---
id: CVE-2026-33020
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 an integer overflow which leads to a heap
  buffer overflow via sixel_frame_convert_to_rgb888() in frame.c, where
  allocation si…
severity: high
cvss: 7.1
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H'
cwe:
  - CWE-122
  - CWE-190
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-2026-33020'
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-2xgm-4x47-2x2p
    label: security-advisories@github.com
  - url: >-
      https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
epss: 0.00224
epssPercentile: 0.11723
ingestedAt: '2026-07-26T10:11:59.430Z'
---

## Overview

libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain an integer overflow which leads to a heap buffer overflow via sixel_frame_convert_to_rgb888() in frame.c, where allocation size and pointer offset computations for palettised images (PAL1, PAL2, PAL4) are performed using int arithmetic before casting to size_t. For images whose pixel count exceeds INT_MAX / 4, the overflow produces an undersized heap allocation for the conversion buffer and a negative pointer offset for the normalization sub-buffer, after which sixel_helper_normalize_pixelformat() writes the full image data starting from the invalid pointer, causing massive heap corruption confirmed by ASAN. An attacker providing a specially crafted large palettised PNG can corrupt the heap of the victim process, resulting in a reliable crash and potential arbitrary 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`