The flaw originates in the Fletcher32 filter implementation in HDF5 when calculating the amount of data to checksum by subtracting a checksum length from the buffer size. The code fails to ensure the buffer is at least four bytes, which allows a size_t underflow. When a chunk has fewer than four bytes, this produces a very large length causing an out‑of‑bounds read in the checksum routine and can crash the application. This leads to a denial of service when the library processes a maliciously crafted HDF5 file.

The HDF Group's HDF5 library, versions 2.3.0 and earlier, contain the vulnerable code. The issue was disclosed in 2026 and affects built‑in tools such as h5ls and h5dump as well as any software that uses this library to read HDF5 files.

The CVSS score of 6.8 indicates moderate severity. No EPSS score is available, and the vulnerability is not currently listed in CISA's KEV catalog. The attack vector is likely local or remote based on file consumption; the adversary must supply a malicious HDF5 file containing a Fletcher32‑filtered chunk smaller than four bytes. Reading such a file with H5Dread (for example, via h5ls or h5dump) triggers the underflow and out‑of‑bounds read, resulting in a crash.