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Vulnerability record · CVE-2026-33018 · published 14 April 2026

CVE-2026-33018: 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 via the load_gif() function in fromgif.c, where a single sixel_frame_t object is reused across all frames of an animated GIF and gif_init_frame() unconditionally frees and reallocates frame->pixels between frames without consulting the object's reference count. Because the public API explicitly provides sixel_frame_ref() to retain a frame and sixel_frame_get_pixels() to access the raw pixel buffer, a callback following this documented usage pattern will hold a dangling pointer after the second frame is decoded, resulting in a heap use-after-free confirmed by ASAN. Any application using sixel_helper_load_image_file() with a multi-frame callback to process user-supplied animated GIFs is affected, with a reliable crash as the minimum impact and potential for code execution. This issue has been fixed in version 1.8.7-r1.

7.0 CVSS 3.1 High EPSS 0.19% · top 92.0% CWE-416 · Use after free
7.0CVSS 3.1 base score
0.19%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 via the load_gif() function in fromgif.c, where a single sixel_frame_t object is reused across all frames of an animated GIF and gif_init_frame() unconditionally frees and reallocates frame->pixels between frames without consulting the object's reference count. Because the public API explicitly provides sixel_frame_ref() to retain a frame and sixel_frame_get_pixels() to access the raw pixel buffer, a callback following this documented usage pattern will hold a dangling pointer after the second frame is decoded, resulting in a heap use-after-free confirmed by ASAN. Any application using sixel_helper_load_image_file() with a multi-frame callback to process user-supplied animated GIFs is affected, with a reliable crash as the minimum impact and potential for code execution. This issue has been fixed in version 1.8.7-r1.

CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:H/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-w46f-jr9f-rgvp 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-2026-33018), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.