{"id":"UBUNTU-CVE-2026-33021","details":"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-\u003epixels 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.","modified":"2026-05-20T16:25:24.038451919Z","published":"2026-04-14T23:16:00Z","upstream":["CVE-2026-33021"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2026-33021"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2026-33021"},{"type":"REPORT","url":"https://github.com/saitoha/libsixel/security/advisories/GHSA-j6m5-2cc7-3whc"},{"type":"REPORT","url":"https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1"}],"affected":[{"package":{"name":"libsixel","ecosystem":"Ubuntu:16.04:LTS","purl":"pkg:deb/ubuntu/libsixel?arch=source&distro=xenial"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.4.2-1","1.5.2-2"],"ecosystem_specific":{"binaries":[{"binary_version":"1.5.2-2","binary_name":"libsixel-bin"},{"binary_name":"libsixel1","binary_version":"1.5.2-2"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-33021.json"}},{"package":{"name":"libsixel","ecosystem":"Ubuntu:18.04:LTS","purl":"pkg:deb/ubuntu/libsixel?arch=source&distro=bionic"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.5.2-2","1.7.3-1"],"ecosystem_specific":{"binaries":[{"binary_name":"libsixel-bin","binary_version":"1.7.3-1"},{"binary_name":"libsixel-examples","binary_version":"1.7.3-1"},{"binary_name":"libsixel1","binary_version":"1.7.3-1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-33021.json"}},{"package":{"name":"libsixel","ecosystem":"Ubuntu:20.04:LTS","purl":"pkg:deb/ubuntu/libsixel?arch=source&distro=focal"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.8.2-1","1.8.2-2.1"],"ecosystem_specific":{"binaries":[{"binary_name":"libsixel-bin","binary_version":"1.8.2-2.1"},{"binary_name":"libsixel-examples","binary_version":"1.8.2-2.1"},{"binary_name":"libsixel1","binary_version":"1.8.2-2.1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-33021.json"}},{"package":{"name":"libsixel","ecosystem":"Ubuntu:22.04:LTS","purl":"pkg:deb/ubuntu/libsixel?arch=source&distro=jammy"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.8.6-2","1.10.3-2","1.10.3-3"],"ecosystem_specific":{"binaries":[{"binary_name":"libsixel-bin","binary_version":"1.10.3-3"},{"binary_version":"1.10.3-3","binary_name":"libsixel-examples"},{"binary_version":"1.10.3-3","binary_name":"libsixel1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-33021.json"}},{"package":{"name":"libsixel","ecosystem":"Ubuntu:24.04:LTS","purl":"pkg:deb/ubuntu/libsixel?arch=source&distro=noble"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.10.3-3","1.10.3-3build1"],"ecosystem_specific":{"binaries":[{"binary_version":"1.10.3-3build1","binary_name":"libsixel-bin"},{"binary_name":"libsixel-examples","binary_version":"1.10.3-3build1"},{"binary_name":"libsixel1","binary_version":"1.10.3-3build1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-33021.json"}},{"package":{"name":"libsixel","ecosystem":"Ubuntu:25.10","purl":"pkg:deb/ubuntu/libsixel?arch=source&distro=questing"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.10.5-1"],"ecosystem_specific":{"binaries":[{"binary_version":"1.10.5-1","binary_name":"libsixel-bin"},{"binary_name":"libsixel-examples","binary_version":"1.10.5-1"},{"binary_version":"1.10.5-1","binary_name":"libsixel1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-33021.json"}},{"package":{"name":"libsixel","ecosystem":"Ubuntu:26.04:LTS","purl":"pkg:deb/ubuntu/libsixel?arch=source&distro=resolute"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.10.5-1","1.10.5-1build1"],"ecosystem_specific":{"binaries":[{"binary_name":"libsixel-bin","binary_version":"1.10.5-1build1"},{"binary_name":"libsixel-examples","binary_version":"1.10.5-1build1"},{"binary_name":"libsixel1","binary_version":"1.10.5-1build1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-33021.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H"},{"type":"Ubuntu","score":"medium"}]}