CVE-2020-21677Public exploitNetwork attack vector

Process crash via oversized high-color buffer copy

Published Aug 10, 2021 · Updated Aug 4, 2024

Heap-based buffer overflow in Libsixel 1.8.4 allows attackers to crash img2sixel by converting a crafted PNG image to SIXEL. In sixel_encoder_output_without_macro at encoder.c line 814, memcpy reads roughly 1.5 GB beyond the end of a roughly 36 MB heap allocation during high-color encoding. Execution requires the crafted image to reach an application that invokes Libsixel's high-color encoder; the demonstrated consequence is process termination.

CVSS severity6.5
Medium
EPSS probability0.90%
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 Libsixel 1.8.4 allows attackers to crash img2sixel by converting a crafted PNG image to SIXEL. In sixel_encoder_output_without_macro at encoder.c line 814, memcpy reads roughly 1.5 GB beyond the end of a roughly 36 MB heap allocation during high-color encoding. Execution requires the crafted image to reach an application that invokes Libsixel's high-color encoder; the demonstrated consequence is process termination.

The record

CVE
CVE-2020-21677
Published
Aug 10, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-787, T1204.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 10, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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 vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/A:P
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/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-787Out-of-bounds Write
T1204.002User Execution: Malicious 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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