Data::RingBuffer::Shared versions before 0.04 for Perl allow a stack buffer overflow via an unvalidated elem_size in ring_read_seq

Data::RingBuffer::Shared versions before 0.04 for Perl allow a stack buffer overflow via an unvalidated elem_size in ring_read_seq. The attach-time validator ring_validate_header checks the capacity-overflow and total_size consistency of the header but never caps elem_size against the destination size. ring_read_seq does memcpy(out, ring_slot(h, seq), elem_size) with elem_size read raw from the mmap'd segment, copying into a fixed 8-byte destination scalar. An elem_size larger than 8 bytes writes past the destination. A local peer that can write the backing file can leave the header valid while setting a large elem_size, so the next read copies a file-controlled length into the fixed 8-byte stack buffer, corrupting adjacent stack frames.