Sandbox threat model

Ghosty can launch shell commands proposed by a model. Approval policy, workspace-aware tools, and an operating-system command wrapper are separate controls: an approval is not a sandbox, and selecting workspace-write does not prove that the current platform has an OS wrapper available.

This document describes only behavior wired into the command execution path. See Authorization order for the policy layers that run before execution reaches this boundary.

Platform overview

Mechanism	Platform	Selection	What Ghosty reports
Seatbelt (sandbox-exec)	macOS	Automatic when the runtime probe succeeds	macos-seatbelt
Bubblewrap (/usr/bin/bwrap)	Linux	prefer_bwrap = true and the file is executable	linux-bwrap
No OS wrapper	Linux without usable opt-in bwrap	Default	none
No OS wrapper	Windows	Current implementation	none
OpenSandbox-compatible service	Any supported host	sandbox_backend = "opensandbox"	External execution path

The repository contains a seccomp implementation module plus a future Windows helper contract. They are not wired into child-command launch, so Ghosty does not advertise them as active sandboxes. Source-only sandbox code is not evidence that a command was restricted.

macOS: Seatbelt

Ghosty probes /usr/bin/sandbox-exec by running a minimal profile. When the probe succeeds and the selected SandboxPolicy requests a sandbox, the child command is wrapped with a generated Seatbelt profile.

The profile can provide:

• broad filesystem reads;
• writes limited by the selected policy, including the workspace and specific runtime/cache paths needed by supported tools;
• network access only when the policy enables it.

If the probe fails or sandbox-exec is unavailable, Ghosty reports no OS sandbox and launches the command without a Seatbelt wrapper. It does not set a Seatbelt marker on that fallback.

Linux: opt-in bubblewrap

Linux command sandboxing is opt-in. Set the top-level configuration key:

prefer_bwrap = true

Ghosty selects bubblewrap only when /usr/bin/bwrap is a regular executable file. The wrapper derives its mounts and network namespace from the resolved SandboxPolicy:

/usr/bin/bwrap \
  --unshare-all \
  [--share-net] \
  --ro-bind / / \
  --dev /dev \
  --proc /proc \
  --tmpfs /tmp \
  [--dev-bind <device-root> <device-root> ...] \
  --bind <writable-root> <writable-root> ... \
  --ro-bind <protected-descendant> <protected-descendant> ... \
  [--ro-bind <extra-ro-root> <extra-ro-root> ...] \
  --chdir <cwd> \
  -- <program> <args>

The sandbox always gets a private /dev (fresh device nodes, so >/dev/null works), a private /proc, and a tmpfs /tmp (#5410). Two optional top-level config keys extend the mounts: bwrap_ro_roots (extra host paths bind-mounted read-only, applied last so they can narrow a policy-writable path) and bwrap_dev_roots (host character/block device nodes bind-mounted read-write; directories are never honored). Missing paths are skipped silently.

That gives the child a read-only root view. For workspace-write, every safe, existing policy root is mounted read-write: the working directory, configured additional roots, /tmp and TMPDIR unless excluded, and verified Git worktree metadata roots. Existing .ghosty and .deepseek descendants are remounted read-only after their writable parent. Missing paths, non-directory paths, and / are not promoted to writable mounts.

For read-only, there are no writable binds, so the working directory remains inside the read-only root view. --unshare-all isolates the network namespace by default. Ghosty adds --share-net only when the policy's network_access is true. danger-full-access and external-sandbox bypass the local wrapper entirely.

If the user does not opt in, or /usr/bin/bwrap is missing or non-executable, Ghosty reports none and launches the command without a Linux OS wrapper. There is no marker-only fallback to a different Linux sandbox.

Install bubblewrap separately when this opt-in fits the workflow:

• Ubuntu/Debian: apt install bubblewrap
• Fedora: dnf install bubblewrap
• Arch: pacman -S bubblewrap

Ghosty does not vendor bubblewrap.

Windows: no advertised OS sandbox

The Windows command path currently reports no OS sandbox. The source tree has a future helper contract for Job Object process-tree cleanup, but it is not wired into selection and must not be described as any of the following:

• read-only filesystem or workspace-write enforcement;
• network blocking;
• registry isolation;
• restricted-token or AppContainer isolation.

Windows host permissions and approval policy still apply, but they are not a Ghosty OS command sandbox.

Linux process hardening is not a command sandbox

At startup on Linux, Ghosty best-effort applies PR_SET_DUMPABLE=0, PR_SET_NO_NEW_PRIVS=1, and RLIMIT_CORE=0 to its own process. Each failure is logged and startup continues. These controls reduce process-inspection, privilege-escalation, and core-dump risk; they do not create filesystem or network isolation for a child command and are not listed as a sandbox backend.

External OpenSandbox execution

When sandbox_backend = "opensandbox" is configured, shell execution is sent to the configured OpenSandbox-compatible HTTP endpoint instead of starting a local child. Ghosty validates the request/response contract, but isolation guarantees belong to the configured service and its operator.

sandbox_backend = "opensandbox"
sandbox_url = "http://localhost:8080"
sandbox_api_key = "YOUR_API_KEY"

sandbox_backend = "none" (or omitting the key) keeps local execution.

Policies and fallbacks

The local sandbox_mode values are:

sandbox_mode = "workspace-write" # read-only | workspace-write | danger-full-access | external-sandbox

• read-only and workspace-write are enforced by Seatbelt or bubblewrap only when that wrapper is selected and available.
• danger-full-access deliberately bypasses the local OS wrapper.
• external-sandbox declares that execution is already externally isolated and bypasses a second local wrapper.
• When no wrapper is selected, the shell command runs without Ghosty OS isolation. Approval rules and workspace-aware native file tools remain separate controls.

Canonical environment overrides exist for sandbox_mode and the external backend:

• GHOSTY_SANDBOX_MODE
• GHOSTY_SANDBOX_BACKEND
• GHOSTY_SANDBOX_URL
• GHOSTY_SANDBOX_API_KEY

There is no GHOSTY_PREFER_BWRAP environment override; use the top-level prefer_bwrap config key.

Diagnostics and failure attribution

ghosty setup --status, ghosty doctor, ghosty doctor --json, and the diagnostics tool report the locally available wrapper after applying the resolved bubblewrap preference. An individual command can still bypass that wrapper when its policy does not request sandboxing. On Linux, merely finding a sandbox-related syscall or source module does not make sandbox_available true.

Denial attribution is intentionally conservative:

• Seatbelt uses its wrapper-specific denial patterns.
• Bubblewrap setup errors must be prefixed by bwrap:; a read-only-filesystem error from the bwrap filesystem view can also identify the boundary.
• A child command's generic Permission denied or Operation not permitted is not, by itself, proof that Ghosty's sandbox blocked it.
• Unsandboxed command failures are never labeled sandbox denials.

Limitations

• Availability is checked before launch; the selected wrapper can still fail because of host policy, container restrictions, or a race after the probe.
• Bubblewrap ignores a configured writable root if it is missing, is not a directory, or canonicalizes to /; a path can also disappear between policy resolution and wrapper launch.
• Seatbelt profiles are generated at runtime and must be tested against the commands they are expected to support.
• No current local wrapper is advertised on Windows.
• An external sandbox backend is only as strong as its configured service.
• No sandbox protects against kernel vulnerabilities or all resource-exhaustion and side-channel attacks.

Implementation references

• crates/tui/src/sandbox/mod.rs — truthful selection and public capability markers
• crates/tui/src/sandbox/seatbelt.rs — macOS wrapper and availability probe
• crates/tui/src/sandbox/bwrap.rs — Linux opt-in wrapper
• crates/tui/src/sandbox/process_hardening.rs — Linux parent-process hardening
• crates/tui/src/sandbox/backend.rs — external backend selection
• crates/tui/src/tools/diagnostics.rs — machine-readable diagnostics