CVE-2026-33021: saitoha libsixel Use After Free

Vulnerability summary

CVE-2026-33021 is a high (CVSS 7.3) Use After Free vulnerability in saitoha libsixel. EPSS estimates a 0.21% chance of exploitation in the next 30 days, higher than 10.7% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 14-04-2026
   PublishedVulnerability first disclosed.
2. 16-04-2026
   Last modifiedVulnerability information updated.

Description

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->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.

• CWE-416Use After FreeThe product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.