libsixel: Use-after-free in sixel_encoder_encode_bytes()

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Summary

CVECVE-2026-33021
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-04-14 23:16:27 UTC
Updated2026-04-17 15:38:09 UTC
Descriptionlibsixel 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.

Risk And Classification

Primary CVSS: v3.1 7.3 HIGH from [email protected]

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H

EPSS: 0.000190000 probability, percentile 0.049880000 (date 2026-04-21)

Problem Types: CWE-416 | CWE-416 CWE-416: Use After Free

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary7.3HIGHCVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
3.1CNADECLARED7.3HIGHCVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
High

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Saitoha Libsixel affected < 1.8.7-r1 Not specified

References

ReferenceSourceLinkTags
github.com/saitoha/libsixel/releases/tag/v1.8.7-r1 [email protected] github.com
github.com/saitoha/libsixel/security/advisories/GHSA-j6m5-2cc7-3whc [email protected] github.com
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