CVE-2026-19028: HDF5 integer underflow in Fletcher32 filter leads to massive out-of-bounds read

H5Zfilterfletcher32 in H5Zfletcher32.c in HDF5 through 2.3.0 computes the data length to checksum by subtracting the 4-byte trailing checksum size from the input buffer size without checking that the buffer is at least 4 bytes, allowing a sizet underflow. This allows attackers to cause a denial of service (massively out-of-bounds read and application crash in H5checksumfletcher32) via a crafted HDF5 file with a Fletcher32-filtered chunk smaller than 4 bytes, triggered via H5Dread, e.g. by the h5ls or h5dump tools.