Buffer overflow in Saitoha Libsixel

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.

Vulnerability class: Buffer Overflow

Published · last modified .

CVSS v3 metric

CVSS v3 base score 7.1 (High). Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H.

EPSS exploit prediction

EPSS: 0.002 (11.7th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 12th percentile — 11.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-33020: held from 0.002 to 0.002.

EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: +0.000

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-33020?
CVE-2026-33020 is a high-severity vulnerability in Saitoha Libsixel, classified under Heap-based Buffer Overflow. CVSS score: 7.1/10. Published 2026-04-14.
How severe is CVE-2026-33020?
High severity. CVSS v3 base score is 7.1 out of 10.