A heap‑buffer overflow in HDF5’s heap-list deserialization routine — H5HL__fl_deserialize in src/H5HLcache.c — was disclosed in March 2025 as CVE‑2025‑2924; the flaw can cause out‑of‑bounds reads and heap corruption when the library processes crafted .h5 files, a proof‑of‑concept was published, and upstream fixes were merged into the HDF5 tree.
Background
HDF5 (Hierarchical Data Format version 5) is a widely used binary container and C library for storing large numerical arrays, hierarchical metadata and chunked dataset content in science, engineering and enterprise systems. Because HDF5 is frequently linked directly into processing toolchains, server‑side ingestion services, command‑line utilities and static embedded images, memory‑safety bugs in the core library can become operational vulnerabilities for any software that opens or parses untrusted .h5 files. The H5HL (heap list) helpers are responsible for managing small, local heaps used by some HDF5 constructs; the deserialization path reconstructs the in‑memory free‑list from on‑disk image data.
Between late March and April 2025 several HDF5 defects affecting the 1.14.6 release were disclosed and tracked as separate CVEs; CVE‑2025‑2924 is the entry that documents the heap overflow rooted in H5HL__fl_deserialize. Public vulnerability indexes and multiple vendor trackers catalog the issue and flag that a public proof‑of‑concept crash exists. Distributors and downstream packagers have treated the issue as significant enough to add to triage queues and, in some cases, to prepare backports.
What the bug is — technical overview
The vulnerable code path
The vulnerable logic sits in H5HL__fl_deserialize(H5HL_t *heap) inside src/H5HLcache.c. The routine walks the on‑disk heap free‑list using an offset value read from the heap image (variable free_block). The deserialization loop calculates a pointer into the dblk_image buffer with:
image = heap->dblk_image + free_block;
and then decodes the next free‑block offset and block size using H5_DECODE_LENGTH_LEN(image, free_block, heap->sizeof_size). In the reported crash case the free_block value can be a very large number (for example, a value derived from a malformed file) which makes the pointer arithmetic move outside the mapped image buffer and allows the subsequent decode macro to read from an invalid region (ASAN redzone). The result is a heap‑buffer overread that can trigger a segmentation fault and corrupt heap state. The GitHub issue includes the relevant code excerpt and ASAN reproduction details.
Why the overflow happens
Two problems combine to produce the condition:
- The code uses a value parsed from the HDF5 image (heap->free_block) as an index/offset into heap->dblk_image without sufficient validation against heap->dblk_size and without guarding the pointer arithmetic before the decode macro runs.
- The H5_DECODE_LENGTH_LEN macro reads bytes in a small loop that under certain size settings may read from just before the intended pointer (it decrements and re‑reads bytes); when the pointer into the image buffer already points just past a boundary, the macro can step into an unreadable redzone.
Taken together, an attacker‑controlled free_block value can cause reads beyond the owning buffer and lead to heap corruption or crashes. The issue was reproducible with AddressSanitizer during testing and reproduced in the PoC steps posted in the upstream issue.
Proof‑of‑Concept and disclosure status
The vulnerability was publicly reported in a GitHub issue that includes PoC reproduction steps — build HDF5 with AddressSanitizer (ASAN) and run the provided crash input to observe a heap buffer overflow. The issue author documented the offending code lines, ASAN output, and a sample crash file pattern used for reproduction. That report, together with a follow‑up pull request containing a fix, demonstrates both exploitability for causing crashes and an upstream acknowledgement of the root cause. Multiple vulnerability aggregators and national CERT trackers also list CVE‑2025‑2924 and note the existence of a public exploit or proof‑of‑concept (PoC). Those aggregators classify the exploit maturity as at least "PoC" and list the attack vector as local (Attack Vector: Local; privileges required: low in many trackers). Scores reported by vendors vary (see the next section).
Affected versions, packaging and vendor status
- Upstream affected: HDF5 releases up to and including 1.14.6 are named in the disclosures as vulnerable.
- Upstream remediations: The HDF Group accepted a pull request that adds defensive checks and bounds validation for the deserialization path; the fix is merged as commit 0a57195… in the hdf5 repository. Administrators and packagers should look for the PR/commit or for a subsequent point release (for example a 1.14.7 or other vendor‑tagged release) that explicitly includes the change.
- Distribution trackers: Debian, Ubuntu, SUSE and other distributors have opened tracking entries and in many cases marked packages as vulnerable while they determine backport strategy. Some distributions list the issue with a "needs evaluation" or "pending" state, and urgency varies — some consider it low/medium priority given exploit complexity and local attack vector, while others rate it more conservatively because of the PoC. Verify your distribution’s package changelog for the exact commit SHA included in patched builds before declaring hosts remediated.
Severity, exploitability and scoring — what vendors report
Public scoring for CVE‑2025‑2924 varies by tracker:
- NVD published an entry noting the vulnerability details and that the exploit has been disclosed. NVD’s final numeric scoring may be in enrichment; check the NVD page for updates.
- Ubuntu published a CVSS v3.1 base score of 3.3 (Low) and lists the vector string as AV:L/AC:L/PR:L/UI:N — their rationale emphasizes the local nature of the attack and the limited direct confidentiality/integrity impact while rating availability as low‑impact.
- Security vendors and independent trackers produced medium scores (CVSSv3 around the mid‑4 to 5 range or CVSSv4 base ~4.8 in some entries) reflecting a consensus that PoC exists but remote exploitation is contingent on deployment patterns. For example, Recorded Future lists CVSS 3.1 around 5.5 (Medium) and INCIBE lists a CVSS 4.0 base of 4.80 (Medium) for the issue. These differences reflect the varied threat models and the maturity of PoC evidence at the time of each assessment.
Important nuance: the presence of a PoC that reliably causes a crash raises the priority for most operational teams because it materially lowers an attacker’s development effort for denial‑of‑service. However,
escalation to reliable arbitrary code execution is environment‑dependent — it requires favorable allocator behavior, absence of modern mitigations, or additional chaining with information‑leak or write primitives in the target application. Treat RCE claims cautiously until independent exploitation writeups demonstrate them.
The upstream fix — what changed
The HDF Group applied a targeted fix that adds sanity and bounds checks to the heap free‑list deserialization path. The PR makes explicit checks on the free_block value before using it as an offset into the dblk_image buffer, prevents pointer arithmetic that could step outside allocated memory, and ensures subsequent length decodes are bounded by heap->dblk_size and the image buffer region. The fix is committed (SHA 0a57195…) and available in the upstream repository; packagers and integrators should reference the commit or wait for an official point release that contains it.
Practical mitigation and remediation playbook
Immediate triage (first 24–72 hours)
- Inventory quickly: locate every binary, package, container image and firmware image that contains HDF5 (dynamic or static). Pay special attention to:
- Server‑side ingestion services that accept user uploads of .h5 files (preview/thumbnail workers, automated conversion pipelines, research data repositories).
- Container images in registries used by CI/CD or production workloads.
- Python wheels, conda packages, MATLAB toolchains or static embedded binaries that may vendor HDF5 source.
- Block untrusted ingestion where feasible: disable or quarantine automatic processing of uploaded .h5 files until patched builds are available. Require authentication and scanning of incoming files, and avoid passing untrusted files to HDF5‑using binaries in the meantime.
- Apply process isolation: if HDF5 processing must continue, run decoders in constrained sandboxes (unprivileged containers, seccomp, AppArmor/SELinux profiles), with strict CPU and memory limits and reduced file permissions. That reduces blast radius of crashes and makes heap grooming attacks much harder.
Patch and rebuild (correct fix)
- Obtain upstream patched sources or distributor packages that explicitly include commit SHA 0a57195… or the canonical PR that closes CVE‑2025‑2924. Confirm the package changelog or vendor advisory mentions the CVE or the commit.
- Rebuild all artifacts that statically link HDF5 (embedded firmware, static binaries, vendor appliances). Replacing a shared system package is insufficient for statically‑linked consumers.
- Redeploy rebuilt binaries and container images, then restart services to ensure running processes use the patched library.
Short‑term compensations if immediate patching isn’t possible
- Enforce strict upload whitelists and file‑type checks; preprocess incoming files in a non‑HDF5 path to validate expected headers and length fields before handing them to the HDF5 library.
- Add input validation wrappers around HDF5 calls where possible (e.g., check heap->free_block and heap->dblk_size values early, and reject suspiciously large sizes). Note: this is only a stopgap unless integrated into the library code itself.
- Increase crash and core‑dump monitoring: alert on repeated SIGSEGVs, frequent worker restarts or spikes in OOM/killed tasks tied to HDF5 processes. Correlate with recent uploads or automated conversion jobs.
Detection, indicators and forensic cues
- Reproducible crashes in processes that call HDF5 APIs (h5dump, h5ls, custom Python/R toolchains that use libhdf5) immediately following file ingestion are a top indicator. Check for SIGSEGVs, ASAN reports, or application core dumps with stack traces referencing H5HL__fl_deserialize or src/H5HLcache.c.
- Look for the presence of known PoC files in inbound upload directories or object stores; the original GitHub issue includes a description of the crash file pattern used for reproduction.
- Telemetry patterns: sudden increases in worker restarts, elevated error rates for dataset reads, or repeated conversion failures on nodes that process HDF5 files are high‑priority signals. Instrument ingestion endpoints to log file hashes and processing outcomes so suspicious files can be quarantined and analyzed.
Exploitation scenarios and attacker model
- Local file attack (most common): An attacker with the ability to place or trick a user into opening a crafted .h5 file will be able to trigger the crash. For desktop users this typically requires social engineering or file exchange. For researchers and engineers, shared dataset repositories or email attachments are realistic vectors.
- Server‑side ingestion: The most consequential scenario is an unauthenticated upload to a public ingestion service that automatically opens or processes .h5 files. In such contexts a crafted file can cause remote, unauthenticated denial‑of‑service by crashing the processing worker. Many cloud preview or conversion pipelines fall into this category.
- Escalation to RCE: While the PoC demonstrates reliable crashes, converting this heap corruption into reliable arbitrary code execution depends heavily on the target environment (allocator behavior, presence/absence of ASLR, hardened allocators, RELRO/PIE, control‑flow protections). Enterprise environments with modern mitigations are less likely to yield RCE; older, statically‑linked, or minimally hardened targets are more at risk. Treat RCE claims as possible but unconfirmed in the general case unless independent exploit writeups demonstrate practicable exploitation.
Operational recommendations — concise checklist
- Inventory: find all HDF5 consumers (binaries, containers, packages, embedded images). Prioritize internet‑facing ingestion endpoints.
- Patch: install vendor or upstream patched packages that include commit 0a57195…; rebuild statically‑linked artifacts.
- Isolate: sandbox HDF5 processing and apply resource limits for workers that must remain online.
- Block: disable automatic untrusted .h5 processing until fixes are applied.
- Monitor: enable crash alerts, scan for PoC files in uploads, and correlate ingestion events with process failures.
Wider context — related HDF5 CVEs and why this matters
CVE‑2025‑2924 is one of several memory‑safety defects disclosed against HDF5 1.14.6 in the same disclosure window. Other reported issues in the 1.14.6 series include heap overflows and allocation errors in unrelated functions (for example, H5VM_memcpyvv and Scale‑Offset filter paths) that together increase the attack surface for environments running that specific release. The clustering of multiple memory bugs in a single release raises the operational urgency: even if one bug is limited to local DoS, the presence of several heap or parsing defects increases the chance that a combination or chaining might yield more powerful primitives for an attacker. For packagers and integrators the practical implication is to avoid selectively ignoring small CVEs — a conservative remediation strategy is to apply the consolidated upstream fixes or upgrade to the next stable point release that bundles all security fixes.
Caveats, verification and items flagged for caution
- Discrepancies in scoring: different vendors have published a range of severity ratings (Low to Medium). These differences stem from distinct threat models and from whether a vendor treats a local PoC crash as a high or lower priority. Enterprises should apply their own risk model that factors exposure (public ingestion vs. offline desktop) rather than rely on any single numeric score.
- RCE claims: public trackers and vendor notes consistently mark denial‑of‑service and heap corruption as primary impacts. Claims of reliable remote code execution remain unverified publicly; treat RCE as environment‑dependent and contingent on additional primitives unless confirmed by multiple independent exploit analyses.
- Patch mapping: not all downstream package updates include the same set of upstream commits. If your team must comply with strict vulnerability management, verify the exact commit SHA or PR is present in the package changelog or vendor advisory before marking systems as remediated. Debian and other distributors list the resolving commit(s) in their trackers; use those references for verification.
For developers and maintainers — code hardening guidance
- Validate external offsets before pointer arithmetic: any decode routine that computes a pointer into an image buffer must validate the offset + required bytes <= buffer length before reading or writing. Prefer explicit bounds checks and early returns on malformed files.
- Favor defensive decoding macros: macros like H5_DECODE_LENGTH_LEN are convenient but brittle when used without prior bounds checks. Add wrapper functions that receive remaining buffer length and refuse reads that would step outside that region.
- Increase fuzzing coverage: add corpus‑based and grammar‑guided fuzz tests that exercise heap deserialization, free‑list parsing and all H5Z filter decode paths. Memory‑safety bugs in binary formats are repeatedly discovered by fuzzers — invest in continuous fuzzing in CI.
- Make static linking visible: document when projects vendor HDF5 source or statically link the library. That helps operations find and patch embedded consumers.
Conclusion
CVE‑2025‑2924 is a concrete, publicly disclosed heap‑overflow in HDF5’s heap‑list deserialization routine that can be triggered by crafted .h5 files and has an upstream fix merged into the HDF5 repository. The immediate operational risk is denial‑of‑service and heap corruption; escalation to reliable remote code execution remains
possible but environment‑dependent. Organizations that accept untrusted HDF5 files — especially public ingestion services, containerized conversion pipelines and any statically‑linked HDF5 consumers — should prioritize inventory, apply the upstream fix or vendor backports (reference commit 0a57195…), rebuild static artifacts, and apply sandboxing and monitoring compensations until patched. Confirm the exact commit is present in patched packages before marking hosts remediated, and treat PoC availability as a signal to accelerate mitigations because it shortens attacker lead time.
Source: MSRC
Security Update Guide - Microsoft Security Response Center