exploit references

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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