GHSA-hg73-4w7g-q96w SandboxJS: Sandbox Escape via Prop Object Leak in New Handler

• npm @nyariv/sandboxjs: < 0.8.36 → fixed in 0.8.36

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 ```typescript addOps 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 ```typescript addOps(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.contextobj.prop); 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: ```typescript 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: ```javascript { globalThis: , Function: , eval: , 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 ```bash mkdir sandboxjs-poc cd sandboxjs-poc npm init -y ``` Step 2: Set module type to ESM ```bash 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 ```bash npm install @nyariv/[email protected] ``` Step 4: Create the minimal exploit ```bash cat > exploit.mjs 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: ```typescript Object.freeze(options.globals); ```