BlackHole CTF Challenge Writeup

Challenge Information

Name: BlackHole
Points: 380
Category: Web
Objective: The objective of the BlackHole challenge is to find the flag hidden within the URL or the web page’s content.

Solution

To solve the BlackHole CTF challenge, follow these steps:

Initial Assessment:
- Identified that the challenge seemed the same as the preliminary one.

Script Adaptation:

Utilized a script from a writeup from that day.

Adapted the script to suit my needs for the challenge.

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import requests
from bs4 import BeautifulSoup
from urllib.parse import urljoin, urlparse, unquote

visited_links = set()

def get_links(url):
    try:
        response = requests.get(url)
        soup = BeautifulSoup(response.text, 'html.parser')
        links = soup.find_all('a', href=True)
        return [urljoin(url, link['href']) for link in links]
    except requests.exceptions.RequestException as e:
        print(f"Error retrieving links from {url}: {e}")
        return []

def is_directory(soup):
    return not "No directories found." in soup.get_text() and soup.find_all(class_="directory-link")

def visit_links_recursive(url):
    if url in visited_links:
        return

    parsed_url = urlparse(url)
    base_domain = "blackhole.ctf.rawsec.com"

    if parsed_url.netloc != base_domain:
        return

    print(f"Visiting: {unquote(url)}")

    try:
        response = requests.get(url)
        soup = BeautifulSoup(response.text, 'html.parser')

        if is_directory(soup):
            print(f"Directory found: {url}")

        visited_links.add(url)

        links = get_links(url)

        for link in links:
            visit_links_recursive(link)

    except requests.exceptions.RequestException as e:
        print(f"Error visiting {url}: {e}")

def main():
    starting_url = 'https://blackhole.ctf.rawsec.com/'
    visit_links_recursive(starting_url)

if __name__ == "__main__":
    main()

Flag.txt Extraction:
- Used the adapted script to find a flag.txt file.
- However, the flag was not directly found in the flag.txt file.
Rereading the URL:
- Revisited the challenge’s instructions to reread the URL.
CyberChef Analysis:
- Analyzed the URL using CyberChef.
- Discovered the flag hidden within the URL.