CVE-2026-33021: libsixel: Use-after-free in sixel_encoder_encode_bytes()

Severity: high (CVSS 7.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-14 · updated 2026-07-25
Weakness: CWE-416

Description

libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain a use-after-free vulnerability in sixel_encoder_encode_bytes() because sixel_frame_init() stores the caller-owned pixel buffer pointer directly in frame->pixels without making a defensive copy. When a resize operation is triggered, sixel_frame_convert_to_rgb888() unconditionally frees this caller-owned buffer and replaces it with a new internal allocation, leaving the caller with a dangling pointer. Any subsequent access to the original buffer by the caller constitutes a use-after-free, confirmed by AddressSanitizer. An attacker who controls incoming frames can trigger this bug repeatedly and predictably, resulting in a reliable crash with potential for code execution. This issue has been fixed in version 1.8.7-r1.

Affected products

saitoha libsixel: before 1.8.7-r1 — fixed in 1.8.7-r1

How to fix

Upgrade to a fixed version: saitoha libsixel 1.8.7-r1. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-33021 or the next vulnerability affects your stack