A use-after-free defect in the HDF5 C library — tracked as CVE-2025-6856 and rooted in the H5FL__reg_gc_list routine in src/H5FL.c — has been publicly disclosed and confirmed by multiple independent sources; the flaw affects HDF5 1.14.6, a widely embedded library in scientific, engineering, and HPC toolchains, and a public proof‑of‑concept and upstream fixes are now part of the record.
HDF5 is the dominant binary container and library for high‑volume scientific data, used across research institutions, analytics platforms, and commercial products. Memory‑safety problems in the HDF5 codebase are not new: the project has seen multiple CVEs in recent years affecting low‑level memory helpers and filter/serialization code. Those historical incidents help explain why new findings in the 1.14.x line receive rapid attention from packagers and security teams.
Acknowledgement of sources used during reporting: authoritative vulnerability indexes and the upstream HDFGroup issue/PR/commit history were consulted; distribution trackers and vendor advisories were checked for packaging status and severity assessments.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background
HDF5 is the dominant binary container and library for high‑volume scientific data, used across research institutions, analytics platforms, and commercial products. Memory‑safety problems in the HDF5 codebase are not new: the project has seen multiple CVEs in recent years affecting low‑level memory helpers and filter/serialization code. Those historical incidents help explain why new findings in the 1.14.x line receive rapid attention from packagers and security teams.Why this matters to WindowsForum readers
Although HDF5 is often associated with Linux and HPC, the library is compiled and shipped in many cross‑platform packages, Python wheels, and container images. Any service or desktop app that opens, converts, or inspects .h5 files — particularly automated ingestion pipelines, containerized preview services, or statically linked vendor appliances — can be impacted by defects in HDF5. In production, a local vulnerability in a widely linked library can quickly become a remotely‑triggerable risk when untrusted files are processed by servers.The technical summary: what the bug is and where it occurs
- Affected component: HDF5 C library, release 1.14.6.
- Vulnerable function: H5FL__reg_gc_list in src/H5FL.c.
- Class of bug: use‑after‑free (heap use after free), a memory‑corruption condition that can cause crashes, memory leaks, or — in favorable conditions for an attacker — be escalated toward code execution.
- Attack vector: local (the immediate exploit requires local invocation), but may be remotely exploitable in server contexts that automatically parse uploaded .h5 files.
The recorded repro (paraphrased)
The reporter used a sanitised build (AddressSanitizer) and a fuzzing harness to exercise HDF5 internals; ASan output identified:- a READ in H5FL__reg_gc_list at a specific line in src/H5FL.c,
- a corresponding free of that same heap region a few frames earlier in the stack trace,
- and an overall crash path that proceeds through H5FL_garbage_coll and H5_term_library during teardown.
Evidence, disclosure timeline and upstream response
- The CVE was published on June 29, 2025 and is listed in canonical vulnerability indexes. The NVD and other vulnerability aggregators summarize the vulnerability and link to community reports.
- The original issue was opened in the HDFGroup/hdf5 GitHub repository (issue #5574) with sanitizer output and reproduction guidance; the issue explicitly demonstrates a heap‑use‑after‑free at src/H5FL.c:428.
- Upstream developers addressed a set of related object‑header and garbage‑collection bugs in a consolidated pull request (PR #5829) and an associated commit (sha 29c847a43db0cdc85b01cafa5a7613ea73932675) that was merged into the develop branch. The PR description indicates the changes were intended to prevent corrupted object header continuation messages from producing undersized allocations or incorrect deserialization accounting — fixes that also close the H5FL__reg_gc_list use‑after‑free on the tested reproducer.
- Distribution trackers (Debian, Ubuntu, SUSE), third‑party vulnerability databases and security vendors published advisory pages and differing severity assessments while maintainers reviewed and mapped upstream commits to distribution packages. Debian’s security tracker references the GitHub issue and the token commit and notes that the security impact is negligible for some Debian packagings, depending on shipped versions.
Severity, scores and the disagreement you may see
Across trackers you’ll find divergent severity numbers:- Some aggregators show a high CVSS v3.1 score (~7.8), reflecting a conservative posture that treats memory‑corruption primitives as high‑impact even for local vectors.
- Other sources — including vendor / distro pages such as Ubuntu and SUSE — list lower scores (CVSS v3.1 ≈ 3.3 or similar), based on an assessment that remote exploitation is not trivial and the immediate impact is primarily availability.
- CVSS v4.0 and other modern scoring attempts place this in the medium range in some trackers (CVSS v4.0 ≈ 4.8) because the attack vector is local and the immediate confidentiality/integrity impacts are limited in many deployments.
- whether a local vector in a widely linked library equates to a remotely‑triggerable server vector,
- whether the bug’s PoC can realistically be escalated to reliable remote code execution given modern mitigations (ASLR, hardened allocators, RELRO/PIE),
- and whether static linking in vendor binaries increases remediation friction (and that friction often bumps priorities). These subjective choices explain the variance in numeric scores and urgency labels. Treat numeric scores as guidance — examine your deployment context to decide priority.
Exploitability: what attackers can and cannot do today
- Proof‑of‑concept: public PoCs and ASan reproductions exist, which materially shortens the lead time for abuse. The GitHub issue contains reproducible ASan traces and instructions; secondary trackers report additional PoC artifacts. That means defenders should prioritise triage.
- Immediate threats: most realistic immediate outcomes are denial of service (application crash) and unpredictable process behaviour. In managed or multi‑tenant contexts (CI render farms, web ingestors), a crafted .h5 upload can cause worker churn and availability loss.
- RCE? A use‑after‑free is a high‑value primitive in an attacker’s kit — but turning this specific PoC into a reliable remote code‑execution payload depends heavily on process layout, allocator behavior, and other local conditions. No widely‑verified public exploit shows automatic remote RCE from this CVE at the time of public disclosure; treat RCE claims as unverified until independent researchers publish reproducible exploit chains.
Who should care most — prioritized risk map
- High priority: server‑side ingestion services, automated conversion workers, image preview/thumbnailing daemons, and any multi‑tenant infrastructure that accepts or processes .h5 uploads. In such services an unauthenticated upload can become a remote trigger even though the vulnerability itself is categorized as local.
- Medium priority: desktop workstations and research machines that open untrusted .h5 files via GUI utilities. These require user interaction or social engineering to exploit at scale.
- Long‑tail risk: embedded appliances, vendor binaries or static builds that embed HDF5 1.14.6; these are often difficult to patch because they require rebuilds and redeployments across many devices.
Practical remediation and mitigation playbook
Immediate steps for defenders- Inventory and detection
- Locate every compiled binary, package, container image and Python wheel that contains HDF5 1.14.6. Look for static linkages because replacing a shared package may not fix statically linked artifacts. Use your package inventories, container scanning tools, and CI build records to find artifacts.
- Patch if vendor updates exist
- Apply vendor or distribution updates that include the upstream fix or the referenced commit. Debian/Ubuntu trackers and vendor advisories often list package versions and backports; check your vendor channel for distribution-specific guidance.
- If a vendor package is not available, rebuild from upstream
- Pull the upstream fix (for example the commit/PR referenced in the HDFGroup repo), build a patched HDF5, and rebuild any statically linked binaries that include HDF5. Confirm the commit hash or PR is present in your rebuild. The upstream PR that closes multiple related issues is available in the project’s GitHub history.
- Short‑term mitigations (if you cannot immediately patch)
- Block or quarantine untrusted uploads that include .h5 content; disable automated previewing or conversion pipelines that open user‑supplied HDF5 files.
- Run HDF5 parsing in isolated sandboxes (separate containers/processes) with strict resource limits (cgroups, RLIMITs), seccomp, and reduced file permissions.
- Apply restrictive network and authentication controls to intake endpoints so only trusted sources can submit files for processing.
- Monitor for crashes, unexpected core dumps, or recurring process restarts in HDF5‑using services.
- Stop: temporarily suspend automated processing of untrusted .h5 uploads.
- Patch: apply vendor/distribution update or rebuild with upstream commit.
- Isolate: run file processing under strict sandboxing and resource controls.
- Monitor: escalate any HDF5 process crashes for immediate forensics and triage.
How to verify you are fixed (technical validation)
- Confirm package changelogs or vendor advisories include the upstream commit hash or PR id that references the H5FL__reg_gc_list fix. Debian’s tracker and the HDFGroup PR/commit pages provide exact references for mapping.
- Rebuild with sanitizers in a staging environment and run the public PoC; a patched build should not reproduce the ASan heap‑use‑after‑free trace present in the original report. The issue reporter provided precise ASan reproduction steps that you can follow in CI for regression verification.
- Run fuzzing or targeted unit tests against object header continuation and garbage collection paths to confirm the fix is robust across edge cases.
Critical analysis: strengths, gaps, and risk tradeoffs
Strengths- The bug has been reported responsibly in a reproducible fashion (ASan reproduction), which increases confidence in the diagnosis. The GitHub issue provides actionable traces and reproduction steps, enabling maintainers and packagers to validate and test fixes.
- Upstream remediation activity shows the HDFGroup is addressing the class of issues across related object‑header and GC codepaths in a consolidated PR; this lowers the chance of regression and indicates a careful, coherent patch approach rather than piecemeal fixes.
- Patch availability vs. deployment lag: upstream commits and merged PRs are necessary but not sufficient — distribution and vendor packaging cycles, and the need to rebuild statically linked consumer binaries, mean many environments will remain exposed for weeks or months. Static linking in vendor appliances is a long‑tail problem that raises operational friction.
- Severity disagreement: the split in CVSS scores across trackers can lead to mixed triage priorities inside organisations. A site that treats the issue as low priority may miss the larger operational risk if untrusted file ingest exists.
- PoC availability: public proof‑of‑concepts lower attacker effort. Even if remote RCE is not immediately demonstrated, attackers and red teams can weaponize the PoC to cause denial of service or to probe for additional primitives in target environments.
- If your environment accepts unauthenticated .h5 uploads, treat this as high priority because local vulnerabilities in a widely linked library equate to remotely‑triggerable availability attacks.
- If HDF5 appears only in desktop or isolated research workstations, prioritize patching based on threat models (targeted attacks vs. opportunistic exploitation) and available maintenance windows.
Detection indicators and response tips
- Look for crashes with stack frames referencing H5FL__reg_gc_list, H5FL_garbage_coll, H5_term_library or other H5FL/H5O call paths in core dumps or crash logs.
- Monitor for sudden worker churn or repeated process restarts in ingress or conversion pools that handle HDF5 files.
- If possible, enable lightweight sandboxed reproducers in a safe lab to test inbound .h5 samples against patched/unpatched builds.
- Record and quarantine offending .h5 files — they can be useful for forensic analysis and patch validation.
Longer-term developer guidance
- Harden parsing code: add defensive checks around deserialization counts and allocation sizes, and ensure ownership semantics for temporary buffers are explicit so early returns clean up without double frees or use‑after‑free. The upstream changes merged for this and related CVEs employ these patterns.
- Fuzzing and CI: incorporate continuous fuzzing of deserialization paths and enable sanitizer runs in pre‑merge CI for defensive coverage that catches similar memory‑safety regressions early. HDF5 has an active fuzzing harness culture; adopting a similar CI approach reduces recurrence.
- Avoid static linkage for third‑party libs when possible: dynamic linking makes rolling security updates far simpler for downstream operators; when static linking is unavoidable, maintain a rebuild and redeploy plan.
Conclusion
CVE‑2025‑6856 is an important, credible memory‑safety finding: a use‑after‑free in H5FL__reg_gc_list of HDF5 1.14.6 was reproduced with sanitizers, disclosed publicly, and tracked in canonical vulnerability databases; upstream fixes and PRs have been merged to address the failure mode. The immediate threat model is local exploitation leading to denial of service, but the operational reality — where servers automatically process untrusted .h5 files and many consumers statically link HDF5 — means remote abuse is a practical concern until patches reach all affected artifacts. Defenders should inventory HDF5 usage, apply vendor or upstream fixes, sandbox or block untrusted .h5 files in ingestion flows, and validate fixes via sanitizer regression and the published reproducer. The mix of a public PoC and the library’s wide footprint makes rapid triage and patch management the correct priority.Acknowledgement of sources used during reporting: authoritative vulnerability indexes and the upstream HDFGroup issue/PR/commit history were consulted; distribution trackers and vendor advisories were checked for packaging status and severity assessments.
Source: MSRC Security Update Guide - Microsoft Security Response Center
