CVE-2026-110601 in libsixel
Summary
by MITRE • 10/14/2026
libsixel before 1.8.7-r1 and 1.9.0 through 1.10.5 contains a heap out-of-bounds write vulnerability in gif_init_frame() in src/fromgif.c, which never checks the GIF transparent color index against the palette size. Attackers can supply a crafted GIF with a small color table and large transparent index, processed with a background color set, to write bytes past the palette buffer, causing heap corruption or crashes.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability identified in libsixel versions prior to 1.8.7-r1 and within the range of 1.9.0 through 1.10.5 represents a critical memory safety failure located specifically within the gif_init_frame function found in the src/fromgif.c source file. This component is responsible for parsing Graphics Interchange Format files, a common image format that utilizes indexed color palettes to define visual data efficiently. The core technical flaw stems from an insufficient validation mechanism when processing the transparent color index attribute of a GIF file. During the initialization phase of frame handling, the software fails to verify whether the specified transparent color index falls within the bounds of the defined palette size. This oversight allows for a heap out-of-bounds write condition, where memory operations are executed beyond the allocated boundaries of the palette buffer.
From an architectural perspective, this defect aligns with CWE-787, which classifies out-of-bounds writes as a severe category of memory corruption vulnerabilities. When an attacker supplies a maliciously crafted GIF file characterized by a deliberately small color table paired with a transparent index value that exceeds the length of that table, the application proceeds to write data into unallocated or adjacent heap memory regions. This behavior bypasses standard boundary checks, leading directly to heap corruption. The consequences of such memory manipulation are severe and unpredictable, potentially resulting in application crashes due to segmentation faults or more sophisticated exploitation scenarios where an attacker might achieve arbitrary code execution by overwriting critical heap metadata or function pointers within the process address space.
The operational impact of this vulnerability is significant for any system relying on libsixel for image processing tasks involving GIF files. Since libraries like libsixel are often integrated into larger applications such as terminal emulators, web browsers, or document viewers that handle user-supplied media, an attacker could trick a victim into opening a specially constructed GIF file. This action would trigger the vulnerable code path during rendering or conversion processes. The resulting heap corruption can destabilize the entire application environment, leading to denial of service conditions where legitimate users are unable to access services due to frequent crashes. In worst-case scenarios involving modern memory protection mitigations that have been bypassed through specific exploitation techniques, this flaw could serve as a foothold for further system compromise.
To mitigate these risks, immediate action is required by updating the libsixel library to version 1.8.7-r2 or later, where the boundary checks for palette indices have presumably been corrected. For systems unable to update immediately due to dependency constraints, implementing input validation at the application layer can provide a temporary defense-in-depth measure. Developers should ensure that any GIF parsing routines validate index values against declared palette sizes before performing memory writes. Furthermore, organizations should monitor security advisories from upstream maintainers for patches addressing this specific issue in the 1.9.x and 1.10.x branches. Enabling compiler-based protections such as stack canaries and heap hardening features like ASAN during development phases can also help detect similar logical errors earlier in the software lifecycle, reducing the likelihood of such vulnerabilities reaching production environments.