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

Summaryinfo

A vulnerability classified as critical was found in saitoha libsixel up to 1.8.7-r0. The affected element is the function load_with_gdkpixbuf of the component SIXEL Encoder. Executing a manipulation can lead to use after free. This vulnerability is handled as CVE-2026-33023. The attack can be executed remotely. There is not any exploit available. Upgrading the affected component is advised.

Detailsinfo

A vulnerability classified as critical was found in saitoha libsixel up to 1.8.7-r0. This vulnerability affects the function load_with_gdkpixbuf 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. CVE summarizes:

libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. In versions 1.8.7 and prior, when built with the --with-gdk-pixbuf2 option, a use-after-free vulnerability exists in load_with_gdkpixbuf() in loader.c. The cleanup path manually frees the sixel_frame_t object and its internal buffers without consulting the reference count, even though the object was created via the refcounted constructor sixel_frame_new() and exposed to the public callback. A callback that calls sixel_frame_ref(frame) to retain a logically valid reference will hold a dangling pointer after sixel_helper_load_image_file() returns, and any subsequent access to the frame or its fields triggers a use-after-free confirmed by AddressSanitizer. The root cause is a consistency failure between two cleanup strategies in the same codebase: sixel_frame_unref() is used in load_with_builtin() but raw free() is used in load_with_gdkpixbuf(). An attacker supplying a crafted image to any application built against libsixel with gdk-pixbuf2 support can trigger this reliably, potentially leading to information disclosure, memory corruption, or code execution. This issue has been fixed in version 1.8.7-r1.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-33023 since 03/17/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. Successful exploitation requires user interaction by the victim. There are known technical details, but no exploit is available.

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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