High
libsixel — security advisory
libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain an integer overflow which leads to a heap buffer overflow via sixel_frame_convert_to_rgb888() in frame.c, where allocation size and pointer offset computations for palettised images (PAL1, PAL2, PAL4) are performed using int arithmetic before casting to size_t. For images whose pixel count exceeds INT_MAX / 4, the overflow produces an undersized heap allocation for the conversion buffer and a negative pointer offset for the normalization sub-buffer, after which sixel_helper_normalize_pixelformat() writes the full image data starting from the invalid pointer, causing massive heap corruption confirmed by ASAN. An attacker providing a specially crafted large palettised PNG can corrupt the heap of the victim process, resulting in a reliable crash and potential arbitrary code execution. This issue has been fixed in version 1.8.7-r1.
- Products
- libsixel
- Affected versions
- 1.10.3-2, 1.10.3-3, 1.10.3-3build1, 1.10.5-1, 1.10.5-1build1, 1.4.2-1, 1.5.2-2, 1.7.3-1, 1.8.2-1, 1.8.2-2.1, 1.8.6-2, 1:1.8.7-r2-1
- Fixed versions
- Not specified
- CVEs
- CVE-2026-33020
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 02:59 UTC · Revision 1
Source evidence
libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain an integer overflow which leads to a heap buffer overflow via sixel_frame_convert_to_rgb888() in frame.c, where allocation size and pointer offset computations for palettised images (PAL1, PAL2, PAL4) are performed using int arithmetic before casting to size_t. For images whose pixel count exceeds INT_MAX / 4, the overflow produces an undersized heap allocation for the conversion buffer and a negative pointer offset for the normalization sub-buffer, after which sixel_helper_normalize_pixelformat() writes the full image data starting from the invalid pointer, causing massive heap corruption confirmed by ASAN. An attacker providing a specially crafted large palettised PNG can corrupt the heap of the victim process, resulting in a reliable crash and potential arbitrary code execution. This issue has been fixed in version 1.8.7-r1.
osv:UBUNTU-CVE-2026-33020
libsixel
Fixed versions: Not specified
libsixel
Fixed versions: Not specified
libsixel
Fixed versions: Not specified
libsixel
Fixed versions: Not specified
libsixel
Fixed versions: Not specified
libsixel
Fixed versions: Not specified
libsixel
Fixed versions: Not specified
libsixel
Fixed versions: Not specified
Complete source record
{
"id": "74f55f1bc789c6d9e18af86c665ce2dfe75f316cb51676d37a5b079c109c1546",
"url": "https://osv.dev/vulnerability/UBUNTU-CVE-2026-33020",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "UBUNTU-CVE-2026-33020",
"summary": "libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain an integer overflow which leads to a heap buffer overflow via sixel_frame_convert_to_rgb888() in frame.c, where allocation size and pointer offset computations for palettised images (PAL1, PAL2, PAL4) are performed using int arithmetic before casting to size_t. For images whose pixel count exceeds INT_MAX / 4, the overflow produces an undersized heap allocation for the conversion buffer and a negative pointer offset for the normalization sub-buffer, after which sixel_helper_normalize_pixelformat() writes the full image data starting from the invalid pointer, causing massive heap corruption confirmed by ASAN. An attacker providing a specially crafted large palettised PNG can corrupt the heap of the victim process, resulting in a reliable crash and potential arbitrary code execution. This issue has been fixed in version 1.8.7-r1.",
"evidence": "osv:UBUNTU-CVE-2026-33020",
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"aliases": [
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"modified": "2026-10-17T12:59:58.534381265Z",
"provider": "osv",
"upstream": [
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"references": [
"https://ubuntu.com/security/CVE-2026-33020",
"https://www.cve.org/CVERecord?id=CVE-2026-33020",
"https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p",
"https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1"
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"security_relevant": true,
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"source_id": "core-osv",
"document_id": 360050,
"published_at": "2026-04-14T22:16:00+00:00"
}