Tags: qr 

Rating: 5.0

# Web Tunnel
Web, 260 points

>I was just going to search some random cat videos on a Saturday morning when my friend came up to me and told me to reach the end of this tunnel (http://tunnel.web.easyctf.com/). Can you do it for me?

Behind the link was a site with a single link to a qr image.

![Example qr code image](./KjZqZwwKROwxELfzz6fN.png)

The qr image contained a string that was similiar to the previous qr code filename. Appending the string to http://tunnel.web.easyctf.com/ revealed another qr code. This pattern seemed to continue so, I wrote a python script to read each of the qr codes. I used [qrtools](https://github.com/primetang/qrtools) to read the qr codes, and StringIO to avoid having to save any of the qr images to a hard drive. For retrieving the qr codes from the web, I used a subprocess call to curl.

``` python
from StringIO import StringIO
import qrtools
import subprocess
import time
import sys

url_suffix = '.png'
url_prefix = 'http://tunnel.web.easyctf.com/'
qr = qrtools.QR()

# Parse the first page
start = subprocess.check_output(['curl', '-q', url_prefix])
start = ''.join(start.split(
'here')[0].replace('\'', '')
url_prefix += start.split('/')[0] + '/'
filename = start.split('/')[1]

while True:
# Read all the qr codes
print filename
qr_file = subprocess.check_output(
['curl', '-q', url_prefix + filename])
qr_io = StringIO(qr_file)
except IOError:
filename = qr.data + url_suffix

The flag was: easyctf{w0w_y0u_reached_th3_3nd_0f_my_tunnel!!!!!}

Original writeup (https://github.com/Migdalo/writeups/tree/master/2017-easyctf/web_tunnel).