CVE-2026-33020

CVE-2026-33020 · libsixel: Integer Overflow in write_png_to_file() leads to Heap-based Buffer Overflow

This page contains facts from public vulnerability sources. Check the original advisory before updating.

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 7.1 · High · EPSS 0.2% · Fix available

Published

2026-04-14T21:53:00.388Z

Updated

2026-04-15T13:30:47.463Z

Stack: system

Check your server →