CVE-2026-59142: Data::HashMap::Shared versions before 0.14 for Perl allow an out-of-bounds read via an unvalidated arena offset and length in shm_str_copy

Data::HashMap::Shared versions before 0.14 for Perl allow an out-of-bounds read via an unvalidated arena offset and length in shmstrcopy.

The attach-time validator shmvalidateheader checks the header scalars and region layout against the file size, but does not validate the array contents it then trusts. shmstrcopy does memcpy(dst, arena + off, len) with off and len read raw from the mmap'd segment and unbounded, on the each, keys, values, pop, shift, take, swap, drain and cursor paths. The get path bounds off and len separately and is not affected.

A local peer that can write the backing file can leave the header valid while poisoning a record's offset and length, so iterating or draining the map copies a file-controlled offset and length out of the arena, reading adjacent memory or crashing the process.