CVE-2026-33018: saitoha libsixel Use After Free

Vulnerability summary

CVE-2026-33018 is a high (CVSS 7.0) Use After Free vulnerability in saitoha libsixel. EPSS estimates a 0.19% chance of exploitation in the next 30 days, higher than 8.4% 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 via the load_gif() function in fromgif.c, where a single sixel_frame_t object is reused across all frames of an animated GIF and gif_init_frame() unconditionally frees and reallocates frame->pixels between frames without consulting the object's reference count. Because the public API explicitly provides sixel_frame_ref() to retain a frame and sixel_frame_get_pixels() to access the raw pixel buffer, a callback following this documented usage pattern will hold a dangling pointer after the second frame is decoded, resulting in a heap use-after-free confirmed by ASAN. Any application using sixel_helper_load_image_file() with a multi-frame callback to process user-supplied animated GIFs is affected, with a reliable crash as the minimum impact and 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 Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.