CVE-2026-33020

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.

• CWE-122
• CWE-190

• https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1Release Notes
• https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2pExploit, Vendor Advisory
• https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2pExploit, Vendor Advisory

Published

Apr 14, 2026

Last Modified

Jul 25, 2026

Status

Analyzed

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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