Vulnerability 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.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Saitoha | Libsixel | < 1.8.7-r1 |
Related Weaknesses (CWE)
References
- https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1Release Notes
- https://github.com/saitoha/libsixel/security/advisories/GHSA-j6m5-2cc7-3whcExploitVendor Advisory
FAQ
What is CVE-2026-33021?
CVE-2026-33021 is a vulnerability with a CVSS score of 7.3 (HIGH). 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(...
How severe is CVE-2026-33021?
CVE-2026-33021 has been rated HIGH with a CVSS base score of 7.3/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-33021?
Check the references section above for vendor advisories and patch information. Affected products include: Saitoha Libsixel.