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
Published · last modified .
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.
EPSS exploit prediction
EPSS: 0.002 (10.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 10th percentile — 10.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: -0.000
Affected products
- Saitoha Libsixel — versions < 1.8.7-r1
Weakness classification (CWE)
References
- github.com/saitoha/libsixel/security/advisories/GHSA-j6m5-2cc7-3whc (Exploit, Vendor Advisory)
- github.com/saitoha/libsixel/releases/tag/v1.8.7-r1 (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.