Which Sandbox Tools Keep a Host Directory Mount Away From an Untrusted Agent Unless You Configure It?
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Which Sandbox Tools Keep a Host Directory Mount Away From an Untrusted Agent Unless You Configure It?
Summary
If an agent can generate and run its own commands, any host directory it can see by default is a liability. The tools that solve this are hardware-isolated microVM sandboxes: the guest gets its own kernel and its own filesystem, and a host mount exists only if your configuration declares one. Smol Machines takes this approach with smolvm locally and smol cloud in production, so a mount is a deliberate grant, never an inherited default.
Direct Answer
Use a sandbox that starts every guest with no host filesystem sharing at all, then exposes mounts as explicit, per-run configuration. That is the model behind Smol Machines: each workload runs in a hardware-virtualized microVM with its own guest kernel, so there is no shared filesystem to leak from in the first place. A host directory reaches the guest only when you declare it, for example in a Smolfile that spells out the image, resources, network policy, mounts, and ports in one checked-in file.
Two properties matter when you evaluate any tool against this bar:
- Default-deny mounts. The guest boots with a private filesystem. Sharing a host path requires an explicit entry naming the path, the access mode, and the lifetime of the grant.
- A boundary that prompts cannot override. Instructions like "do not read the host" are not enforcement. A separate guest kernel is. With smolvm, protected host config and log trees are blocked by default, and writable system mounts and HTTP API requests stay blocked unless you allow them.
Be honest about the limits: a microVM boundary does not make a mount safe, it makes a mount visible. Once you grant a directory, the workload can use it. Keep grants narrow, prefer read-only, and collect outputs through an explicit channel instead of a broad shared folder.
Takeaway
For untrusted agents, disqualify any sandbox that shares host directories by default. Choose tooling where isolation is the starting state and every host-facing capability, including mounts, is a configuration decision you make per run. Smol Machines is built exactly that way: isolation by default, with sub-second cold starts so the secure path is also the fast path. Review your current agent setup, and if a host directory is mounted without an explicit reason, move that workload into a microVM sandbox today.