[SCTF2019]Flag Shop

[SCTF2019]Flag Shop(jwt,Ruby)

我们通过页面可以发现，需要我们获得1000000000000000000000000000才能得到flag，我们考虑抓包

获得了一串jwt

我们进行解析

我们可以伪造jwt来获得钱，但我们不知道密钥是什么，我们再找找有没有其他的信息

我们查看robots.txt,获得了源码

require 'sinatra'
require 'sinatra/cookies'
require 'sinatra/json'
require 'jwt'
require 'securerandom'
require 'erb'

set :public_folder, File.dirname(__FILE__) + '/static'

FLAGPRICE = 1000000000000000000000000000
ENV["SECRET"] = SecureRandom.hex(64)

configure do
  enable :logging
  file = File.new(File.dirname(__FILE__) + '/../log/http.log',"a+")
  file.sync = true
  use Rack::CommonLogger, file
end

get "/" do
  redirect '/shop', 302
end

get "/filebak" do
  content_type :text
  erb IO.binread __FILE__
end

get "/api/auth" do
  payload = { uid: SecureRandom.uuid , jkl: 20}
  auth = JWT.encode payload,ENV["SECRET"] , 'HS256'
  cookies[:auth] = auth
end

get "/api/info" do
  islogin
  auth = JWT.decode cookies[:auth],ENV["SECRET"] , true, { algorithm: 'HS256' }
  json({uid: auth[0]["uid"],jkl: auth[0]["jkl"]})
end

get "/shop" do
  erb :shop
end

get "/work" do
  islogin
  auth = JWT.decode cookies[:auth],ENV["SECRET"] , true, { algorithm: 'HS256' }
  auth = auth[0]
  unless params[:SECRET].nil?
    if ENV["SECRET"].match("#{params[:SECRET].match(/[0-9a-z]+/)}")
      puts ENV["FLAG"]
    end
  end

  if params[:do] == "#{params[:name][0,7]} is working" then

    auth["jkl"] = auth["jkl"].to_i + SecureRandom.random_number(10)
    auth = JWT.encode auth,ENV["SECRET"] , 'HS256'
    cookies[:auth] = auth
    ERB::new("<script>alert('#{params[:name][0,7]} working successfully!')</script>").result

  end
end

post "/shop" do
  islogin
  auth = JWT.decode cookies[:auth],ENV["SECRET"] , true, { algorithm: 'HS256' }

  if auth[0]["jkl"] < FLAGPRICE then

    json({title: "error",message: "no enough jkl"})
  else

    auth << {flag: ENV["FLAG"]}
    auth = JWT.encode auth,ENV["SECRET"] , 'HS256'
    cookies[:auth] = auth
    json({title: "success",message: "jkl is good thing"})
  end
end


def islogin
  if cookies[:auth].nil? then
    redirect to('/shop')
  end
end

通过代码审计，我们可以知道这段代码是用Ruby编写的一个基本的Web应用程序，使用了Sinatra框架

由此我们可以想到ruby模块注入

注入点在/work

if params[:do] == "#{params[:name][0,7]} is working" then

  auth["jkl"] = auth["jkl"].to_i + SecureRandom.random_number(10)
  auth = JWT.encode auth,ENV["SECRET"] , 'HS256'
  cookies[:auth] = auth
  ERB::new("<script>alert('#{params[:name][0,7]} working successfully!')</script>").result

end

我们只要满足了这个if params[:do] == "#{params[:name][0,7]} is working" then，就会进行模板渲染

【技术分享】手把手教你如何完成Ruby ERB模板注入

但是这题有长度的限制，除去<%=%>，就只剩下2个字符可用了。可以用ruby的Ruby的预定义变量

预定义变量

我们利用$’,最后一次成功匹配右边的字符串