The vulnerability CVE-2026-28231 affects pillow_heif, a Python library that enables HEIF image processing as a plugin for the Pillow imaging library. The root cause is an integer overflow in the buffer validation logic within the encoding path implemented in the _pillow_heif.c source file. Specifically, when encoding images with very large dimensions, the integer overflow allows the calculation of buffer sizes to wrap around, bypassing bounds checks. This results in a heap out-of-bounds read during the encoding process. The out-of-bounds read can lead to two primary impacts: information disclosure, where heap memory contents may be leaked into the encoded image data, and denial of service, where the process crashes due to invalid memory access. The vulnerability is exploitable remotely without any authentication or user interaction, making it accessible in default configurations. The issue was addressed and fixed in pillow_heif version 1.3.0 by correcting the buffer validation to prevent integer overflow and enforce proper bounds checking. No public exploits or active exploitation campaigns have been reported as of the publication date.

This vulnerability can have significant consequences for organizations that use pillow_heif for HEIF image processing, particularly in web services or applications that accept user-supplied images. The information disclosure risk could expose sensitive server memory contents, potentially leaking confidential data. The denial of service impact could disrupt services by crashing processes handling image encoding, affecting availability. Since exploitation requires no privileges or user interaction, attackers can remotely trigger the vulnerability, increasing the risk of automated attacks. However, the scope is limited to systems using vulnerable pillow_heif versions, which are primarily Python environments processing HEIF images. The medium CVSS score reflects moderate severity due to the combination of information leakage and service disruption without integrity compromise.

To mitigate this vulnerability, organizations should immediately upgrade pillow_heif to version 1.3.0 or later, where the integer overflow and bounds check issues are fixed. Additionally, implement input validation to restrict image dimensions and file sizes before processing to reduce the risk of triggering integer overflows. Employ runtime protections such as memory safety tools (e.g., AddressSanitizer) during development and testing to detect out-of-bounds reads. Monitor application logs for crashes or anomalies related to image processing. If upgrading is not immediately feasible, consider isolating image processing workloads in sandboxed environments to limit impact from potential crashes or data leaks. Finally, maintain an inventory of software dependencies to quickly identify and patch vulnerable components.