CVE-2026-28231: Pillow-heif Info Disclosure Vulnerability

CVE-2026-28231 Overview

CVE-2026-28231 is an integer overflow vulnerability in pillow_heif, a Python library for working with HEIF images and serving as a plugin for Pillow. Prior to version 1.3.0, an integer overflow in the encode path buffer validation of _pillow_heif.c allows an attacker to bypass bounds checks by providing large image dimensions, resulting in a heap out-of-bounds read (CWE-125). This vulnerability can lead to information disclosure where server heap memory leaks into encoded images, or denial of service through process crashes. Notably, no special configuration is required to trigger this vulnerability — it occurs under default settings.

Critical Impact

Attackers can exploit this integer overflow to leak sensitive heap memory into encoded images or crash the application, affecting confidentiality and availability of systems processing HEIF images.

Affected Products

• bigcat88 pillow-heif versions prior to 1.3.0

Discovery Timeline

• 2026-02-27 - CVE CVE-2026-28231 published to NVD
• 2026-03-04 - Last updated in NVD database

Technical Details for CVE-2026-28231

Vulnerability Analysis

The vulnerability resides in the encode path of _pillow_heif.c, specifically in the buffer validation logic. When processing images with exceptionally large dimensions, the library performs size calculations that can result in an integer overflow. This arithmetic overflow causes the computed buffer size to wrap around to a smaller value than actually required, effectively bypassing the bounds checks designed to prevent memory access violations.

Once the bounds check is circumvented, subsequent memory operations read beyond the allocated heap buffer boundaries. The out-of-bounds read (CWE-125) allows the library to access adjacent heap memory that may contain sensitive data from previous allocations. This data can then be inadvertently incorporated into the encoded output image, creating an information disclosure vector.

The attack is particularly concerning because it requires no special configuration — the vulnerability triggers under default library settings when processing maliciously crafted image dimensions through the standard encoding workflow.

Root Cause

The root cause is insufficient integer overflow protection in the buffer size calculations within _pillow_heif.c. When large image dimension values are multiplied to calculate the required buffer size, the result can exceed the maximum value representable by the integer type, causing it to wrap around to a small positive number. This bypasses subsequent size validation checks that compare the calculated size against safety thresholds.

Attack Vector

An attacker can exploit this vulnerability by providing a specially crafted image with large dimension values to any application using pillow_heif for HEIF encoding. The attack is network-accessible when the vulnerable library is used in web applications or services that accept user-uploaded images for processing.

The exploitation flow involves:

1. Crafting an image with dimension values chosen to trigger integer overflow during buffer size calculation
2. Submitting the malicious image to a service using pillow_heif for encoding
3. The integer overflow causes allocation of an undersized buffer
4. Subsequent read operations access memory beyond buffer boundaries
5. Leaked heap data may be embedded in the output image, or the process may crash

Since no special configuration is required and this triggers under default settings, any deployment of pillow_heif versions prior to 1.3.0 that processes untrusted image input is potentially vulnerable.

Detection Methods for CVE-2026-28231

Indicators of Compromise

• Unexpected application crashes in image processing workflows with large dimension HEIF images
• Anomalous memory consumption patterns during HEIF encoding operations
• Corrupted or unusually large encoded image outputs
• Process termination signals (SIGSEGV, SIGBUS) originating from pillow_heif components

Detection Strategies

• Monitor application logs for crashes or segmentation faults during image processing operations
• Implement input validation to check image dimensions before passing to pillow_heif encoding functions
• Use memory sanitizers (AddressSanitizer) in development environments to detect out-of-bounds access
• Deploy runtime application self-protection (RASP) solutions to detect memory corruption attempts

Monitoring Recommendations

• Enable crash dump collection and analysis for applications using pillow_heif
• Set up alerts for abnormal memory access patterns in image processing services
• Monitor for unusually large image dimension values in incoming requests
• Track pillow_heif version deployment across environments to identify vulnerable instances

How to Mitigate CVE-2026-28231

Immediate Actions Required

• Upgrade pillow_heif to version 1.3.0 or later immediately
• Audit all applications and services using pillow_heif to identify vulnerable deployments
• Implement input validation to reject images with excessively large dimensions before processing
• Consider temporarily disabling HEIF encoding functionality if immediate upgrade is not possible

Patch Information

The vulnerability has been fixed in pillow_heif version 1.3.0. The patch addresses the integer overflow in the encode path buffer validation logic within _pillow_heif.c. Organizations should upgrade to this version or later to remediate the vulnerability.

The fix is available in commit 8305a15d3780c533b762578cbe987d27a2c59c7a. Additional details can be found in the GitHub Security Advisory GHSA-5gjj-6r7v-ph3x and the v1.3.0 release notes.

Workarounds

• Implement pre-processing validation to reject images with dimensions that could trigger integer overflow
• Apply resource limits on image processing services to constrain maximum allowable dimensions
• Isolate HEIF processing in sandboxed environments to limit impact of potential exploitation
• Use Web Application Firewalls (WAF) with rules to filter requests containing excessively large image parameters

# Upgrade pillow_heif to patched version
pip install --upgrade pillow-heif>=1.3.0

# Verify installed version
pip show pillow-heif | grep Version