# CVE-2026-33021: libsixel: Use-after-free in sixel_encoder_encode_bytes()

> An attacker can trigger a use-after-free in libsixel by controlling input frames, potentially leading to code execution or crashes.

- **Severity:** High 7.3 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 14 Apr 2026
- **Updated:** 16 Apr 2026
- **Also known as:** DEBIAN-CVE-2026-33021, GHSA-j6m5-2cc7-3whc, UBUNTU-CVE-2026-33021

## What is CVE-2026-33021?

CVE-2026-33021 is a high-severity use after free vulnerability in saitoha libsixel, published on 14 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### 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.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-33021?

It affects 1 product.


Products: Saitoha Libsixel.

## How do I fix CVE-2026-33021?

Upgrade to a fixed version.

## Is CVE-2026-33021 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33021 exists.

## Other vulnerabilities in saitoha libsixel

- [CVE-2019-19635](https://insights.sbomify.com/CVE-2019-19635/): Memory corruption in n/a (critical 9.8)
- [CVE-2019-19636](https://insights.sbomify.com/CVE-2019-19636/): Integer overflow in n/a (critical 9.8)
- [CVE-2019-19637](https://insights.sbomify.com/CVE-2019-19637/): Integer overflow in n/a (critical 9.8)
- [CVE-2019-19638](https://insights.sbomify.com/CVE-2019-19638/): Integer overflow in n/a (critical 9.8)
- [CVE-2022-27044](https://insights.sbomify.com/CVE-2022-27044/): Memory corruption in n/a (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33021 and fix it if it is affected.

What it is: CVE-2026-33021 is a high-severity use after free vulnerability in saitoha libsixel, published on 14 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33021 exists.

Affected product: saitoha libsixel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33021/ (Markdown: https://insights.sbomify.com/CVE-2026-33021/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33021
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33021
- OSV: https://osv.dev/vulnerability/CVE-2026-33021

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33021/
Summaries are generated and may contain mistakes; the official description above is the record.