Windows write-restriction sandbox backend for the [harness sandbox seam](../sandbox/): a Node.js/[koffi](https://koffi.dev/) port of the mechanism in [huoyaoyuan/windows-acl-restrict-poc](https://github.com/huoyaoyuan/windows-acl-restrict-poc) (`10e4dfb`, the fixed revision), mounted as the win32 rung of the [`@deepseek-ai/dsh-sandbox-local`](../sandbox-local/) chain (`workspace-write` / `read-only` modes); the same package carries the Linux/macOS backends.
Mechanism in one line: the caller's token is duplicated into a `WRITE_RESTRICTED` token whose restricting SIDs include an orphan SID (`S-1-4-x-y`) that only this sandbox instance has added to the workspace and temp directories' DACLs. Windows then grants a write only where BOTH the caller's normal access AND the restricting-SID intersection allow it — the orphan SID is the write allowlist, and it grants nothing anywhere else on the system.
Building directly on the raw ACL mechanism is the recorded design choice: it implements both confinement modes without the problems the rejected container options carry — see the [design note](../../../.agents/notes/implemented/feature/2026-08-08-windows-acl-restricted-token-sandbox.md) ([mxc](https://github.com/microsoft/mxc/blob/main/docs/process-container/os-version-support.md) needs an OS floor of Windows 11 24H2 and wholesale host DACL writes for arbitrary-path reads; AppContainer cannot do arbitrary-path reads at all).
sandbox.dispose()// revokes all standing grants; reports every cleanup failure
```
Every Win32 API call in this package is checked; failures throw `Win32Error` carrying the API name, the exact Win32 code, the `FormatMessageW` system text, and the failing path/context. This is deliberate: the POC ignored every return value and, when `CreateRestrictedToken` failed, silently ran the child with the FULL unrestricted token (fail-open). This port fails closed by construction.
## The confinement runner
The seam-facing shape is the **runner entry** (`./runner`), the argv-prefix wrapper `@deepseek-ai/dsh-sandbox-local` spawns in place of the caller's command — the same architecture as bwrap/landlock-run/sandbox-exec, so the sandbox seam's `confine()` contract needs no change. Stable argv contract:
The runner creates the restricted token, spawns the wrapped argv under it with the caller's stdio passed straight through (the caller's pipes, made inheritable around the spawn — Node clears stdio inheritability at startup, which raw spawns must compensate for), wraps the child in a `KILL_ON_JOB_CLOSE` job (a dead runner kills the child), ignores its own console Ctrl+C so the child handles its own, mirrors the child's exit code, and revokes all grants on exit. Every runner-side failure prints `windows-acl-run: <detail>` to stderr and exits 127 — the seam's `RUNNER_FAILURE_RULES` match that signature, so a runner refusal is never mistaken for a denial.
Modes:
-`workspace-write`: the workspace and temp directories carry the orphan-SID Write grant; every other write is denied by the token intersection.
-`read-only`: STRICT zero grants — nothing is writable. The NUL device is a securable object and is NOT granted (unlike Linux's `/dev/null` sink): `Set-Content NUL` and native `> NUL` writes fail with access denied, while PowerShell's `> $null` redirection keeps working (it discards without opening NUL). Documented behavior, not a prompt promise — the model-facing surface makes no sink claims for read-only mode.
The `AclSandbox` class (`tempDir: null` disables the temp grant) remains the programmatic API for direct spawns.
## Header verification
All constants, signatures, and struct layouts were verified against the Windows headers on the development machine (MinGW `winnt.h` / `accctrl.h` / `aclapi.h` / `securitybaseapi.h` / `sddl.h` / `processthreadsapi.h` / `fileapi.h` / `namedpipeapi.h` / `synchapi.h` / `winbase.h`) and are cross-checked at runtime by [`verify/abi-probe.cpp`](verify/abi-probe.cpp) (sizes, offsets, enum values, static asserts):
The koffi struct definitions assert their sizes against the probe at module load, so a header/koffi layout drift fails loudly instead of corrupting memory.
## Verified boundaries (inherent to restricted tokens, not this port)
- **Writes are restricted; reads, network, and process visibility are not.** `WRITE_RESTRICTED` intersects write accesses only, so a confined child can read any caller-readable file and open sockets. `read-only` mode therefore cannot be expressed by this mechanism alone; pair it with a read-side policy or an AppContainer/`S-1-15-2` capability token for stronger confinement.
- **Console isolation is unavailable.** Under the restricted token, children created with `CREATE_NO_WINDOW` / `CREATE_NEW_CONSOLE` die during DLL initialization with `STATUS_DLL_INIT_FAILED` (`0xC0000142`). The POC tried to fix this by adding the console logon SID (`S-1-2-1`) to the restricting list; on Windows 11 26200 `CreateWellKnownSid(WinLocalLogonSid)` fails with `ERROR_INVALID_PARAMETER` (87), the correct `WinConsoleLogonSid` yields a valid `S-1-2-1` but the child still dies, and the POC's final revision removed both the SID and console isolation. Children therefore share the host console; stdio redirection is pipe-based and unaffected.
- **ACL grants are standing directory mutations.** They persist if the process dies mid-run; `dispose()` revokes them, and `init()` revokes already-applied grants when a later step fails. The POC's documented manual cleanup (`icacls <dir> /remove '*S-1-4-…'`) fails on this platform with `ERROR_NONE_MAPPED` (1332) — revoke through this module instead.
- **Granted directories must be caller-owned.** The owner's implicit `WRITE_DAC` is what lets the sandbox edit the DACL without elevation.
- **The temp grant follows `GetTempPathW`** — pass `tempDir` explicitly whenever possible. `GetTempPathW` reads the NATIVE environment block, which host runtimes that manage `process.env` through worker pools may not keep in sync (verified with vitest: a worker-side `process.env.TMP` change never reached the native block). A defaulted grant landing on the real temp dir inherits `(OI)(CI)` over every subdirectory of temp, silently widening the allowlist — point it at a per-sandbox directory instead.
Indirectly, through [`dsh-bash-sandbox`](../../bash/bash-sandbox/README.md), [`dsh-pwsh-sandbox`](../../bash/pwsh-sandbox/README.md), and their tools, which render this backend's enforcement and denial facts (the confined stderr the tool layer classifies through `denialSignatures`) while the [`dsh-sandbox`](../sandbox/README.md) seam owns the `SANDBOX_UNAVAILABLE` text and runner selection.
#### KV Cache effect
None directly; the denial surface belongs to the tool layer.
- **One write allowlist per instance** — the orphan SID is the unit of the allowlist; reusing one sandbox instance across two workspaces widens both grants to both roots. Create one instance per workspace root.
- **Cleanup is best-effort by design** — `dispose()` attempts every revocation and aggregates failures into an `AggregateError`; a cleanup failure leaves a standing (but orphan-SID-only) ACE that this process's next `init()`/`dispose()` cycle or `icacls` (via the ACE, not the trustee name) can still remove.
- **Each confined command mutates two directory DACLs** — a grant on entry and a revoke on exit, on the workspace root and the temp root: a handful of Win32 calls per command (inheritance is evaluated lazily per access, not a per-file walk). The runner pays this per command; reusing one grant per session is deferred work if the churn ever matters.
- **Read-side confinement and network policy are out of scope** — `WRITE_RESTRICTED` intersects write accesses only; pair this backend with a read-side policy for stronger confinement.
