Home Assistant Core vulnerable to Path Traversal via backup upload during onboarding

Home Assistant Core before 2026.6.0 contains a path traversal vulnerability that allows unauthenticated attackers to write arbitrary files to any directory on the host filesystem by uploading a crafted backup archive during the initial onboarding window. Attackers can manipulate the 'name' field inside the uploaded archive's backup.json to supply an absolute path, causing pathlib.Path.truediv to discard the configured backup directory prefix and write attacker-controlled content to arbitrary locations, with full filesystem access when the process runs as root.