ctf-writeup/AmateursCTF 2023/funny factorials

funny factorials

I made a factorials app! It's so fancy and shmancy. However factorials don't seem to properly compute at big numbers! Can you help me fix it?

About the Challenge

We got a python code called app.py. Here is the content of app.py

from flask import Flask, render_template, request
import sys

app = Flask(__name__)

def factorial(n):
    if n == 0:
        return 1
    else:
        try:
            return n * factorial(n - 1)
        except RecursionError:
            return 1

def filter_path(path):
    # print(path)
    path = path.replace("../", "")
    try:
        return filter_path(path)
    except RecursionError:
        # remove root / from path if it exists
        if path[0] == "/":
            path = path[1:]
        print(path)
        return path

@app.route('/')
def index():
    safe_theme = filter_path(request.args.get("theme", "themes/theme1.css"))
    f = open(safe_theme, "r")
    theme = f.read()
    f.close()
    return render_template('index.html', css=theme)

@app.route('/', methods=['POST'])
def calculate_factorial():
    safe_theme = filter_path(request.args.get("theme", "themes/theme1.css"))

    f = open(safe_theme, "r")
    theme = f.read()
    f.close()
    try:
        num = int(request.form['number'])
        if num < 0:
            error = "Invalid input: Please enter a non-negative integer."
            return render_template('index.html', error=error, css=theme)
        result = factorial(num)
        return render_template('index.html', result=result, css=theme)
    except ValueError:
        error = "Invalid input: Please enter a non-negative integer."
        return render_template('index.html', error=error, css=theme)

if __name__ == '__main__':
    sys.setrecursionlimit(100)
    app.run(host='0.0.0.0')

There is / endpoint that accepts 2 HTTP request method, POST and GET. The website retrieves the value of the theme parameter, read the content of the corresponding file, and then return it in the index.html. Here is the preview of the website

How to Solve?

As you can see, we need to input the flag location into theme parameter to obtain the flag. But, there is a function called filter_path to prevent the path traversal vulnerability.

def filter_path(path):
    # print(path)
    path = path.replace("../", "")
    try:
        return filter_path(path)
    except RecursionError:
        # remove root / from path if it exists
        if path[0] == "/":
            path = path[1:]
        print(path)
        return path

filter_path function takes a path string, removes any ../ sequences, and recursively continues filtering until no ../ remains. It also removes a leading forward slash if present, and then returns the filtered path. For example:

Our input: ../../../../test
The output: test

Even though you added ../ in the middle of payload, the function will remove it because of the recursion. But lets take a look again in this part

# remove root / from path if it exists
if path[0] == "/":
    path = path[1:]
print(path)
return path

If the first character in our input is /, the function will remove the /. But if we double the / and the payload will be like this

//flag.txt

The function only remove the first character and the function will return /flag.txt.

amateursCTF{h1tt1ng_th3_r3curs10n_l1mt_1s_1mp0ssibl3}