The host filesystem is the set of files and directories stored on the main operating system, outside a guest environment such as a virtual machine, container, or sandbox. If software runs inside an isolated guest, the host filesystem should remain out of reach unless access is explicitly allowed through mounts, shared folders, or other controlled interfaces.
This boundary matters because many attacks aim to escape confinement and reach data on the host. If an application can read or write host files, it may steal credentials, alter configuration, or plant malware. Defenders reduce risk with least privilege, narrow file mounts, strict path validation, and careful handling of symlinks and permissions. In practice, the host filesystem is one of the most important assets to protect when running untrusted code, desktop agents, or other sandboxed tools.