Bug 2458527 (CVE-2026-33021)

Summary: CVE-2026-33021 libsixel: libsixel: Use-after-free vulnerability allows for potential code execution via premature pixel buffer freeing.
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
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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, a SIXEL encoder/decoder implementation. An attacker who controls incoming frames can exploit a use-after-free vulnerability. This occurs because a caller-owned pixel buffer is prematurely freed during a resize operation, leaving a dangling pointer. This can lead to a reliable crash and has the potential for arbitrary code execution.
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Bug Depends On: 2458614    
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Description OSIDB Bzimport 2026-04-14 23:01:44 UTC 
libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain a use-after-free vulnerability in sixel_encoder_encode_bytes() because sixel_frame_init() stores the caller-owned pixel buffer pointer directly in frame->pixels without making a defensive copy. When a resize operation is triggered, sixel_frame_convert_to_rgb888() unconditionally frees this caller-owned buffer and replaces it with a new internal allocation, leaving the caller with a dangling pointer. Any subsequent access to the original buffer by the caller constitutes a use-after-free, confirmed by AddressSanitizer. An attacker who controls incoming frames can trigger this bug repeatedly and predictably, resulting in a reliable crash with potential for code execution. This issue has been fixed in version 1.8.7-r1.