Libsixel CVE-2026-33021

Local attacker or application using libsixel 1.8.7 or earlier must call sixel_encoder_encode_bytes() with a pixel buffer, trigger a resize operation via sixel_frame_convert_to_rgb888(), then access the original buffer. No user interaction required; requires vulnerable code path execution.

The CVSS v3.1 score of 7.3 (AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H) indicates a local attack vector with low complexity requiring no privileges or user interaction, delivering high availability impact and low confidentiality/integrity impact. While the local-only attack vector limits remote exploitation scenarios, the PR:N (no privileges required) component is significant-any local process or user can trigger the vulnerability without special permissions. The 'RCE' tag suggests potential for code execution beyond simple denial-of-service, though this would require precise heap manipulation. No public exploit code is identified at time of analysis, and the vulnerability is not listed in CISA KEV, indicating no confirmed active exploitation in the wild. However, the technical description notes attackers can trigger the bug 'repeatedly and predictably,' suggesting high reliability for weaponization. The SSVC framework would classify this as having moderate technical impact (local scope limits blast radius) but high automatable potential given the deterministic trigger. Real-world risk depends heavily on deployment context: applications processing untrusted SIXEL images from local users or sandboxed processes face elevated risk, while server-side deployments with strict input controls have lower exposure. The availability of a patch (1.8.7-r1) significantly reduces risk for organizations with robust patch management.

A local attacker with the ability to supply SIXEL image files to a vulnerable application (such as a terminal emulator or image converter) crafts a malicious SIXEL file containing specific frame dimensions designed to trigger resize operations during encoding. When the victim application processes this file through sixel_encoder_encode_bytes(), the library frees the caller's pixel buffer and replaces it internally. The attacker then leverages timing or additional crafted frames to cause the application to access the now-freed memory region, potentially achieving code execution if heap feng shui techniques allow controlled data placement in the freed region, or at minimum causing a reliable application crash for denial of service.