build-policy.mjs

import fs from "node:fs";
import crypto from "node:crypto";

const hashAlgo = "sha512";

function hash(file) {
	return new Promise(resolve => {
		fs.createReadStream(file).pipe(
			crypto.createHash(hashAlgo).setEncoding("base64")
		).on("finish", function() {
			resolve(`${hashAlgo}-${this.read()}`); 
		});
	});
}

const fsExploitFile = "./fs-exploit.mjs";
const relativeFs = "./relative-fs.mjs";

const [fsExploitHash, safeFsHash] = await Promise.all([
	hash(fsExploitFile),
	hash(relativeFs)
]);

fs.writeFileSync("policy.json", JSON.stringify({
	resources: {
		[fsExploitFile]: {
			integrity: fsExploitHash,
			dependencies: {
				"node:fs": relativeFs,
				"node:path": true,
				"node:process": true
			}
		},
		[relativeFs]: {
			integrity: safeFsHash,
			dependencies: {
				"node:fs": true,
				"node:path": true,
				"node:process": true
			}
		}
	}
}, null, 2));

fs-exploit.mjs

import fs from "node:fs";
import path from "node:path";
import process from "node:process";

fs.readFileSync("./secret.txt");

// May not read from parent directory
try {
	fs.readFileSync("../secret.txt");
} catch (e) {
	console.error(e);
}

// May not read from same directory on a different drive
if (process.platform === "win32") {
	try {
		const cwd = process.cwd();
		const [drive, directory] = cwd.split(":");
		const driveCharCode = drive.toUpperCase().charCodeAt(0);
		const nextDriveLetter = String.fromCharCode(driveCharCode === 90 /*Z*/ ? 65 /*A*/ : driveCharCode + 1);
		const sameDirectoryWrongDrive = path.join(`${nextDriveLetter}:${directory}`, "secret.txt");
		fs.readFileSync(sameDirectoryWrongDrive);
	} catch (e) {
		console.error(e);
	}
}

policy.json

{
  "resources": {
    "./fs-exploit.mjs": {
      "integrity": "sha512-FIqIT3ls3kDsaSuLaE9nZnlMHj5qJJqnjXJZykg/3V85N+SYAisZQbqO81b9UFpbnKwO8fJnTlDNDYKaKD/Nkw==",
      "dependencies": {
        "node:fs": "./relative-fs.mjs",
        "node:path": true,
        "node:process": true
      }
    },
    "./relative-fs.mjs": {
      "integrity": "sha512-JLzd/IljL8i9h1wGTiE4uwvJS9k+K1W5jral+4aUdw7ZcIX5EeVMHHYWUU80S1XfrlcJ8h1qFWxo8vTUEW8q2A==",
      "dependencies": {
        "node:fs": true,
        "node:path": true,
        "node:process": true
      }
    }
  }
}

relative-fs.mjs

import fs from "node:fs";
import path from "node:path";
import process from "node:process";

const isWindows = process.platform === "win32";
const cwd = process.cwd();
const drive = isWindows ? getDrive(cwd) : null;

function getDrive(file) {
	return file.split(":")[0].toUpperCase();
}

function isRelativePath(file) {
	const normalized = path.normalize(file);
	if (isWindows && path.isAbsolute(normalized) && drive !== getDrive(normalized)) {
		return false;
	}
	const relative = path.relative(cwd, normalized);
	return !relative.startsWith("..");
}

function readFileSync(file, options) {
	if (!isRelativePath(file)) {
		throw new Error(`ENOENT: no such file or directory, open '${file}'`);
	}
	return fs.readFileSync(file, options);
}

export default { readFileSync };

secret.txt

Secret

test.cmd

@node ./build-policy.mjs
@node --experimental-policy=policy.json ./fs-exploit.mjs