ISSessions CTF 2021 Programming - Image1

Image1

Bad CAPTCHA, I’m a robot.

Solution

For this, I knew I had to use some sort of OCR that was smart enough to handle the graphic changing and inputting a raw image.

Tesseract it is!

import pytesseract
import requests
import cv2
from lxml import html
import hashlib

def image1():
    s = requests.session()
    pytesseract.pytesseract.tesseract_cmd = r'F:\Program Files\Tesseract-OCR\tesseract.exe'
    url = "http://progimage.ctf.issessions.ca:15000/"
    page = s.get(url)
    tree = html.fromstring(page.content)
    htmlimg = tree.xpath('///img[@alt="captch challenge graphic"]/@src')
    imgfile = requests.get(url + htmlimg[0])

    file = open("prog1.gif", "wb")
    file.write(imgfile.content)
    file.close()

    img = cv2.VideoCapture("prog1.gif")
    ret, frame = img.read()
    text = (pytesseract.image_to_string(frame)).strip()
    print(text)
    resp = s.get(url + "?answer=" + text)
    print(resp.content)

This python code works as follows:

• Initialize Tesseract’s home
• Get the full page contents
• Strip out the src attribute from the image
• Download the image
• Write it to a file locally
• Create an OpenCV video capture, since this is a gif file, and grab the first frame only
• Print out the text read from the image by Tesseract, just as a debug
• Send it back to the server
• Profit — aka, Flag.