A file path boundary is the intended directory limit that should contain every read, write, rename, or delete operation. In secure software, a request may supply a filename or path, but the application must ensure the final resolved path stays inside the approved root directory, not in arbitrary parts of the filesystem.
This matters because boundary failures often lead to path traversal, unauthorized overwrite, data theft, or deletion of sensitive files. Attackers try inputs like ../, encoded separators, absolute paths, or symlink tricks to escape the storage root. Defenders protect the boundary by canonicalizing paths, resolving them against a fixed base directory, blocking traversal patterns, checking symlinks, and running services with narrow filesystem permissions. In systems like registries and upload services, a strong file path boundary keeps stored content predictable, auditable, and contained.