A heap‑based buffer overflow has been disclosed in the HDF5 library: CVE‑2025‑2923 documents a flaw in the function H5F_addr_encode_len (file src/H5Fint.c) that can write past an allocated buffer when processing crafted data, producing a reliable crash and a low‑to‑medium severity local attack vector; a public proof‑of‑concept and upstream fixes are available but distribution packaging and backport timelines vary, leaving many deployments exposed until they validate and deploy the upstream changes.
HDF5 (Hierarchical Data Format version 5) is a ubiquitous binary container and library used across scientific computing, machine learning pipelines, instrumentation, and many enterprise data workflows. Because HDF5 is commonly linked directly into analysis tools, data‑processing services, and command‑line utilities (for example, h5dump, h5repack, and language bindings used by Python/R/MATLAB), a memory‑safety bug in the core C library can create operational risk far beyond a single desktop application. Several HDF5 vulnerabilities were discovered in the same release window around 1.14.6, highlighting the importance of triage across entire dependency trees.
CVE‑2025‑2923 was assigned after an AddressSanitizer trace and researcher report showed that H5F_addr_encode_len performs unchecked writes through the pointer argument pp, writing up to one byte past the end of the target buffer under certain malformed input conditions. The issue is local in the sense that exploitation requires the attacker to cause the vulnerable code path to run on the host, but that local requirement does not eliminate real‑world remote attack scenarios where untrusted HDF5 files are accepted by server‑side ingestion or preview pipelines. Public trackers and vendor pages classify the CVSS v3.1 base impact as low to medium (examples vary), reflecting the immediate consequence of denial‑of‑service and memory corruption rather than an asserted, immediate remote code execution (RCE) primitive.
void H5F_addr_encode_len(size_t addr_len, uint8_t pp /in,out/, haddr_t addr)
The code loops addr_len times and writes a byte at **(pp)++ for each iteration. When the function is called with an internal tracer pointer that points near the end of an allocation, and addr_len exceeds the remaining space, the loop writes beyond the allocation bounds — in effect a classic heap‑based buffer overflow by unchecked indexed writes. The researcher’s report reproduces the function body and the AddressSanitizer evidence showing a one‑byte out‑of‑bounds write in the else branch when addresses are undefined.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
HDF5 (Hierarchical Data Format version 5) is a ubiquitous binary container and library used across scientific computing, machine learning pipelines, instrumentation, and many enterprise data workflows. Because HDF5 is commonly linked directly into analysis tools, data‑processing services, and command‑line utilities (for example, h5dump, h5repack, and language bindings used by Python/R/MATLAB), a memory‑safety bug in the core C library can create operational risk far beyond a single desktop application. Several HDF5 vulnerabilities were discovered in the same release window around 1.14.6, highlighting the importance of triage across entire dependency trees.CVE‑2025‑2923 was assigned after an AddressSanitizer trace and researcher report showed that H5F_addr_encode_len performs unchecked writes through the pointer argument pp, writing up to one byte past the end of the target buffer under certain malformed input conditions. The issue is local in the sense that exploitation requires the attacker to cause the vulnerable code path to run on the host, but that local requirement does not eliminate real‑world remote attack scenarios where untrusted HDF5 files are accepted by server‑side ingestion or preview pipelines. Public trackers and vendor pages classify the CVSS v3.1 base impact as low to medium (examples vary), reflecting the immediate consequence of denial‑of‑service and memory corruption rather than an asserted, immediate remote code execution (RCE) primitive.
Technical anatomy: how H5F_addr_encode_len fails
The vulnerable pattern
The weakness is localized and straightforward: the function signature in question isvoid H5F_addr_encode_len(size_t addr_len, uint8_t pp /in,out/, haddr_t addr)
The code loops addr_len times and writes a byte at **(pp)++ for each iteration. When the function is called with an internal tracer pointer that points near the end of an allocation, and addr_len exceeds the remaining space, the loop writes beyond the allocation bounds — in effect a classic heap‑based buffer overflow by unchecked indexed writes. The researcher’s report reproduces the function body and the AddressSanitizer evidence showing a one‑byte out‑of‑bounds write in the else branch when addresses are undefined.
Why this specific coding choice matters
- The function assumes the caller has reserved at least addr_len bytes starting at *pp, but the code does not validate the remaining buffer space before the loop.
- The write pattern uses post‑increment on the pointer target (i.e., (pp)++), which advances the caller’s pointer as bytes are written; that makes recovery and detection by adjacent code more difficult after corruption has occurred.
- The vulnerability is not structural or algorithmic; it is a missing bounds check in a hot path that manipulates addresses during serialization/deserialization of HDF5 internal structures — a location that is often exercised when loading files or processing dataset metadata.
Reproducer and PoC details
The original issue submission includes a short, replicable harness demonstrating how to reproduce the crash using clang’s sanitizers and a small fuzzing harness (the repository contains a zipped PoC and ASAN output). The researcher included step‑by‑step reproduction instructions: build the HDF5 library with AddressSanitizer enabled, compile a small fuzzer harness that attempts to open a crafted HDF5 file, and trigger the overflow via H5Fopen on malformed input. That public PoC makes the condition concrete and reproducible for integrators validating their builds.Affected versions and vendor status
- Affected upstream: HDF5 releases up to and including 1.14.6; the vulnerability is tracked against that release window. Multiple public vulnerability aggregators list HDF5 ≤ 1.14.6 as vulnerable.
- Upstream response: The HDF Group issue and an associated pull request were opened and merged to address broken handling in related object header continuation code; the commit referenced by downstream trackers is 29c847a... and corresponds to fixes landed in the HDF5 GitHub repository. Packagers are mapping these commits into distribution updates or backported package releases.
- Distribution packaging: Debian, Ubuntu and other distributions have recorded the CVE in their trackers; some distributors have marked the issue as postponed or needs evaluation while they await consolidated upstream fixes and coordinate backports. This leads to discrepancies in when end‑users receive patched packages. Administrators should verify the exact package changelog and commit hashes in their distribution’s hdf5 package before declaring a host remediated.
Exploitability, impact and severity — practical analysis
Exploitability model
- Attack vector: Local (AV:L) — an attacker must cause the vulnerable code to run on the target. In practice, that can be achieved by delivering a crafted .h5 file to a service or application that will parse or open it.
- Complexity: Low — the PoC demonstrates reproducible crashes under ASAN and shows an exact write past the buffer; the conditions to trigger the specific loop are not exotic.
- Privileges required: Low — code runs in the context of the process that loads the file; an attacker only needs to be able to feed an HDF5 file to a vulnerable process.
Immediate impacts
- Denial of Service (DoS): The most certain impact. The overflow can crash processes that open crafted files, causing service disruption in ingestion and processing pipelines.
- Memory corruption / data integrity: Heap corruption can produce silent data corruption in long‑running services or during processing, potentially affecting downstream results.
- Potential for RCE (caveat): While heap overflows are a useful primitive for exploit developers, turning a one‑byte buffer overflow into reliable remote code execution typically requires favorable allocator behavior, an information leak for ASLR bypass, or additional exploitable conditions. Public advisories cautiously treat RCE as possible but unproven in the wild for this CVE. Treat claims of immediate RCE as speculative unless independent exploit write‑ups demonstrate a working chain.
Severity scoring nuance
Different vulnerability trackers assign different numerical severity values. For CVE‑2025‑2923:- NVD enumerates the CVE record and provides enrichment text (NVD indicates the issue is real and disclosed).
- Several CNAs and databases show CVSS values in the low to medium range (for instance, CVSS v3.1 ≈ 3.3; CVSS v4.0 values around 4.8 appear in some aggregator feeds). These numeric differences reflect factors such as attack vector (local) and impact (DoS and data integrity rather than guaranteed confidentiality loss). Operators should not rely solely on a single numeric score; instead evaluate exposure in context (server‑side ingesters vs local desktop usage).
Patches, commits and packaging: what to look for
The HDF Group issue that reported the overflow links to a pull request and specific upstream commits intended to remediate the underlying root causes in HDF5’s serialization and object‑header handling. The canonical artifacts to confirm in a patch or vendor package are:- Inclusion of the specific fixes merged to the HDF5 repository (for example, the commit referenced by distribution trackers with prefix 29c847a).
- Pull request merges that address H5F_addr_encode_len call sites or the code paths that compute and pass buffer pointers and lengths into that function.
- Distribution changelogs referencing CVE‑2025‑2923 or listing the upstream commit ID in the hdf5 package revision notes.
Practical mitigation and remediation checklist
For engineering, operations, and security teams responsible for systems that consume HDF5 data, prioritize actions as follows:- Inventory and exposure assessment
- Identify all binaries, containers, and services that link against libhdf5 (static and dynamic linking).
- Search for language bindings and packages that bring HDF5 into your environment (for example, Python packages like h5py often bundle or require a platform HDF5 library).
- Tag any service that accepts uploaded .h5 files, parses HDF5 content, or auto‑ingests HDF5 datasets (these are highest priority).
- Patch and rebuild
- Apply upstream HDF Group fixes or deploy distribution packages that explicitly reference CVE‑2025‑2923 or the upstream commit id.
- Rebuild any statically linked binaries or firmware images that embed HDF5 so they include the corrected code.
- Validate builds by checking the library version and examining the commit history in your build artifacts.
- Temporary mitigations if immediate patching is impossible
- Block or sandbox ingestion of untrusted HDF5 files — e.g., quarantine uploads, run processing inside restricted containers, and apply strict file‑type screening.
- Enforce access controls on upload endpoints and reduce privileges for processes that parse HDF5 content.
- Implement monitoring and crash detection for services that process HDF5 files — a sudden increase in process crashes or core dumps often indicates attempted exploitation.
- Detection and validation
- Reproduce the PoC in a controlled lab to verify that patched builds no longer crash under the same inputs (follow the sanitizers-based harness pattern from the upstream report).
- Add unit or integration tests that call serialization/deserialization code paths with edge cases to detect regressions.
- Communication and change management
- Inform application owners and developers that HDF5 revisions are required and map which internal products depend on the library.
- Coordinate patch windows for servers that accept external HDF5 content to minimize downtime while applying rebuilt artifacts.
Why this matters for WindowsForum readers
Although HDF5 is most visible in scientific and HPC ecosystems, many Windows applications and toolchains include HDF5 (for example, analysis tools, instrument data converters and some cross‑platform libraries). Windows users should:- Check any installed scientific stacks (Anaconda/Miniconda environments often include h5py and HDF5 binaries) and update packages once patched HDF5 binaries are available for their platform.
- For enterprise Windows servers used in file‑preview, document management, or automation pipelines that accept HDF5 content, apply the same sandboxing and patching discipline described above.
- Remember that even if your desktop environment isn’t directly exposed to remote uploads, a collaborator or vendor might send a crafted file; exercise caution opening untrusted .h5 attachments and validate files in an isolated environment.
Strengths and limitations of the disclosure
Notable strengths
- The vulnerability report included a clear memory‑sanitizer trace and a replicable PoC harness. That speeded up triage and allowed maintainers to reason about the exact failure mode quickly.
- Upstream maintainers merged a focused fix addressing the root cause and related handling of object header continuation messages, showing responsive patching and targeted remediation.
Limitations and remaining risks
- Distribution and vendor rollouts vary: some distributions postponed backports and marked the issue for later resolution, which leaves many production deployments vulnerable until packaging is completed and tested. Administrators should not assume that a vendor release number implies the fix is present without explicit commit confirmation.
- The PoC reduces dwell time for attackers but the short write‑past‑end primitive may still require additional environmental conditions to achieve reliable RCE. Organizations should treat the PoC as a clear sign of risk: DoS is trivial, and escalation to RCE is possible given additional chaining.
- Embedded and statically linked uses are especially brittle: firmware and appliance images that embed an earlier HDF5 tree may require rebuilds — a nontrivial lifecycle operation for constrained devices.
Immediate operational recommendations (concise)
- Prioritize patching for any server or service that accepts HDF5 files from untrusted sources (uploads, shared buckets, APIs).
- Rebuild static artifacts and firmware images that embed HDF5 with the upstream commit that fixes the issue.
- If patching cannot occur immediately: quarantine and sandbox HDF5 processing, restrict file inputs, and enable crash monitoring and alerting.
- Validate patched images by reproducing the PoC in a controlled environment using sanitizers, and confirm the ASAN trace no longer reproduces.
Conclusion
CVE‑2025‑2923 is a concrete, reproducible heap‑based buffer overflow in HDF5’s H5F_addr_encode_len that exposes processes parsing HDF5 content to crashes and memory corruption. The disclosure includes a practical PoC and upstream patches, which is a positive sign for defenders; however, patching timelines and packaging choices across distributions and Redistributors mean many production environments remain at risk until maintainers rebuild and deploy fixed artifacts. Defenders should treat this as a high‑priority triage item for any service that automatically opens or processes HDF5 files — inventory presence, apply upstream fixes or vendor packages that contain the upstream commit, sandbox ingestion, and validate patched builds with the provided PoC harness.Source: MSRC Security Update Guide - Microsoft Security Response Center
