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

Official 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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p x_refsource_CONFIRM
• https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1 x_refsource_MISC