CVE-2021-20308Public exploitNetwork attack vector

Code execution via malformed GIF heap overflow

Published Apr 5, 2021 · Updated Aug 3, 2024

Integer overflow in HTMLDOC 1.9.11 and earlier allows remote attackers to execute arbitrary code or crash the converter via a crafted GIF. image_load_gif() multiplies attacker-controlled width, height, and depth values without an overflow check, allocates an undersized pixel buffer, and lets gif_read_image() overrun it. The GIF must be processed by HTMLDOC; the public demonstration produces a segmentation fault, while code execution is reported but not demonstrated.

CVSS severity9.8
Critical
EPSS probability2.48%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Integer overflow in HTMLDOC 1.9.11 and earlier allows remote attackers to execute arbitrary code or crash the converter via a crafted GIF. image_load_gif() multiplies attacker-controlled width, height, and depth values without an overflow check, allocates an undersized pixel buffer, and lets gif_read_image() overrun it. The GIF must be processed by HTMLDOC; the public demonstration produces a segmentation fault, while code execution is reported but not demonstrated.

The record

CVE
CVE-2021-20308
Published
Apr 5, 2021
Updated
Aug 3, 2024
Vendor
HTMLDOC Project
Product
HTMLDOC
Classifications
CWE-190, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 5, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=htmldoc 1.9.11 and older

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

HTMLDOC Project · HTMLDOCversion=htmldoc 1.9.11 and older

Published CVSS scores

7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
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.

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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-190Integer Overflow or Wraparound
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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