# Agent Note: Windows sandbox rung: raw ACL restricted tokens over mxc and AppContainer

Status: implemented

English | [中文](2026-08-08-windows-acl-restricted-token-sandbox.zh.md)

## Problem

The original [sandbox decision](2026-07-06-sandbox.md) left `PLATFORM_CHAINS.win32` empty, so shipped Windows profiles degraded to danger-full-access because no confining executor existed. The win32 rung must govern the two file-effect modes in the sandbox vocabulary — `read-only` (no explicit writable root) and `workspace-write` (writes under the workspace root plus a backend-defined temp area) — while reporting any effects its mechanism cannot govern; reads, network, and process visibility remain outside this vocabulary.

## Decision

Implement the rung directly on the raw ACL mechanism: duplicate the caller's token into a `WRITE_RESTRICTED` token (`CreateRestrictedToken` with `WRITE_RESTRICTED` + `DISABLE_MAX_PRIVILEGE` + `LUA_TOKEN`) whose restricting SIDs carry distinct workspace and private-temp capabilities. `WRITE_RESTRICTED` intersects write accesses only, so reads keep the caller's ambient access while a write must also match one of these capability ACEs. The mechanism is the one huoyaoyuan/windows-acl-restrict-poc (`10e4dfb`) demonstrates; this port checks every API call and fails closed (the POC fail-opened on ignored failures). The per-workspace SID is derived deterministically from the canonical workspace path (`workspaceWriteSid` — sha256 → `S-1-4-x-y`); its standing workspace ACE is the cross-session reuse cache, and an exact-ACE skip prevents repeated eager tree propagation. Each live session/workspace pair instead gets a random private temp directory and a domain-separated SID derived from that path (`tempWriteSid`); its ACE is revocable, TMP/TEMP point at that directory, and the token default DACL names the temp SID so newly created temp objects do not acquire the shared workspace capability. A fork therefore cannot write its sibling's temp tree. A fresh provider chooses a new path and SID even for the same resumed session, so crash residue is inert litter rather than a collision or inherited capability; agentless calls create and remove the same shape per invocation. The ambient temp root is never an implicit grant. A workspace equal to or containing the temp root fails before any ACL mutation because its inheritable standing ACE would otherwise reach every private child; the direct API rejects overlap in either direction between a writable root and the actual private temp directory. PowerShell can complete its startup AppLocker probe through this private-temp capability, so `workspace-write` remains FullLanguage absent a host-wide policy; `read-only` cannot create the probe files and conservatively enters ConstrainedLanguage. That split is PowerShell startup behavior, not part of the ACL boundary. The token lists are read-only = [logon SID, Everyone] and workspace-write = [logon SID, Everyone, workspace SID, optional temp SID]. Logon SID + Everyone are keep-alive invariants (early DLL init dies with 0xC0000142 and CNG crashes pwsh with 0xE0434352 without them). Because Everyone remains, an external object granting Everyone write access clears both checks; because NTFS ACLs belong to file objects, a granted workspace hard link also grants an external alias. Rejecting all hard links would reject ordinary pnpm workspaces, so the provider reports `enforcement: 'partial'` and the native suite pins both gaps. Read-only carries no capability SID, so standing workspace ACEs remain inert across a mode downgrade. Authenticated Users is absent from both lists — CIM is unavailable, closing the C:\-root tree-creation escape — and INTERACTIVE/LOCAL are absent, denying Public-tree writes. New anonymous pipes and sync objects inherit the temp SID (or workspace SID when temp is disabled, Everyone under read-only) through `SetTokenInformation(TokenDefaultDacl)`; named pipes keep the Win32 layer's owner/SYSTEM/Admins-full, Everyone/ANONYMOUS-read-only template, so piped grandchild stdio remains denied. It ships as [`@deepseek-ai/dsh-sandbox-windows-acl`](../../../../packages/sandbox/sandbox-windows-acl/README.md), the `win32` rung of [`dsh-sandbox-local`](../../../../packages/sandbox/sandbox-local/README.md), and [`@deepseek-ai/dsh-pwsh-sandbox`](../../../../packages/bash/pwsh-sandbox/README.md) as the confining executor.

## How the restriction works (why no new identity)

The identity routes restrict by *who* runs the child; this rung restricts by *token derivation*. An identity route (landstrip's restricted-user, AppContainer) runs the child under a fresh account or container SID that starts with zero ACEs on the host's files — everything, reads included, defaults to denied, and every path the child may touch must then be opened back up by writing ACEs for that identity: the wholesale DACL mutation that disqualified both alternatives. The restricted token keeps the caller's own SID and logon session: [`CreateRestrictedToken`](https://learn.microsoft.com/en-us/windows/win32/api/securitybaseapi/nf-securitybaseapi-createrestrictedtoken) derives a token that adds the restricting SIDs and the `WRITE_RESTRICTED` flag, so Windows performs the access check twice — once against the normal SIDs, once against the restricting SIDs — and grants write-class access only where both checks pass. Reads pass on the normal check alone (the caller's SIDs already carry read access everywhere the caller can read), which is why this rung needs no read grants and no new account; writes must additionally clear the capability-SID check, which only the workspace and temp ACEs satisfy. `DISABLE_MAX_PRIVILEGE | LUA_TOKEN` synthesize the limited-user effect of a fresh account token-side, so even an elevated caller derives a filtered token. The same primitive could restrict reads (`SidsToDisable` turning SIDs deny-only), but a read-restricted token would need per-path read grants — reintroducing exactly the cost the identity routes pay — and the sandbox vocabulary never requires read confinement.

## Alternatives considered

### Why not mxc (Microsoft xContainer)?

Two disqualifiers. First, the OS floor is too new: the [mxc OS-version policy](https://github.com/microsoft/mxc/blob/main/docs/process-container/os-version-support.md) sets the product floor at Windows 11 24H2 (build 26100), and the BaseContainer tier (T1, `Experimental_CreateProcessInSandbox`) exists only on 25H2+ (build 26600+) with the OS feature enabled — on every supported release at or below 25H2 the filesystem policy falls back to T3, AppContainer plus host-side DACL ACE augmentation. Second, supporting arbitrary-path reads under either tier means granting read access by writing ACLs over every path the child may read: a model that reads the whole workspace and arbitrary files would require wholesale host DACL mutation — a standing side effect and a cost a write-only restriction does not need.

### Why not AppContainer?

An AppContainer token carries no ambient read access: every readable path must be pre-granted through capabilities or explicit ACEs, so arbitrary-path reads — the harness's read model — are unsupported without the same wholesale grants. The restricted token needs no read grants at all: it intersects write access only.

### Why not landstrip?

The [landstrip evaluation](../../rejected/feature/2026-07-26-evaluate-landstrip-for-windows-sandbox-rung.md) was rejected before implementation (not battle-tested; the in-house launcher plan won), and its Windows backend is AppContainer-shaped, inheriting the same arbitrary-read problem.

## Consequences

Bought: write-only confinement with no new OS floor (`CreateRestrictedToken` predates the mxc releases by two decades), reads/network/process visibility untouched exactly as the mode vocabulary requires, and fail-closed errors carrying the API name and exact Win32 code. Sessions share the intentionally standing workspace capability but not their revocable temp capabilities; restart residue cannot block or authorize a resumed session. Cost: enforcement is structurally partial because Everyone-granted writes and NTFS hard-link aliases cannot be path-confined by this token shape; no read-side or network isolation; console isolation unavailable (hidden-console children die with `STATUS_DLL_INIT_FAILED`; children share the host console); standing workspace ACE mutations (the reuse cache, plus inert residue when a workspace is renamed) and random temp litter after an unclean shutdown until OS hygiene reclaims it; EAGER full-tree workspace propagation (`SetNamedSecurityInfoW` walks every descendant immediately — tens of seconds on large workspaces), paid once per workspace per machine; CIM unavailable in both confined modes (Authenticated Users is absent, closing the C:\-root tree-creation escape); FAT-class non-ACL targets still writable; NULL-DACL directories not identity-preserving under a grant/revoke round trip; `whoami` and token-inspection cmdlets failing under the restricted token; read-only pwsh entering ConstrainedLanguage while workspace-write remains FullLanguage absent host policy; and named-pipe opens remaining denied, so libuv piped-stdio grandchildren fail with EPERM while inherited/ignored stdio and anonymous pipes work. The package README owns these operational limits.

## Testing

The product-visible Windows roster flip is win32-only, so keyless snapshots that must replay on macOS/Linux cannot cover it; bundle composition specs plus the win32 real-runner suites are the substitute evidence, and the CI Windows lane owns the assembled signal. `sandbox-local/tests/acl-grants.spec.ts` pins random temp allocation, per-session/workspace reuse, fork/workspace separation, crash-resume non-collision, paired argv SIDs, failure cleanup, and standing-versus-revocable lifecycle with Win32 mocked. On Windows, `workspace-sid.spec.ts` pins workspace/temp derivation and domain separation; `acl.spec.ts` pins real DACL lifecycle; and `runner.spec.ts` pins paired-SID validation, sibling temp denial under a shared workspace SID, per-call agentless temp creation/removal, TMP/TEMP rewriting, mode downgrade, Public denial, Everyone/hard-link partial boundaries, mode-specific PowerShell language behavior, and grandchild stdio. ARM64 and emulated x64 native runs own the architecture-specific acceptance evidence.

## Related

The [pwsh executor decision](2026-08-01-pwsh-tool-and-executor.md) owns the pwsh-sandbox/tool-pwsh dialect split this rung consumes.
