misc Writeup

Unintended solution: answer leak in file name.

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Challenge Description

Accessing the URL given by the challenge, we are greeted with a fingerprint and are prompted to determine if it’s a male or female fingerprint.

Solution

The intended solution was to train a model to classify fingerprints. However, if you right-click the image and open it in a new tab, it will download a file that contains the gender the fingerprint belongs to.

Automation Script

To automate the process, I coded a simple Python script to go through the 90 fingerprints by downloading the image and extracting the file name. The flag will be printed at the end.

import requests
from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.chrome.service import Service
from selenium.webdriver.support.ui import Select
from webdriver_manager.chrome import ChromeDriverManager

# Set up the WebDriver
options = webdriver.ChromeOptions()
options.add_argument('--headless')  # Run in headless mode if you don't need a GUI
driver = webdriver.Chrome(service=Service(ChromeDriverManager().install()), options=options)

url = "http://34.107.71.117:30921/"
driver.get(url)

for i in range(0, 90):
    image = driver.find_element(By.TAG_NAME, 'img')
    src = image.get_attribute('src')
    headers = {
        'Cookie': f'user_challenges={driver.get_cookie("user_challenges")["value"]}'
    }
    response = requests.get(src, headers=headers, stream=True)
    content_disposition = response.headers['Content-Disposition']
    file_name = content_disposition.split('filename=')[-1].strip('"')
    if file_name.split('_')[2] == 'M':
        choice = 'M'
    else:
        choice = 'F'
    select = Select(driver.find_element(By.ID, 'label'))
    select.select_by_value(choice)
    driver.find_element(By.XPATH, '//form').submit()
    print(driver.page_source)

# Close the WebDriver session
driver.quit()

Flag

CTF{97c87b18d5fd447d1e180aeee8e474e74ac950cd567489cf51004cf12ead8fae}