<- WEB

baby breaking grad

4 minutes to read

We are provided with this webpage:

baby breaking grad 1

Let’s click in the button:

baby breaking grad 2

Source code analysis

Alright, since we are provided with source code, let’s take a look. It is a Node.js project using Express JS. This is index.js:

const express       = require('express');
const app           = express();
const bodyParser    = require('body-parser');
const routes        = require('./routes');
const path          = require('path');

app.use(bodyParser.json());
app.set('views','./views');
app.use('/static', express.static(path.resolve('static')));

app.use(routes);

app.all('*', (req, res) => {
    return res.status(404).send('404 page not found');
});

app.listen(1337, () => console.log('Listening on port 1337'));

This file is quite standard. Let’s take a look at the routes (routes/index.js):

const randomize         = require('randomatic');
const path              = require('path');
const express           = require('express');
const router            = express.Router();
const StudentHelper     = require('../helpers/StudentHelper');

router.get('/', (req, res) => {
    return res.sendFile(path.resolve('views/index.html'));
});

router.post('/api/calculate', (req, res) => {
    let student = req.body;

    if (student.name === undefined) {
        return res.send({
            error: 'Specify student name'
        })
    }

    let formula = student.formula || '[0.20 * assignment + 0.25 * exam + 0.25 * paper]';

    if (StudentHelper.isDumb(student.name) || !StudentHelper.hasPassed(student, formula)) {  
        return res.send({
            'pass': 'n' + randomize('?', 10, {chars: 'o0'}) + 'pe'
        });
    }

    return res.send({
        'pass': 'Passed'
    });
});

module.exports = router;

Here we see that /api/calculate is the unique endpoint we have. We are able to pass name and formula in the request body, and they will be processed by StudentHelper.isDumb and StudentHelper.hasPassed:

const evaluate = require('static-eval');
const parse = require('esprima').parse;

module.exports = {
    isDumb(name){
        return (name.includes('Baker') || name.includes('Purvis'));  
    },

    hasPassed({ exam, paper, assignment }, formula) {
        let ast = parse(formula).body[0].expression;
        let weight = evaluate(ast, { exam, paper, assignment });

        return parseFloat(weight) >= parseFloat(10.5);
    }
};

The above functions allow us to calculate some grade. For the first one, we must enter a name that does not contain Baker or Purvis. Then, for Student.hasPassed, the object student must contain attributes exam, paper and assignment. After that, the formula is parsed and executed on the previous attributes.

Notice that there’s a default formula:

    let formula = student.formula || '[0.20 * assignment + 0.25 * exam + 0.25 * paper]';

However, the key here is that we can define a new formula that will be processed by evaluate (static-eval). This is a problem because we have the chance to execute JavaScript code.

Let’s start by passing the subject (grade greater than 10.5):

$ curl 134.122.103.40:30580/api/calculate -d '{"formula":"[11]","name":"x","exam":"","paper":"","assignment":""}' -H 'Content-Type: application/json'  
{"pass":"Passed"}

Finding a working exploit

There are no vulnerabilities that apply to static-eval version 2.0.2 in snyk.io:

{
  "name": "breaking-grad",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "nodeVersion": "v12.18.1",
  "scripts": {
    "start": "node index.js",
    "dev": "nodemon .",
    "test": "echo \"Error: no test specified\" && exit 1"  
  },
  "keywords": [],
  "authors": [
    "makelaris",
    "makelarisjr"
  ],
  "dependencies": {
    "body-parser": "^1.19.0",
    "express": "^4.17.1",
    "randomatic": "^3.1.1",
    "static-eval": "2.0.2"
  }
}

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However, we can find some articles showing how to bypass a restrictive sandbox and get Remote Code Execution (RCE) in static-eval via Prototype Pollution. For example, this blogpost from Faraday. Nevertheless, the key is to look at the static-eval GitHub repository, because we can find a commit for version 2.0.3 that corrected a security issue in version 2.0.2:

baby breaking grad 4

In fact, the pull request adds a new test case that looks like an exploit:

baby breaking grad 5

Let’s use Burp Suite (Repeater) to test a bit:

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Now, let’s try the exploit:

baby breaking grad 8

It seems to work, since there are no errors shown. Hence, we might be able to get RCE since we can execute arbitrary JavaScript code. For instance, let’s use child_process to execute sleep 5:

baby breaking grad 9

Alright, so we have blind RCE. We know this because we won’t see the output of our commands:

baby breaking grad 10

Well, not all the output… Here we have to consider two things. First, the flag filename is randomized in the entrypoint.sh:

#!/bin/bash

# Secure entrypoint
chmod 600 /entrypoint.sh

# Generate random flag filename
FLAG=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 5 | head -n 1)  
mv /app/flag /app/flag$FLAG

exec "$@"

Which can be bypassed using a wildcard /app/flag*:

baby breaking grad 11

And the second thing is that errors are shown. Therefore, instead of printing the flag, let’s try to execute the flag as a command in a subshell (so that the shell errors out):

baby breaking grad 12

Flag

And there is the flag (notice that &#39; is a single quote in HTML encoding):

baby breaking grad 13

HTB{f33l1ng_4_l1ttl3_blu3_0r_m4yb3_p1nk?...you_n33d_to_b3h4v'eval!!}