Vulnerability · Published Apr 14, 2026 · Assigned by GitHub_M
CVE-2026-33021libsixel: Use-after-free in sixel_encoder_encode_bytes()
High 7.3
In plain English
A high-severity use-after-free in libsixel from saitoha. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 16% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | libsixel | no fix listed | DEBIAN-CVE-2026-33021 |
| Debian:13 | libsixel | no fix listed | DEBIAN-CVE-2026-33021 |
| Debian:14 | libsixel | 1:1.8.7-r2-1 | DEBIAN-CVE-2026-33021 |
| Ubuntu:22.04:LTS | libsixel | no fix listed | UBUNTU-CVE-2026-33021 |
| Ubuntu:24.04:LTS | libsixel | no fix listed | UBUNTU-CVE-2026-33021 |
| Ubuntu:25.10 | libsixel | no fix listed | UBUNTU-CVE-2026-33021 |
| Ubuntu:26.04:LTS | libsixel | no fix listed | UBUNTU-CVE-2026-33021 |
| Ubuntu:26.10 | libsixel | no fix listed | UBUNTU-CVE-2026-33021 |
Affected products (as reported)
Weakness: CWE-416 · Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
References
- https://github.com/saitoha/libsixel/security/advisories/GHSA-j6m5-2cc7-3whc x_refsource_CONFIRM
- https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1 x_refsource_MISC