Use After Free in Saitoha Libsixel
CVE-2026-33021
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.
Vulnerability class: Use-After-Free
EPSS: 0.002 (16.1th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 7.3 (High). Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H.
Affected products
- Saitoha Libsixel — versions < 1.8.7-r1
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Exploit, Vendor Advisory)
- security-advisories@github.com (x_refsource_MISC, Release Notes)
Frequently asked questions
- What is CVE-2026-33021?
- CVE-2026-33021 is a high-severity vulnerability in Saitoha Libsixel, classified under Use After Free. CVSS score: 7.3/10. Published 2026-04-14.
- How severe is CVE-2026-33021?
- High severity. CVSS v3 base score is 7.3 out of 10.