🛡️ @openclaw/fs-safe

Capability-style filesystem roots for Node.js apps that handle untrusted relative paths.

Think Go's os.Root / OpenInRoot or Rust's cap-std, but for Node. Hand root() a trusted directory and you get back a handle whose every method resolves relative paths against it and refuses to escape — through .., symlink swaps, hardlink aliases, or TOCTOU rename races between check and use.

import { root } from "@openclaw/fs-safe";

const fs = await root("/safe/workspace");
await fs.write("notes/today.txt", "hello\n");   // ok
await fs.write("../escape.txt", "x");            // throws FsSafeError("outside-workspace")

That's the whole pitch. root() is the product; the rest of the package — JSON stores, atomic writes, secret files, archive extraction, temp workspaces — is supporting cast for the same boundary.

Full docs and reference at fs-safe.io.

Contents

Why this exists · Not a sandbox · Install · Quick start · Reading · Subpaths · Failure semantics · Atomic writes · Stores · Secure absolute reads · Walking · Archive extraction · Path scopes · Errors · Safety model · Limitations

Why this exists

Most Node code that has to touch caller-controlled paths reaches for:

path.resolve(root, input).startsWith(root)

That validates a string. It does not pin the file you opened, defend against a symlink retarget between check and use, reject hardlinked aliases of out-of-tree inodes, or verify that a write landed where you intended after a rename. The pieces to do those things exist scattered across the ecosystem — write-file-atomic for atomic writes, tar / jszip for archive extraction, various safefs-style convenience wrappers — but none of them give you one root handle with traversal-resistant semantics across every operation.

The same idea has landed in other languages. Go added os.Root and OpenInRoot; Rust has had cap-std for years. Node's fs is path-string-oriented and exposes flags like O_NOFOLLOW but not an ergonomic "operate inside this root" API. fs-safe fills that gap.

	Root boundary	Atomic writes	Symlink/hardlink defense	TOCTOU resistance	Archive extraction
path.resolve().startsWith()	string check only	–	–	–	–
write-file-atomic	–	✓	–	–	–
Go os.Root / Rust cap-std	✓	platform	✓	✓	–
@openclaw/fs-safe	✓	✓	✓	✓ (POSIX fd-relative)	✓ (ZIP/TAR)

Not a sandbox

This is a library-level guardrail, not OS-level isolation. It does not replace containers, seccomp, AppArmor, or filesystem permissions. It is for code that already runs with the privileges of its workspace and wants to stop trivial path tricks from escaping it. If your threat model is a hostile process, you need OS isolation; if your threat model is "an agent, plugin, upload handler, or CLI will eventually be tricked into writing somewhere it shouldn't," fs-safe catches that.

Install

pnpm add @openclaw/fs-safe

Node 20.11 or newer. Core root/path/json/temp helpers avoid framework dependencies. Archive helpers use optional jszip and tar dependencies for ZIP/TAR support; installs that omit optional dependencies can still use every non-archive subpath.

On POSIX, root() uses one process-global persistent Python helper for the fd-relative operations Node does not expose ergonomically (renameat, unlinkat, recursive mkdirat-style walks, and parent-fd writes). Configure it before first use when you need a strict environment policy:

import { configureFsSafePython } from "@openclaw/fs-safe";

configureFsSafePython({ mode: "auto" });    // default: use helper, fall back if unavailable
configureFsSafePython({ mode: "off" });     // never spawn Python; use Node fallbacks
configureFsSafePython({ mode: "require" }); // fail closed if helper cannot start

Equivalent env vars: FS_SAFE_PYTHON_MODE=auto|off|require and FS_SAFE_PYTHON=/path/to/python3. Without Python, fs-safe keeps lexical and canonical root checks, no-follow opens, atomic temp+rename writes, and post-write identity verification. What you lose is the strongest POSIX fd-relative protection against a same-process-user racer swapping parent directories between validation and mutation. Windows already uses the Node fallback path. See the Python helper policy for deployment guidance.

Quick start

import { root } from "@openclaw/fs-safe";

const fs = await root("/safe/workspace", {
  hardlinks: "reject",
  symlinks: "reject",
  mkdir: true,
  mode: 0o600,
});

await fs.write("notes/today.txt", "hello\n");
const text = await fs.readText("notes/today.txt");
const config = await fs.readJson("config.json");
await fs.copyIn("uploads/upload.png", "/tmp/upload.png");
await fs.move("notes/today.txt", "notes/archive/today.txt", { overwrite: true });
await fs.remove("notes/archive/today.txt");

root() takes the trusted directory; relative paths in subsequent calls are resolved against it. Defaults you pass to root() apply to every call below; per-call options override them.

When you need metadata or a FileHandle:

const { buffer, realPath, stat } = await fs.read("notes/today.txt");
const opened = await fs.open("notes/today.txt");

create() is the don't-clobber variant of write() and throws already-exists when the target already exists:

await fs.create("notes/README.md", "seed\n"); // throws if it already exists

write() replaces file contents by default; pass { overwrite: false } or use create() when an existing file should be an error. move() defaults to no clobber because it can otherwise delete an unrelated target while also consuming the source. Pass { overwrite: true } when replacing the target is intended.

Use ensureRoot() when a computed relative directory target resolves to the root itself ("" or ".") and you want the operation to be accepted. root() still requires the trusted root directory to already exist.

Reading

Pick the narrowest read shape that gives you what you need:

await fs.readJson("config.json"); // parsed value; validate it at your boundary
await fs.readText("notes/today.txt");
await fs.readBytes("image.png");
await fs.read("notes/today.txt"); // { buffer, realPath, stat }
const opened = await fs.open("large.log"); // FileHandle for streaming

For streams, use open() and the returned FileHandle:

await using opened = await fs.open("large.log");
{
  const stream = opened.handle.createReadStream();
  // consume stream
}

Root reads default to DEFAULT_ROOT_MAX_BYTES (16 MiB). Pass a larger maxBytes for expected large reads, or Number.POSITIVE_INFINITY when the caller has a separate size budget.

reader() returns a callback that reads absolute or relative paths through the same root boundary. It is useful for APIs that accept a (path) => Promise<Buffer> loader. Absolute paths outside the root are rejected with outside-workspace. readAbsolute() has the same absolute-path behavior directly.

When you need a writable FileHandle, use openWritable() and prefer await using for cleanup:

await using opened = await fs.openWritable("logs/current.log", { writeMode: "append" });
{
  await opened.handle.appendFile("line\n");
}

nonBlockingRead is the only I/O scheduling knob in RootDefaults; it applies to read/open operations because it changes how file descriptors are opened. Filesystem safety policy remains explicit through hardlinks and symlinks.

stat(), exists(), and list() are boundary-checked, but they cannot pin a later operation to the same filesystem object. Use read(), open(), write(), create(), copyIn(), move(), or remove() for operations that must be race-resistant at the point of use.

Subpaths

The main entry point is intentionally small: root, the root option/result types, and FsSafeError. Use subpaths for everything else. Low-level helpers that OpenClaw needs to compose higher-level APIs are grouped under @openclaw/fs-safe/advanced instead of being separate public leaf contracts.

Subpath	Contents
@openclaw/fs-safe/root	root(), Root, RootDefaults, related types
@openclaw/fs-safe/config	process-global Python helper configuration
@openclaw/fs-safe/path	canonical path checks: isPathInside, safeRealpathSync, isNotFoundPathError, isSymlinkOpenError
@openclaw/fs-safe/json	tryReadJson, readJson, readJsonIfExists, writeJson, sync variants
@openclaw/fs-safe/store	fileStore, fileStoreSync, and jsonStore
@openclaw/fs-safe/secret	strict and try-style secret file read/write helpers
@openclaw/fs-safe/atomic	replaceFileAtomic, replaceFileAtomicSync, replaceDirectoryAtomic, movePathWithCopyFallback
@openclaw/fs-safe/temp	tempWorkspace, tempWorkspaceSync, withTempWorkspace, resolveSecureTempRoot
@openclaw/fs-safe/secure-file	fd-pinned absolute file reads with owner, mode, ACL, trusted-dir, size, and timeout checks
@openclaw/fs-safe/file-lock	acquireFileLock, withFileLock, createFileLockManager, and related lock types
@openclaw/fs-safe/permissions	POSIX mode and Windows ACL inspection plus remediation formatting helpers
@openclaw/fs-safe/walk	budget-bounded directory walking with symlink policy, filters, and truncation accounting; not root-bounded
@openclaw/fs-safe/archive	extractArchive, resolveArchiveKind, ArchiveLimitError, preflight helpers
@openclaw/fs-safe/advanced	lower-level composition helpers such as path scopes, root-file open, install paths, filename sanitizing, temp-file targets, sibling-temp writes, local-root readers, regular-file helpers, pathExists, and withTimeout; less stable than focused public subpaths
@openclaw/fs-safe/errors	FsSafeError, FsSafeErrorCode
@openclaw/fs-safe/types	shared types: DirEntry, PathStat, …
@openclaw/fs-safe/test-hooks	hooks the test suite uses to inject races; only active under NODE_ENV=test

Failure semantics in the name

When two helpers behave differently on the same input, the difference is in the name, not the docs.

import { readJson, tryReadJson } from "@openclaw/fs-safe/json";

await tryReadJson("./config.json"); // returns null on missing or invalid
await readJson("./manifest.json");  // throws on missing or invalid

For one-off structured reads under a trusted root, readRootJsonObjectSync() performs the root-bounded open and JSON object validation in one step. Use readRootStructuredFileSync() when the parser lives outside fs-safe, such as JSON5-backed plugin manifests.

Atomic writes

replaceFileAtomic() writes a sibling temp file, optionally fsyncs it, and renames it over the destination. Mode preservation, rename retry / copy fallback on EPERM, parent-directory fsync, and a beforeRename hook for backup or observer flows are all opt-in.

import { replaceFileAtomic } from "@openclaw/fs-safe/atomic";

await replaceFileAtomic({
  filePath: "/safe/workspace/state.json",
  content: JSON.stringify(state, null, 2),
  mode: 0o600,
  syncTempFile: true,
  syncParentDir: true,
});

replaceFileAtomicSync() covers the synchronous case with the same options shape. Both accept an injectable fileSystem for tests.

Stores

Use fileStore().json() for small state files that need explicit fallback reads, atomic writes, and optional sidecar locking around read-modify-write updates:

import { fileStore } from "@openclaw/fs-safe/store";

const files = fileStore({ rootDir: "/safe/workspace/state", private: true });
const store = files.json("settings.json", { lock: true });

await store.updateOr({ enabled: false }, (current) => ({ ...current, enabled: true }));

jsonStore({ filePath }) is the absolute-path convenience wrapper for the same primitive.

Use update() when missing state is part of your model; use updateOr() for the common merge-into-defaults case. Standalone helpers use options bags because they do not carry a bound root and often need multiple authority, path, and policy knobs.

Use fileStore() for cache/blob/media-style directories where callers need safe relative paths, size limits, atomic replacement, stream writes, and TTL cleanup behind one root. Pass private: true for credentials, auth profiles, tokens, and per-agent private state; private mode keeps the same store shape while routing writes through the secret-file atomic path.

import { fileStore } from "@openclaw/fs-safe/store";

const media = fileStore({
  rootDir: "/safe/workspace/media",
  maxBytes: 5 * 1024 * 1024,
  mode: 0o600,
});

await media.write("inbound/photo.jpg", bytes);
await media.writeJson("state/photo.json", { id: "photo" });
const cached = await media.readJsonIfExists("state/photo.json");
const opened = await media.open("inbound/photo.jpg");
await media.pruneExpired({ ttlMs: 10 * 60 * 1000, recursive: true });

The store subpath also includes durable JSON queue helpers for the common "one JSON file per work item" pattern: atomic entry writes, pending-entry loads, acknowledgement via .delivered markers, failed-entry moves, and stale temp cleanup. Retry, dedupe, and transport semantics stay with the caller.

tempWorkspace() exposes write(), writeText(), writeJson(), copyIn(), and read() for single-file scratch workflows without hand-rolled path joins, plus a store: FileStore view of the workspace dir for the richer cases (writeStream, readJsonIfExists, store.json<T>(rel)).

tempFile() is the smaller one-file temp helper. It is intentionally an advanced primitive: use tempWorkspace() for the stable temp surface and reach for tempFile() only when you need a raw file target.

import { tempFile } from "@openclaw/fs-safe/advanced";

await using target = await tempFile({ prefix: "download", fileName: "payload.bin" });
await fs.promises.writeFile(target.path, bytes);
const checksumPath = target.file("payload.sha256");

Secure absolute file reads

Use readSecureFile() when the caller gives you an absolute credential path instead of a root-relative workspace path. It opens the file first, validates the same handle it will read from, checks trusted directories, owner, POSIX mode or Windows ACLs, hardlink count, size, and optional timeout, then reads through the pinned handle.

import { readSecureFile } from "@openclaw/fs-safe/secure-file";

const { buffer } = await readSecureFile({
  filePath: "/var/lib/app/token",
  label: "auth token",
  trust: { trustedDirs: ["/var/lib/app"] },
  io: { maxBytes: 16 * 1024, timeoutMs: 5_000 },
});

Use permissions: { allowInsecure: true } only for migration or explicit local-development flows where a warning is preferable to refusing the file.

Directory walking

walkDirectory() and walkDirectorySync() replace ad-hoc recursive readdir() loops with entry and depth budgets, a symlink policy, and stable relative paths.

import { walkDirectory } from "@openclaw/fs-safe/walk";

const scan = await walkDirectory("/safe/workspace", {
  maxDepth: 4,
  maxEntries: 10_000,
  symlinks: "skip",
  include: (entry) => entry.kind === "file",
});

for (const file of scan.entries) {
  console.log(file.relativePath);
}

Check scan.truncated before treating the result as complete.

Archive extraction

extractArchive() handles ZIP and TAR behind one API, with traversal checks, blocked-link-type rejection, and entry-count and byte budgets.

import { extractArchive, resolveArchiveKind } from "@openclaw/fs-safe/archive";

const kind = resolveArchiveKind(uploadPath);
if (!kind) throw new Error(`unsupported archive: ${uploadPath}`);

await extractArchive({
  archivePath: uploadPath,
  destDir: "/safe/workspace/plugin",
  kind,
  timeoutMs: 15_000,
  limits: {
    maxArchiveBytes: 256 * 1024 * 1024,
    maxEntries: 50_000,
    maxExtractedBytes: 512 * 1024 * 1024,
    maxEntryBytes: 256 * 1024 * 1024,
  },
});

Extraction stages into a private directory and merges through the same safe-open boundary used by direct writes, so a symlinked entry can't trick the merge into following an out-of-tree path.

Advanced path scopes

For code that already has a trusted absolute path and wants lower-level boundary validation without going through root():

import { pathScope } from "@openclaw/fs-safe/advanced";

const uploads = pathScope("/safe/uploads", { label: "uploads directory" });
const files = await uploads.files(["photo.jpg"]);
const target = await uploads.writable("report.pdf");

Errors

Every failure surfaces as an FsSafeError with a closed code union you can branch on:

import { FsSafeError } from "@openclaw/fs-safe/errors";

try {
  await fs.write("../escape.txt", "x");
} catch (err) {
  if (err instanceof FsSafeError && err.code === "outside-workspace") {
    // handle
  }
  throw err;
}

Codes are grouped by category:

if (err instanceof FsSafeError) {
  if (err.category === "policy") {
    // Unsafe caller input or filesystem state.
  } else {
    // Operational problem such as helper startup, timeout, or unverifiable permissions.
  }
}

Current FsSafeErrorCode values are already-exists, hardlink, helper-failed, helper-unavailable, invalid-path, insecure-permissions, not-empty, not-file, not-found, not-owned, not-removable, outside-workspace, path-alias, path-mismatch, permission-unverified, symlink, timeout, too-large, and unsupported-platform.

Safety model

• root-bounded APIs resolve paths against a configured root and reject canonical escapes
• reads open with O_NOFOLLOW where available, then verify fd identity matches the path identity before returning the buffer or handle
• writes use pinned parent-directory helpers and atomic replacement on POSIX, with verified post-write identity
• remove, mkdir, move, stat, list, and parent-fd writes use one persistent fd-relative Python helper on POSIX, with Node fallbacks when the helper is disabled or unavailable
• archive extraction stages into a private directory and merges through the same boundary checks used by direct writes

Limitations

• Windows uses the safest Node-level behavior available; some fd-relative POSIX hardening is unavailable there.
• Hardlink rejection depends on platform metadata. Treat it as defense-in-depth, not authorization.
• fs-safe does not validate file contents or archive payload semantics beyond filesystem safety constraints. Schemas, signatures, and authorization belong in the layer above.

License

MIT.