CVE-2021-34121Public exploit

Memory disclosure via Roman-numeral array overread

Published Jul 18, 2023 · Updated Oct 28, 2024

An out-of-bounds read in HTMLDOC 1.9.12 allows local users to expose process memory by opening a crafted document. In toc.cxx, parse_tree() divides a heading counter by 100 and uses the result to index ten-element Roman-numeral arrays; a value of 1000 selects index 10 beyond their bounds. The upstream report demonstrates an invalid read but not standalone code execution; execution requires an additional vulnerability chain.

CVSS severity7.8
High
EPSS probability0.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

An out-of-bounds read in HTMLDOC 1.9.12 allows local users to expose process memory by opening a crafted document. In toc.cxx, parse_tree() divides a heading counter by 100 and uses the result to index ten-element Roman-numeral arrays; a value of 1000 selects index 10 beyond their bounds. The upstream report demonstrates an invalid read but not standalone code execution; execution requires an additional vulnerability chain.

The record

CVE
CVE-2021-34121
Published
Jul 18, 2023
Updated
Oct 28, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-125, T1204.002
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 18, 2023CVE publishedPublication date reported by the CVE source.
  2. Oct 28, 2024Record 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 vectorlocal
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.8NIST NVDCVSS: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.

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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)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-125Out-of-bounds Read
T1204.002Malicious File

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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