CVE-2026-33020: saitoha libsixel Heap-based Buffer Overflow

Vulnerability summary

CVE-2026-33020 is a high (CVSS 7.1) Heap-based Buffer Overflow vulnerability in saitoha libsixel. EPSS estimates a 0.22% chance of exploitation in the next 30 days, higher than 12.2% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 14-04-2026
   PublishedVulnerability first disclosed.
2. 15-04-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-122Heap-based Buffer OverflowA 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().
• CWE-190Integer Overflow or WraparoundThe product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.