HDF5 CVE-2025-6818 Heap Overflow: Risks and Remediation for 1.14.6

A heap-based buffer overflow has been publicly disclosed in HDF5 1.14.6 — tracked as CVE-2025-6818 — rooted in the H5O__chunk_protect routine inside src/H5Ochunk.c, creating a locally exploitable crash and potential memory‑corruption vector that defenders must treat seriously in any environment that accepts untrusted .h5 inputs.

Background / Overview​

HDF5 (Hierarchical Data Format v5) is a ubiquitous binary container and C library used across scientific computing, engineering pipelines, and many enterprise data workflows to store large arrays, matrices, metadata and chunked datasets. The library is commonly linked directly into data processing tools, high‑performance computing applications, containerized ingestion services, and desktop utilities that read or write HDF5 files. Because it runs inside user‑mode processes that often accept external files, memory‑safety defects in HDF5 can translate directly into denial‑of‑service, data corruption, or — under very favorable circumstances — code execution.
CVE‑2025‑6818 was published in late June 2025 and identifies a heap‑based buffer overflow in the function H5O__chunk_protect (file: src/H5Ochunk.c) of HDF5 1.14.6. Public trackers and vendor advisories describe the attack vector as local and note that an exploit or proof‑of‑concept (PoC) has been disclosed, increasing urgency for exposed systems. Different trackers have produced different severity scores (CVSS v3.x and v4.0 variants), but most agree on the root technical facts and the need for remediation.

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Why this matters: HDF5’s role and the operational impact​

HDF5 is not a niche dependency — it is embedded in scientific Python stacks, MATLAB toolchains, R packages, command‑line utilities (h5dump, h5ls, h5repack), containerized conversion services, and some vendor appliances. A single library vulnerability can therefore cascade across many products and deployment models.

• Data ingestion and preview services that automatically open or parse uploaded HDF5 files are high‑risk because an unauthenticated upload can remotely result in local processing of malformed content (server‑side attack surface).
• Desktop applications that open HDF5 files still present local‑attack risk: tricking an operator to open a malicious .h5 file can provoke crashes or local exploitation.
• Statically linked binaries and embedded firmware images that vendor HDF5 make patching harder because you must rebuild and redeploy artifacts, not just update an OS package.

The practical worst‑case outcomes are: immediate denial‑of‑service, silent data corruption, or — if heap corruption can be chained with other weaknesses or predictable allocator behavior — escalation toward code execution. Public PoC material that reproduces a crash is already available, which reduces the time to weaponization for opportunistic attackers and automated scanners.

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Technical anatomy: H5Ochunk.c and H5O__chunk_protect​

What the function does (high level)​

The HDF5 library organizes large datasets as chunks for efficient I/O and storage. The H5O__chunk_protect routine is part of chunk handling and protection semantics: it deals with locating and protecting chunk objects in the object metadata cache during read/write operations. When chunk metadata or chunk allocation/lookup logic accepts length or index values from file headers, the code must carefully validate sizes and boundaries before memory allocation or copying.

The bug class​

CVE‑2025‑6818 is described consistently as a heap‑based buffer overflow (CWE‑122 / CWE‑119 variants in vendor trackers). At a high level, the implementation failed to ensure that a copy or write into a heap buffer could not exceed the allocated size, permitting an overwrite of adjacent heap memory. Heap overflows are dangerous because they can corrupt allocator metadata, vtables/pointers, or adjacent object state — the exact outcome depends heavily on the platform, allocator, and runtime mitigations in place.

Exploitability nuance​

• The published entries indicate the exploit requires local access and low privileges in most tracker assessments, meaning an attacker who can influence an HDF5‑consuming process (for example, via file upload to a background ingestion service, or by convincing a user to open a file) can trigger the condition.
• Public PoC(s) demonstrating crashes exist; several vulnerability databases mark the PoC as available or the exploit as known. PoC presence increases detection and triage speed but also lowers the bar for attackers.
• Turning a heap overflow into reliable remote code execution (RCE) is not automatic. It depends on allocator behavior, ASLR, hardened allocators, compiler hardening flags, DEP/NX, PIE/RELRO, and whether the vulnerable process can be groomed into a favorable memory layout. Many vendor writeups caution that RCE is speculative without further proof. Treat RCE claims cautiously until independently reproduced.

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Scoring, vendor assessments, and discrepancies​

When a vulnerability is catalogued across multiple feeds, you will often see variance in CVSS scores and priority assignments. That is the case for CVE‑2025‑6818:

• NVD (National Vulnerability Database) shows the canonical CVE entry and indicates the heap overflow in H5O__chunk_protect; their entry records the public details but was not always fully enriched with a single CVSS vector at initial capture.
• Several vendor and distro trackers provide concrete CVSS values:
• Ubuntu’s security page lists CVSS v3.1 = 3.3 (Low) and marks the issue as low priority in some Ubuntu release contexts.
• SUSE lists a CVSS v3.1 value of 3.3 (Low) and calls the issue resolved in its distributions.
• Multiple security vendors (Snyk, Wiz, AquaSec/OpenCVE) list CVSS v4.0 ≈ 4.8 (Medium) and CVSS v3.1 ≈ 3.3 (Low), and characterize the vector as local with low privileges required.

Why the differences? Vendors re‑score based on packaging context and expected exposure. If a distribution ships the library only in non‑exposed images, or the vulnerable code path requires specific compile‑time options or dataset features, a vendor will often downscore the urgency. Conversely, upstream or security vendors focusing on raw exploitability may emphasize PoC presence and potential for DoS, producing higher urgency.
Operational takeaway: prioritize remediation by exposure, not by a single numeric score. Treat internet‑facing HDF5 ingestion, automated decode/preview services, and containerized pipelines as higher priority despite discrepant CVSS numbers.

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Affected versions and packaging status​

• Confirmed affected: HDF5 1.14.6 (the version explicitly named in multiple entries).
• Distribution packaging: multiple ecosystems flagged the issue; Ubuntu and SUSE published tracking pages with varying priorities and remediation status. Packagers and integrators should verify whether a given distribution package includes the upstream commit that fixes the vulnerability.
• Static linking caveat: many downstream applications statically embed HDF5. For those artifacts you must rebuild and redeploy the binary with the patched library; updating a system package is insufficient for statically linked consumers. Community trackers and distro notes reinforce rebuilding static deliverables as part of remediation.

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Evidence, PoC status and confidence in the technical details​

Multiple trackers report a publicly disclosed PoC or exploit evidence that reproduces a crash in HDF5 1.14.6 involving H5O__chunk_protect. Public PoCs materially reduce the lead time for attackers and increase the practical urgency for internet‑facing services and batch processing pipelines that automatically consume untrusted .h5 files. However, independent verification of a working RCE exploit chain for this specific CVE has not been widely published at the time of disclosure — treat claims of reliable RCE as unverified until multiple trusted sources demonstrate them. Confidence assessment:

• Existence of the bug: High — multiple vendor/distro trackers and NVD list the CVE and the vulnerable function.
• PoC availability: High — PoC/crash artifacts are reported in several trackers.
• Likelihood of immediate RCE in modern hardened environments: Low to Medium — possible with the right conditions but not yet demonstrated publicly; assume DoS as the immediate practical threat.

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Practical mitigation and remediation playbook​

Immediate triage and medium‑term remediation should be practical and prioritized by exposure.

Quick emergency steps (first 24–72 hours)​

• Inventory: identify all binaries, containers, and systems that include HDF5 1.14.6. Search package manifests, SBOMs, container images, wheels, and static link artifacts. Prioritize internet‑facing services and automated processing nodes.
• Isolate: stop or sandbox automated processing of untrusted HDF5 files. Move decoding to an isolated processing tier (container or VM) with strict resource limits and least privilege.
• Apply vendor patches: where an OS/package vendor has shipped an update that addresses CVE‑2025‑6818, apply and validate that package on exposed nodes. Check changelogs for the upstream commit SHA or specific CVE mention to ensure the fix is included.
• Rebuild statically linked artifacts: if your product vendors HDF5 statically, rebuild binaries against a patched HDF5 and redeploy. Do not assume package updates will fix statically embedded libraries.
• Monitor: enable crash and core‑dump collection for HDF5‑linked processes; set alerts for unusual crash spikes or elevated restart counts.

Medium‑term and hardening measures​

• Add input validation: enforce strict upper bounds on chunk sizes and dataset metadata in any code that calls into HDF5, rejecting suspicious or unexpected parameters before handing them to the library.
• CI/CD: integrate address sanitizer (ASAN), memory sanitizers, and fuzzing for any code paths that parse or write .h5 files. Automate regression checks for historical memory‑safety classes of bugs.
• SBOM and supply‑chain hygiene: ensure every build artifact has an SBOM so you can rapidly identify vulnerable images and statically linked binaries.
• Run processing in dedicated, minimal‑privilege containers with restricted filesystem access and seccomp/AppArmor/SELinux profiles.

Detection guidance​

• Look for reproducible crash signatures in core dumps that indicate heap corruption originating from H5Ochunk.c or H5O__chunk_protect.
• Create detection rules that flag uploads or file processing events with suspicious chunk‑metadata values or repeated failure patterns while parsing HDF5 chunks.
• Deploy runtime integrity monitoring around processes that perform large numbers of HDF5 opens (e.g., ingestion daemons, preview generators).

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Risk analysis: strengths and remaining risks​

Strengths defenders can leverage:

• The vulnerability is catalogued publicly and PoCs exist, which helps defenders reproduce and validate fixes. That makes detection and patch validation tractable.
• Upstream and downstream packaging teams have been active in triage and commits; patched artifacts either exist or can be produced by merging the upstream fix.

Residual risks and weaknesses:

• Static linking and long‑tail deployments: many scientific images, embedded devices, and legacy toolchains may not be updated quickly, producing a prolonged attack surface.
• PoC availability increases the chance of opportunistic attacks and automated scanning against exposed HDF5 services.
• Claims of RCE remain speculative for this specific CVE; however, heap-based corruption is a high‑value primitive and should be treated as a severe risk in critical environments until proven otherwise.

Flagged uncertainties:

• If any public writeups assert reliable remote RCE from CVE‑2025‑6818, treat those claims as unverified until corroborated by multiple, independent technical analyses or vendor confirmation. The current public record emphasizes DoS and heap corruption first.

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Checklist for ops, devs and security teams (actionable)​

• Inventory every HDF5 binary, library and container (system packages, wheels, R wheels, conda packages, statically linked ELF/PEs).
• For internet‑facing ingestion services: disable automatic decode/preview for .h5 until fixed binaries are deployed.
• Apply vendor patches where available; verify the package changelog or upstream commit SHA to confirm the fix covers H5O__chunk_protect or the specific CVE.
• Rebuild and redeploy statically linked artifacts with patched HDF5.
• Add monitoring for crashes, core dumps and unusual process restarts; collect and retain suspect sample files offline for analysis.
• Add targeted input bounds checks and deny lists for malformed chunk metadata in caller code.
• Run fuzzers and ASAN in CI for HDF5‑consuming components and add test cases based on the public PoC.

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Conclusion​

CVE‑2025‑6818 is a real, publicly disclosed heap‑based buffer overflow in HDF5 1.14.6’s chunk handling path (H5O__chunk_protect). Multiple authoritative trackers and vendor pages confirm the vulnerability, and PoC evidence is reported — making this an operational concern for any organization that processes untrusted HDF5 data or vendors static HDF5‑linked binaries. While immediate exploitation appears to yield denial‑of‑service and heap corruption in most environments, the presence of a heap overflow in a widely embedded library justifies a conservative remediation posture: inventory, isolate, patch/rebuild, and harden. Prioritize internet‑facing ingestion services and automated processing tiers, and treat any claims of trivial RCE as unverified until proven by independent technical analysis.

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Source: MSRC Security Update Guide - Microsoft Security Response Center