Can you read the admin's email?
User: billy
Pass: TheKid

Solution

There's a mail system, if you input the given credentials..

..it will login and display a email sent from administrador.

...

I dug up the system and found no other functionality.

So, to read the email you do this POST request:

POST /viewmail/ HTTP/1.1  
Host: lab.shellterlabs.com:32867  
Content-Length: 63  
Cache-Control: max-age=0  
Origin: http://lab.shellterlabs.com:32867  
Upgrade-Insecure-Requests: 1  
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36  
Content-Type: application/x-www-form-urlencoded  
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8  
Referer: http://lab.shellterlabs.com:32867/  
Accept-Encoding: gzip, deflate  
Accept-Language: en-US,en;q=0.8  
Cookie: sessionid="billy:6405552caf11c7b1aa5b18a3346ae1f13eafa516"; csrftoken=s3iaLUoYZpWFkJaaMjvj8YOsCATokC9e  
Connection: close

csrfmiddlewaretoken=s3iaLUoYZpWFkJaaMjvj8YOsCATokC9e&email_id=1  

The csrfmiddlewaretoken has not working here, It just need to be equal the cookie csrftoken. But when you change the sessionid on cookie the request will fail:

Well, the challenge consists in read the email of the administrator. The footer gives the clue, the SHX Mail Administrator username is nealcaffrey.

To read the nealcaffrey email we need to forge his sessionid, but how?

I started by fuzzing the form fields and when I sent some symbols it triggered a django debuggin page displaying details from an exception and leaking part of the source code.

Nice, the administrator forgotten to disable Debug in a production environment, very common..

def generate_session_token(username):  
    user_hash = sha1(username).digest()
    return stream_cipher(user_hash).encode('hex')

This code is showing us how the sessionid is created:

• The user_hash is the hex of sha1("billy")
• sessionid is the hex result of some Stream Cipher algorithm over the user_hash

What is a Stream Cipher?

Wikipedia says: A stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the ciphertext stream.

We don't know which algorithm is used, there's so much popular stream cipher algorithms out there, but the most common one is a simple XOR byte-per-byte and I bet the challenger used it. But to test it we need first recover the key used in encryption.

Recovering the key and the flag

• Create the billy_user_hash
• XOR byte-per-byte billy_user_hash with sessionid_hash
• Now we can XOR the key with any username_user_hash, including nealcaffrey_user_hash to get a valid admin sessionid

Final script

Just for note, you can replace this xor_bytearray() function and part of this code by a simple hex(hex1 ^ hex2), because Python do a xor byte-wise by default in HEX values!

..and the flag

Thanks ShellterLabs and RoboCore, awesome contest!