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Vulnerability record · CVE-2026-33020 · published 14 April 2026

CVE-2026-33020: Saitoha libsixel heap-based buffer overflow vulnerability

SSaitoha · Libsixel

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.

7.1 CVSS 3.1 High EPSS 0.22% · top 88.3% CWE-122 · Heap-based buffer overflowCWE-190 · Integer overflow
7.1CVSS 3.1 base score
0.22%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
3References, 2 tagged exploit
25 Jul 2026Last modified by NVD

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.

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1 Release Notes
https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p ExploitVendor Advisory
https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p ExploitVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.8CVE-2019-19637Saitoha libsixel integer overflow vulnerabilityAn issue was discovered in libsixel 1.8.2. There is an integer overflow in the function sixel_decode_raw_impl at fromsixel.c.EPSS 1.2%9.8CVE-2019-19638Saitoha libsixel integer overflow vulnerabilityAn issue was discovered in libsixel 1.8.2. There is a heap-based buffer overflow in the function load_pnm at frompnm.c, due to an integer overflow.EPSS 1.2%9.8CVE-2019-19635Saitoha libsixel out-of-bounds write vulnerabilityAn issue was discovered in libsixel 1.8.2. There is a heap-based buffer overflow in the function sixel_decode_raw_impl at fromsixel.c.EPSS 1.2%9.8CVE-2019-19636Saitoha libsixel integer overflow vulnerabilityAn issue was discovered in libsixel 1.8.2. There is an integer overflow in the function sixel_encode_body at tosixel.c.EPSS 1.2%8.8CVE-2022-27044Saitoha libsixel out-of-bounds write vulnerabilitylibsixel 1.8.6 is affected by Buffer Overflow in libsixel/src/quant.c:876.EPSS 1.0%8.8CVE-2022-27046Saitoha libsixel use after free vulnerabilitylibsixel 1.8.6 suffers from a Heap Use After Free vulnerability in in libsixel/src/dither.c:388.EPSS 1.0%8.8CVE-2020-21547Saitoha libsixel out-of-bounds write vulnerabilityLibsixel 1.8.2 contains a heap-based buffer overflow in the dither_func_fs function in tosixel.c.EPSS 1.1%8.8CVE-2020-21548Saitoha libsixel out-of-bounds write vulnerabilityLibsixel 1.8.3 contains a heap-based buffer overflow in the sixel_encode_highcolor function in tosixel.c.EPSS 1.1%

Source: NIST National Vulnerability Database (record CVE-2026-33020), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.