CVE-2026-33020 libsixel: Integer Overflow in write_png_to_file() leads to Heap-based Buffer Overflow
An integer overflow in libsixel allows attackers to cause heap corruption and potential code execution via crafted images.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-33020?
CVE-2026-33020 is a high-severity vulnerability in saitoha libsixel, published on 14 Apr 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
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.
Which versions are affected by CVE-2026-33020?
It affects 1 product.
Products include Saitoha Libsixel.
How do I fix CVE-2026-33020?
Upgrade to a fixed version.
Is CVE-2026-33020 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33020 exists.
Other vulnerabilities in saitoha libsixel
Sources
Also known as DEBIAN-CVE-2026-33020, GHSA-2xgm-4x47-2x2p, UBUNTU-CVE-2026-33020.