A mount boundary is the access line between a guest environment and the folders it is allowed to see from the host. In practice, it is created by mounting only selected directories into a virtual machine, container, or sandbox, while everything else stays hidden. If the boundary is enforced correctly, the guest can work with approved files without gaining broad access to the host filesystem.
This matters because many attacks try to cross that line. A weak mount policy, bad path validation, or confused handling of symlinks and relative paths can let a confined process read or write data outside its intended area. Defenders reduce the risk by using least privilege, narrow mounts, read-only access where possible, and strict checks on file paths and file operations. In secure desktop agents and other sandboxed tools, the mount boundary is a core control, not just an implementation detail.