saitoha libsixel up to 1.8.6 SIXEL Encoder sixel_frame_convert_to_rgb888 heap-based overflow

Summaryinfo

A vulnerability classified as critical has been found in saitoha libsixel up to 1.8.6. Affected by this vulnerability is the function sixel_frame_convert_to_rgb888 of the component SIXEL Encoder. This manipulation causes heap-based overflow. This vulnerability appears as CVE-2026-33020. The attack may be initiated remotely. There is no available exploit. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability classified as critical was found in saitoha libsixel up to 1.8.6. Affected by this vulnerability is the function sixel_frame_convert_to_rgb888 of the component SIXEL Encoder. The manipulation with an unknown input leads to a heap-based overflow vulnerability. The CWE definition for the vulnerability is CWE-122. A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc(). As an impact it is known to affect 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 an integer overflow which leads to a heap buffer overflow via sixel_frame_convert_to_rgb888() in frame.c, where allocation size and pointer offset computations for palettised images (PAL1, PAL2, PAL4) are performed using int arithmetic before casting to size_t. For images whose pixel count exceeds INT_MAX / 4, the overflow produces an undersized heap allocation for the conversion buffer and a negative pointer offset for the normalization sub-buffer, after which sixel_helper_normalize_pixelformat() writes the full image data starting from the invalid pointer, causing massive heap corruption confirmed by ASAN. An attacker providing a specially crafted large palettised PNG can corrupt the heap of the victim process, resulting in a reliable crash and potential arbitrary 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-2026-33020 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.

The vulnerability scanner Nessus provides a plugin with the ID 306636 (Linux Distros Unpatched Vulnerability : CVE-2026-33020), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 1.8.7-r1 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (306636). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Vendor

• saitoha

Name

• libsixel

Version

• 1.8.0
• 1.8.1
• 1.8.2
• 1.8.3
• 1.8.4
• 1.8.5
• 1.8.6

License

• open-source

Website

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

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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