Bug 2458500 (CVE-2026-33020)

Summary: CVE-2026-33020 libsixel: libsixel: Arbitrary code execution via crafted palettised PNG image processing
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in libsixel. An integer overflow vulnerability in the `sixel_frame_convert_to_rgb888()` function can lead to a heap buffer overflow when processing specially crafted large palettised PNG images. An attacker can exploit this by providing a malicious image, which causes heap corruption in the victim's process. This can result in a denial of service due to a reliable crash and potentially allow for arbitrary code execution.
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Bug Depends On: 2458613    
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Description OSIDB Bzimport 2026-04-14 22:01:47 UTC 
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.