---
id: CVE-2026-33021
title: libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel
summary: >-
  libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel.
  Versions 1.8.7 and prior contain a use-after-free vulnerability in
  sixel_encoder_encode_bytes() because sixel_frame_init() stores the
  caller-owned pixel buffe…
severity: high
cvss: 7.3
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H'
cwe:
  - CWE-416
vendor: saitoha
product: libsixel
affected:
  - libsixel < 1.8.7-r1
patched:
  - libsixel 1.8.7-r1
published: '2026-04-14'
updated: '2026-07-25'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-33021'
references:
  - url: 'https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1'
    label: security-advisories@github.com
  - url: >-
      https://github.com/saitoha/libsixel/security/advisories/GHSA-j6m5-2cc7-3whc
    label: security-advisories@github.com
tags:
  - nvd
epss: 0.00211
epssPercentile: 0.10121
ingestedAt: '2026-07-26T10:11:59.883Z'
---

## Overview

libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain a use-after-free vulnerability in sixel_encoder_encode_bytes() because sixel_frame_init() stores the caller-owned pixel buffer pointer directly in frame->pixels without making a defensive copy. When a resize operation is triggered, sixel_frame_convert_to_rgb888() unconditionally frees this caller-owned buffer and replaces it with a new internal allocation, leaving the caller with a dangling pointer. Any subsequent access to the original buffer by the caller constitutes a use-after-free, confirmed by AddressSanitizer. An attacker who controls incoming frames can trigger this bug repeatedly and predictably, resulting in a reliable crash with potential for code execution. This issue has been fixed in version 1.8.7-r1.

## Affected

- `libsixel < 1.8.7-r1`

## Remediation

Upgrade past the affected range:

- `libsixel 1.8.7-r1`