CVE-2026-33021

CWE-416Published: April 14, 2026· Updated: Apr 17, 2026

7.3

CVSS v3.1

EPSS:0.01%probability of exploitation in 30 daysPercentile:2.7th

Official 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.

NVD Source

Technical Analysis

CVE-2026-33021 requires local access, meaning attackers must already have a foothold on the target system.

The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.

A successful exploit results in availability disruption (denial of service), with a CVSS base score of 7.3.

From a weakness classification perspective (CWE-416): Use-after-free vulnerabilities involve accessing memory after it has been freed, often enabling arbitrary code execution.