README.md

01 - A letter from Santa

Description

Level: Easy
Author: coderion

Finally, after 11 months of resting, Santa can finally send out his presents and challenges again. He was writing a letter to his youngest baby elf, who's just learning his ABC/A-Z's. Can you help the elf read the message?

Solution

For this challenge we are given a ZIP file that contains the source of a website. The website looks like this:

We can select a letter from the alphabet (which somehow influences the message) as well as enter an arbitrary text. For the letter b in combination with the letter f, we get this output:

fffffffffffffffffffffffff

Some fs are backed out though:

The blacked out characters only seemed to depend on the letter selected of the alphabet. Moreover, the string output had a length of 25 which hinted at the dimensions of a QR code. I decided to write the following script:

import requests
import string

URL = "https://030029e6-ad43-4951-9442-33c64b7ac112.idocker.vuln.land/"
data = ""

for char in string.ascii_lowercase:
    params = {
        "alphabet_select": char,
        "user_input": "XXX"
    }
    response = requests.post(URL, data=params)
    data += response.text
    data += "<br/>"

with open("output.html", "w") as f:
    f.write(data)

It goes over all letters and uses them as input once. The output of the above program is a html page which looks like this:

As already expected, we can see a QR code which we can scan to get the flag HV23{qr_c0des_fun}.