CVE-2025-6856: Hdfgroup HDF5 Use-After-Free Vulnerability

CVE-2025-6856 Overview

CVE-2025-6856 is a use-after-free vulnerability in HDF5 version 1.14.6, a widely deployed data model and file format library used in scientific computing. The flaw resides in the H5FL__reg_gc_list function in src/H5FL.c, which manages free-list garbage collection for regularly sized memory blocks. An attacker with local access and low privileges can trigger memory corruption by manipulating input processed by the affected function. The exploit has been publicly disclosed, though no active exploitation has been reported. The issue is tracked under [CWE-119] (improper restriction of operations within memory buffer bounds).

Critical Impact
Local attackers can trigger a use-after-free condition in HDF5 1.14.6, potentially causing application crashes or limited availability impact in processes that parse untrusted HDF5 data.

Affected Products

HDF Group HDF5 version 1.14.6
Applications embedding the affected HDF5 library
Scientific computing stacks and data pipelines relying on HDF5 1.14.6

Discovery Timeline

2025-06-29 - CVE-2025-6856 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-6856

Vulnerability Analysis

The vulnerability affects the H5FL__reg_gc_list function, which is part of HDF5's free-list memory management subsystem. This function walks a list of regularly sized memory blocks queued for garbage collection and releases them back to the allocator. A use-after-free condition arises when the function operates on a memory region that has already been released, allowing subsequent memory operations to reference invalid pointers.

Exploitation requires local access to a system processing attacker-influenced HDF5 files or data streams. The observed impact is limited to availability, consistent with crashes rather than code execution. Public references include a crash report and issue thread on the HDF Group GitHub repository, documenting the fault path.

The EPSS score is 0.203%, indicating low predicted exploitation likelihood.

Root Cause

The root cause is improper memory lifetime management within the free-list garbage collection routine. H5FL__reg_gc_list dereferences or continues to reference a memory block after it has been freed, violating the invariants expected by the allocator. This class of defect is classified under [CWE-119], improper restriction of operations within the bounds of a memory buffer.

Attack Vector

The attack vector is local. An attacker must be able to invoke or influence a program that calls into HDF5's free-list subsystem, typically by supplying a crafted HDF5 file or by triggering allocation patterns that force garbage collection over freed regions. User interaction is not required beyond initiating processing of the malicious input.

No public proof-of-concept exploit code has been verified. Refer to the GitHub Issue #5574 and the HDF5 Crash Report for technical reproduction details.

Detection Methods for CVE-2025-6856

Indicators of Compromise

Repeated crashes or segmentation faults in processes linked against libhdf5 version 1.14.6
Core dumps referencing H5FL__reg_gc_list or nearby free-list routines in H5FL.c
Unexpected termination of scientific workloads shortly after opening untrusted .h5 or .hdf5 files

Detection Strategies

Inventory hosts and containers running HDF5 1.14.6 using software composition analysis or package manifests
Enable AddressSanitizer or Valgrind in development and test builds to surface use-after-free conditions during HDF5 parsing
Monitor endpoint telemetry for abnormal termination of processes known to link HDF5 libraries

Monitoring Recommendations

Alert on unexpected crash signatures involving HDF5 stack frames captured through OS crash reporting
Track file provenance for HDF5 inputs entering shared analysis environments, flagging files from untrusted sources
Correlate process crash events with recent file open operations targeting HDF5 datasets

How to Mitigate CVE-2025-6856

Immediate Actions Required

Identify all systems, containers, and virtual environments running HDF5 1.14.6 through package inventories
Restrict local access to systems that process HDF5 files from untrusted sources
Validate the origin of HDF5 files before processing them in sensitive workloads
Monitor the HDF Group GitHub repository for an upstream fix and release notes

Patch Information

At the time of NVD publication, no vendor advisory URL is listed for CVE-2025-6856. Track the upstream issue at GitHub Issue #5574 for remediation status and apply a fixed HDF5 release once available. Users of downstream distributions should watch their package maintainer's advisories for a rebuilt library.

Workarounds

Avoid processing HDF5 files supplied by untrusted or unauthenticated users on affected 1.14.6 installations
Isolate HDF5 parsing workloads in sandboxed or containerized environments with restricted privileges
Consider downgrading to a prior stable HDF5 release not affected by this defect, if compatible with application requirements

bash

# Identify HDF5 1.14.6 installations on Linux hosts
dpkg -l | grep -i hdf5
rpm -qa | grep -i hdf5
ldconfig -p | grep libhdf5

# Python environments
pip list | grep -i h5py
conda list | grep -i hdf5