saitoha libsixel up to 1.8.7-r0 SIXEL Encoder sixel_encoder_encode_bytes use after free

Summaryinfo

A vulnerability, which was classified as critical, has been found in saitoha libsixel up to 1.8.7-r0. Affected is the function sixel_encoder_encode_bytes of the component SIXEL Encoder. This manipulation causes use after free. This vulnerability is handled as CVE-2026-33021. The attack can be initiated remotely. There is not any exploit available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in saitoha libsixel up to 1.8.7-r0. It has been declared as critical. Affected by this vulnerability is the function sixel_encoder_encode_bytes of the component SIXEL Encoder. The manipulation with an unknown input leads to a use after free vulnerability. The CWE definition for the vulnerability is CWE-416. Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-33021 since 03/17/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 1.8.7-r1 eliminates this vulnerability. The upgrade is hosted for download at github.com.

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Productinfo

Vendor

• saitoha

Name

• libsixel

Version

• 1.8.7-r0

License

• open-source

Website

• Product: https://github.com/saitoha/libsixel/

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