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A vulnerability present in libpng for nearly 30 years has been identified as a potential risk to millions of systems.
The flaw, a heap buffer overflow, can cause application crashes and, under certain conditions, may allow arbitrary code execution when processing specially crafted PNG images.
“In the worst case scenario, with proper heap grooming, an attacker could read and write up to num_palette / 2 bytes past the end of the index_to_palette and palette_to_index buffers, potentially leading to information disclosure or arbitrary code execution,” said researchers.
How the Libpng Vulnerability Works
Libpng is the official PNG reference library and is widely embedded in operating systems, browsers, graphics applications, and embedded devices.
Because of this widespread integration, vulnerabilities in libpng can have broad downstream impact.
The vulnerability (CVE-2026-25646) has existed since the original implementation of the affected function — then called png_set_dither() — meaning all historical versions of libpng are affected.
The flaw is a heap buffer overflow in the png_set_quantize() function, which reduces the number of colors in an image to match display constraints.
The vulnerability stems from a logic error in how color indices are handled during the quantization process.
It is triggered when a PNG image contains a PLTE (palette) chunk but no hIST (histogram) chunk, the calling application requests color quantization, and the palette includes more than twice the number of colors supported by the user’s display.
Importantly, all of these conditions are valid under the PNG specification, meaning a malicious image does not need to violate format rules to exploit the bug.
Internally, the function builds a hash table that groups similar colors using a color distance metric.
The failure occurs because the code stores current color indices when populating this table but later treats them as original indices during validation.
This mismatch prevents the algorithm from properly pruning palette entries, causing it to enter an infinite loop.
As the loop continues, the search variable max_d increments beyond the fixed size of the hash table — 769 pointers — forcing reads past the end of a heap-allocated buffer.
In the most likely scenario, this results in a deterministic denial-of-service crash when the application processes a crafted PNG file.
However, the researchers noted that with careful heap grooming, attackers may be able to read or write up to num_palette / 2 bytes beyond allocated buffers, creating the potential for information disclosure or arbitrary code execution.
Proof-of-concept code has been released for the vulnerability.
Reducing Risk from the Libpng Flaw
Organizations should approach this vulnerability as both a patching issue and a broader supply chain risk.
Because libpng is widely embedded across applications and platforms, remediation requires visibility into dependencies and runtime protections.
- Patch to libpng version 1.6.55, rebuild affected applications, and verify patched library versions across all managed systems and embedded deployments.
- Conduct a comprehensive dependency review, including SBOM validation, to identify bundled or statically linked versions of libpng within third-party and internally developed software.
- Isolate and sandbox image processing components, especially in web-facing or user-upload workflows, to limit the blast radius of potential exploitation.
- Enable compiler and runtime memory protections such as ASLR, stack canaries, and endpoint exploit detection to reduce the likelihood of successful heap corruption attacks.
- Restrict and validate image uploads by enforcing server-side file checks, re-encoding or sanitizing PNG files, and limiting unnecessary quantization features where feasible.
- Enhance monitoring for abnormal PNG parsing behavior, application crashes, or suspicious memory activity associated with image processing services.
- Test incident response plans with tabletop exercises around vulnerable third-party libraries.
These steps help limit exposure and strengthen resilience.
CVE-2026-25646 highlights the enduring risk within widely trusted open-source components and the systemic exposure created by deeply embedded libraries.
A vulnerability that remained undiscovered for decades in a core image-processing library reinforces the need for continuous dependency visibility and disciplined update governance.
These risks further reinforce the value of zero-trust architectures that assume no component — internal or external — should be inherently trusted.
