CVE-2026-33020: Saitoha Libsixel buffer overflow (CVSS 7.1)

Description

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.

CVSS 3.1 breakdown

Attack vectorLocal

Attack complexityLow

Privileges requiredNone

User interactionRequired

ScopeUnchanged

ConfidentialityNone

IntegrityHigh

AvailabilityHigh

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

Affected products

Saitoha Libsixel< 1.8.7-r1

Remediation

Fixed in

Saitoha Libsixel≥ 1.8.7-r1

Patch & vendor advisories

https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2p

References

https://github.com/saitoha/libsixel/releases/tag/v1.8.7-r1Release Notes
https://github.com/saitoha/libsixel/security/advisories/GHSA-2xgm-4x47-2x2pExploitVendor Advisory

Related CVEs

Recent high-severity vulnerabilities sharing a weakness type.

Frequently asked

Is CVE-2026-33020 being actively exploited?

CVE-2026-33020 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.22%.

How severe is CVE-2026-33020?

CVE-2026-33020 has a CVSS 3.1 base score of 7.1, rated high. EPSS estimates a 0.22% probability of exploitation.

Which products are affected by CVE-2026-33020?

Affected products include Saitoha Libsixel.

How do I fix CVE-2026-33020?

Upgrade affected software to a fixed version, for example Saitoha Libsixel 1.8.7-r1 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-33020 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

See how prioritisation works Book a demo