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Vulnerability record · CVE-2026-33021 · published 14 April 2026

CVE-2026-33021: Saitoha libsixel use after free vulnerability

SSaitoha · Libsixel

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.

7.3 CVSS 3.1 High EPSS 0.21% · top 89.8% CWE-416 · Use after free
7.3CVSS 3.1 base score
0.21%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
2References, 1 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 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.

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/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-j6m5-2cc7-3whc 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-2026-33021), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.