DICE CTF 2022 – knock-knock

knock-knock

by BrownieInMotion

Knock knock? Who's there? Another pastebin!!

knock-knock.mc.ax

Downloads

index.js

Dockerfile

We start by checking the web page and notice that we can create a pastebin:

Browsing

We can see the content of our paste by sending a GET request on /note and each note has a unique id and we notice that token parameter is apparently some sort of signature:

Random Content

As you can see if we tamper the token we get invalid token error:

Invalid Token

Time to look at the source code. We proceed by checking the index.js file and we notice that on launch the flag is set directly in a pastebin. The id parameter is incremental which means that our flag is on id=0

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 })

Another interesting finding here is that crypto.randomUUID is called without parentheses () to execute which made me test my theory directly in node as shown below:

Node Results

As you can see above crypto.randomUUID is just the source code of the function. This means that they key used to sign the id is static. We can simply sign 0 in order to see the flag by running the code below:

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')
  }
}
let test = new Database()
console.log(test.generateToken(0)) // Generates this: 7bd881fe5b4dcc6cdafc3e86b4a70e07cfd12b821e09a81b976d451282f6e264

By sending id as 0 and the token we just got we can see the flag now: