CVE-2026-33020
libsixel: Integer Overflow in write_png_to_file() leads to Heap-based Buffer Overflow
Description
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.
INFO
Published Date :
April 14, 2026, 10:16 p.m.
Last Modified :
July 25, 2026, 11:10 a.m.
Remotely Exploit :
No
Source :
[email protected]
Affected Products
The following products are affected by CVE-2026-33020
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | HIGH | [email protected] | ||||
| CVSS 3.1 | HIGH | MITRE-CVE |
Solution
- Update libsixel to version 1.8.7-r1.
- Ensure all image processing uses safe integer types.
- Validate image data before processing.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-33020.
| URL | Resource |
|---|---|
| https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1 | Release Notes |
| https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p | Exploit Vendor Advisory |
| https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p | Exploit Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-33020 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-33020
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-33020 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-33020 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Jul. 25, 2026
Action Type Old Value New Value Added Translation Title: libsixel de saitoha, Description: libsixel es una implementación de codificador/decodificador SIXEL derivada de sixel de kmiya. Las versiones 1.8.7 y anteriores contienen un desbordamiento de entero que conduce a un desbordamiento de búfer de montón a través de sixel_frame_convert_to_rgb888() en frame.c, donde los cálculos de tamaño de asignación y desplazamiento de puntero para imágenes paletizadas (PAL1, PAL2, PAL4) se realizan utilizando aritmética de enteros antes de la conversión a size_t. Para imágenes cuyo recuento de píxeles excede INT_MAX / 4, el desbordamiento produce una asignación de montón de tamaño insuficiente para el búfer de conversión y un desplazamiento de puntero negativo para el sub-búfer de normalización, después de lo cual sixel_helper_normalize_pixelformat() escribe los datos completos de la imagen comenzando desde el puntero inválido, causando una corrupción masiva del montón confirmada por ASAN. Un atacante que proporciona un PNG paletizado grande especialmente diseñado puede corromper el montón del proceso víctima, lo que resulta en un fallo fiable y una potencial ejecución de código arbitrario. Este problema ha sido solucionado en la versión 1.8.7-r1. -
CVE Modified by [email protected]
Jun. 17, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'saitoha', 'product': 'libsixel', 'versions': [{'status': 'affected', 'version': '< 1.8.7-r1'}]}] -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jun. 17, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-33020', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'no'}, {'technicalImpact': 'total'}], 'version': '2.0.3', 'timestamp': '2026-04-15T13:30:43.489988Z'} -
Initial Analysis by [email protected]
Apr. 23, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:saitoha:libsixel:*:*:*:*:*:*:*:* versions up to (excluding) 1.8.7-r1 Added Reference Type GitHub, Inc.: https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1 Types: Release Notes Added Reference Type CISA-ADP: https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p Types: Exploit, Vendor Advisory Added Reference Type GitHub, Inc.: https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p Types: Exploit, Vendor Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Apr. 15, 2026
Action Type Old Value New Value Added Reference https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p -
New CVE Received by [email protected]
Apr. 14, 2026
Action Type Old Value New Value Added Description 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. Added CVSS V3.1 AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H Added CWE CWE-190 Added CWE CWE-122 Added Reference https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1 Added Reference https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p