CVE-2026-25646: LIBPNG Buffer Overflow Vulnerability

CVE-2026-25646 Overview

CVE-2026-25646 is an out-of-bounds read vulnerability in libpng, the widely-used reference library for reading, creating, and manipulating PNG (Portable Network Graphics) raster image files. The vulnerability exists in the png_set_quantize() API function and affects versions prior to 1.6.55.

When the png_set_quantize() function is called with no histogram and the number of colors in the palette exceeds twice the maximum supported by the user's display, certain palettes cause the function to enter an infinite loop that reads past the end of an internal heap-allocated buffer. Critically, the PNG images that trigger this vulnerability are valid according to the PNG specification, making detection more challenging.

Critical Impact

This heap-based buffer overflow (CWE-122) can lead to denial of service through infinite loops, potential information disclosure via out-of-bounds memory reads, and possible application crashes when processing maliciously crafted PNG images.

Affected Products

• libpng versions prior to 1.6.55
• Applications using the png_set_quantize() API function
• Systems processing user-supplied PNG images with color quantization enabled

Discovery Timeline

• 2026-02-10 - CVE CVE-2026-25646 published to NVD
• 2026-02-11 - Last updated in NVD database

Technical Details for CVE-2026-25646

Vulnerability Analysis

This vulnerability is classified as a heap-based buffer overflow (CWE-122) that manifests as an out-of-bounds read condition. The root of the issue lies in the png_set_quantize() function's handling of color palettes when no histogram is provided.

The function is designed to reduce the number of colors in a PNG image to match display capabilities. However, when processing palettes where the color count significantly exceeds (more than double) the user-specified maximum, the algorithm fails to properly validate buffer boundaries. This leads to memory being read beyond the allocated heap buffer while the function becomes trapped in an infinite loop.

The attack surface is particularly concerning because the triggering conditions use valid PNG images per the specification—standard image validation would not flag these files as malformed.

Root Cause

The vulnerability stems from insufficient bounds checking in the png_set_quantize() function when processing color palettes without a histogram. The function's loop control logic does not properly account for edge cases where the palette size ratio triggers continuous iteration past the allocated buffer boundaries.

Attack Vector

This vulnerability can be exploited via network delivery of malicious PNG files. An attacker can craft a PNG image with a specific color palette configuration that, when processed by an application calling png_set_quantize(), triggers the vulnerable code path. Exploitation requires no authentication or user interaction beyond the application processing the PNG file.

Attack scenarios include:

• Uploading malicious PNG files to web applications that perform server-side image processing
• Sending PNG attachments to applications with automatic image rendering
• Embedding PNG images in documents processed by vulnerable applications

The following patch was applied to address this vulnerability:

/* pngrtran.c - transforms the data in a row for PNG readers
 *
- * Copyright (c) 2018-2025 Cosmin Truta
+ * Copyright (c) 2018-2026 Cosmin Truta
 * Copyright (c) 1998-2002,2004,2006-2018 Glenn Randers-Pehrson
 * Copyright (c) 1996-1997 Andreas Dilger
 * Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.

Source: GitHub Commit Update

The complete patch includes proper bounds validation in the png_set_quantize function to prevent the out-of-bounds read and infinite loop conditions. For full technical details, see the GitHub Security Advisory.

Detection Methods for CVE-2026-25646

Indicators of Compromise

• Application processes hanging or consuming excessive CPU when processing PNG files
• Memory access violations or segmentation faults during PNG image processing
• Abnormal memory consumption patterns in image processing services
• Process crashes with stack traces referencing png_set_quantize() or related libpng functions

Detection Strategies

• Monitor application logs for crashes or hangs during PNG processing operations
• Implement static analysis scanning to identify libpng version in deployed applications
• Use software composition analysis (SCA) tools to inventory libpng dependencies across the environment
• Deploy anomaly detection for CPU-bound processes that may indicate infinite loop exploitation

Monitoring Recommendations

• Enable resource monitoring for services that process user-supplied images
• Set up alerts for abnormal memory read patterns in image processing pipelines
• Monitor for increased crash rates in applications with libpng dependencies
• Implement timeout mechanisms for image processing operations to detect infinite loops

How to Mitigate CVE-2026-25646

Immediate Actions Required

• Upgrade libpng to version 1.6.55 or later immediately
• Audit all applications for dependencies on vulnerable libpng versions
• Consider temporarily disabling color quantization features if patching is delayed
• Implement input validation to restrict PNG color palette sizes before processing

Patch Information

The vulnerability is fixed in libpng version 1.6.55. The patch addresses the bounds checking issue in the png_set_quantize() function. The security fix was committed by Joshua Inscoe and is available through:

• GitHub Commit with Fix
• GitHub Security Advisory
• Openwall OSS Security Discussion

Workarounds

• Avoid using the png_set_quantize() function with user-supplied PNG images until patched
• Implement pre-processing validation to reject PNG images with excessive palette sizes
• Use application-level sandboxing to isolate PNG processing operations
• Deploy resource limits (CPU time, memory) on image processing services to contain infinite loop impact

# Check installed libpng version
pkg-config --modversion libpng

# Update libpng on Debian/Ubuntu systems
sudo apt update && sudo apt install libpng16-16

# Verify the updated version
ldconfig -p | grep libpng