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CVE-2026-33021 is a use-after-free vulnerability in libsixel affecting version 1.8.7 and earlier. The flaw is in the encoder path, specifically sixel_encoder_encode_bytes(), due to unsafe ownership handling of the pixel buffer. During initialization, sixel_frame_init() stores a caller-supplied pixel buffer pointer directly in frame->pixels instead of making a defensive copy or otherwise preserving clear ownership semantics. If a resize operation occurs later, sixel_frame_convert_to_rgb888() unconditionally frees that buffer and replaces it with a newly allocated internal buffer. Because the original allocation is still assumed by the caller to remain valid, subsequent caller access to the original pixel buffer dereferences freed memory. The issue was confirmed with AddressSanitizer. According to the provided content, an attacker able to control incoming frames can trigger the condition repeatedly and predictably, causing a reliable crash and potentially enabling code execution.
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