CVE-2026-66759: Gimp: out-of-bounds read in file-icns plugin causes information disclosure or crash on crafted icns images

A flaw was found in the file-icns plugin in GIMP, affecting versions 2.99.14 and newer. When applying a decompressed mask during ICNS image processing, the plugin reads from the mask data buffer without verifying if the cursor exceeds the allocated resource size. If a crafted file contains a truncated mask resource, the icnsdecompress function continues reading past the bounds of the buffer. This out-of-bounds read vulnerability results in information disclosure of heap contents, where memory contents are leaked as alpha channel pixel values, or a crash leading to a denial of service if unmapped memory is accessed.