CVE-2026-33020: high-severity vulnerability in saitoha libsixel
libsixel: Integer Overflow in write_png_to_file() leads to Heap-based Buffer Overflow
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21Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 7.1epss 0.2%
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.
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H
Affected products
saitoha · libsixelRelated CVEs — saitoha libsixel
In the same product, most dangerous first.
CVE-2026-33023HIGHlibsixel: Use-after-free in load_with_gdkpixbuf()EPSS 0.3%CVE-2026-33019HIGHlibsixel: Integer overflow leads to Out-of-bounds Read in img2sixelEPSS 0.3%CVE-2025-9300MEDIUMsaitoha libsixel img2sixel encoder.c sixel_debug_print_palette stack-based overflowEPSS 0.3%CVE-2026-33021HIGHlibsixel: Use-after-free in sixel_encoder_encode_bytes()EPSS 0.2%CVE-2026-110601MEDIUMlibsixel before 1.8.7-r1 and 1.9.0 through 1.10.5 Heap Out-of-Bounds Write via GIFEPSS 0.2%CVE-2026-33018HIGHlibsixel: Use-After-Free in load_gif()EPSS 0.2%