cdxy.me
Footprints on Cyber Security and Python

入口代码审计

<?php

error_reporting(0);

$dir = 'sandbox/' . sha1($_SERVER['REMOTE_ADDR']) . '/';
if(!file_exists($dir)){
  mkdir($dir);
}
if(!file_exists($dir . "index.php")){
  touch($dir . "index.php");
}

function clear($dir)
{
  if(!is_dir($dir)){
    unlink($dir);
    return;
  }
  foreach (scandir($dir) as $file) {
    if (in_array($file, [".", ".."])) {
      continue;
    }
    unlink($dir . $file);
  }
  rmdir($dir);
}

switch ($_GET["action"] ?? "") {
  case 'pwd':
    echo $dir;
    break;
  case 'phpinfo':
    echo file_get_contents("phpinfo.txt");
    break;
  case 'reset':
    clear($dir);
    break;
  case 'time':
    echo time();
    break;
  case 'upload':
    if (!isset($_GET["name"]) || !isset($_FILES['file'])) {
      break;
    }

    if ($_FILES['file']['size'] > 100000) {
      clear($dir);
      break;
    }

    $name = $dir . $_GET["name"];
    if (preg_match("/[^a-zA-Z0-9.\/]/", $name) ||
      stristr(pathinfo($name)["extension"], "h")) {
      break;
    }
    move_uploaded_file($_FILES['file']['tmp_name'], $name);
    $size = 0;
    foreach (scandir($dir) as $file) {
      if (in_array($file, [".", ".."])) {
        continue;
      }
      $size += filesize($dir . $file);
    }
    if ($size > 100000) {
      clear($dir);
    }
    break;
  case 'shell':
    ini_set("open_basedir", "/var/www/html/$dir:/var/www/html/flag");
    include $dir . "index.php";
    break;
  default:
    highlight_file(__FILE__);
    break;
}
  • upload参数上传的文件可以跳路径。
  • 触发代码执行的点只有shell参数,意味着我们要控制index.php的内容。

思路

如何在index.php已经存在的情况下,覆盖该文件逻辑,并绕过php后缀过滤。

  • http://gosecure.net/2016/04/27/binary-webshell-through-opcache-in-php-7/

这个思路出过比赛,这里使用action=phpinfo参数查看配置,果然开启了opcache,但和以往题目不同的是,环境对cache的timestamp做了验证validate_timestamps = 1

幸运的是上面链接仍然给出了bypass timestamp的方法,即获取到文件创建时的timestamp,然后写到cache的bin里面。

此外,再获取到目标环境的system_id,即可构造出可用的恶意opcache。

获取timestamp

注意到开始的php代码中有两个参数:

  • time:获取当前timestamp
  • reset:删除当前目录下文件

二者结合即可精确拿到timestamp

import requests
print requests.get('http://202.120.7.217:9527/index.php?action=time').content
print requests.get('http://202.120.7.217:9527/index.php?action=reset').content
print requests.get('http://202.120.7.217:9527/index.php?action=time').content

运行后1和3的结果一致。

获取system_id

上文链接中给出的github项目给出了system_id的生成代码:

  • https://github.com/GoSecure/php7-opcache-override

所需的数据均可从phpinfo提取,计算结果:

PHP version : 7.0.28
Zend Extension ID : API320151012,NTS
Zend Bin ID : BIN_SIZEOF_CHAR48888
Assuming x86_64 architecture
------------
System ID : 7badddeddbd076fe8352e80d8ddf3e73

构造恶意opcache

在phpinfo中寻找opcache相关配置,并按照pwd参数的路径,在本地启动一个同版本、同配置、同目录的php项目,然后将index.php内容写入需要执行的代码。

访问之,在/tmp/cache目录生成cache文件,然后将文件导入010editor,将system_id和timestamp两个字段修改为题目数据。

pic

代码执行

然后通过upload参数,配合路径穿越,将index.php.bin上传到opcache所在位置(由于.bin是后缀,正好绕过了正则):

/../../../../../../tmp/cache/7badddeddbd076fe8352e80d8ddf3e73/var/www/sandbox/209a9184b3302dc0ff24bc20b7b8844eab478cb6/index.php.bin

然后请求shell参数,当index.php被加载时,实际加载的是我们上传的opcache,回显可以看到opcache中php代码执行结果。

文件修复

通过scandir发现路径,然后拿到这个bin文件。

@print_r(file_get_contents('flag/93f4c28c0cf0b07dfd7012dca2cb868cc0228cad'));

看了下可见字符,该文件存在OPCACHE头,是/var/www/html/flag.php的opcache文件。但无法正常解析,与正确的文件对了下格式,补全一个00即可正常解析。

pic

粗粒度指令还原

使用前文链接github中给出的opcache分析工具,可以还原部分指令。

这个工具要安装旧版本依赖。

pip install construct==2.8.22
pip install treelib
pip install termcolor
python opcache_disassembler.py -c -a64 ../../flag.php.bin

结果如下(代码里包含了我加的缩进和猜测):

function encrypt() {
  #0 !0 = RECV(None, None); //两个接收参数
  #1 !0 = RECV(None, None);
      #2 DO_FCALL_BY_NAME(None, 'mt_srand');   mt_srand(1337)
      #3 SEND_VAL(1337, None);
      #4 (129)?(None, None);
  #5 ASSIGN(!0, '');
  #6 (121)?(!0, None);
  #7 ASSIGN(None, None);
  #8 (121)?(!0, None);
  #9 ASSIGN(None, None);
      #10 ASSIGN(None, 0); for($i
      #11 JMP(->-24, None); 循环开始
          #12 DO_FCALL_BY_NAME(None, 'chr');
              #13 DO_FCALL_BY_NAME(None, 'ord'); ord($a[$i])
                  #14 FETCH_DIM_R(!0, None);
                  #15 (117)?(None, None);
              #16 (129)?(None, None);

              #17 DO_FCALL_BY_NAME(None, 'ord'); ord($b[$i])
                  #18 MOD(None, None);
                      #19 FETCH_DIM_R(!0, None);
                      #20 (117)?(None, None);
              #21 (129)?(None, None);

              #22 BW_XOR(None, None);

              #23 DO_FCALL_BY_NAME(None, 'mt_rand');   mt_rand(0,255)
                  #24 SEND_VAL(0, None);
                  #25 SEND_VAL(255, None);
              #26 (129)?(None, None);

              #27 BW_XOR(None, None);

          #28 SEND_VAL(None, None); chr的传参
          #29 (129)?(None, None);
          #30 ASSIGN_CONCAT(!0, None);
      #31 PRE_INC(None, None);      i++
      #32 IS_SMALLER(None, None);   for 条件 i<?
      #33 JMPNZ(None, ->134217662); 循环结束
  #34 DO_FCALL_BY_NAME(None, 'encode');
  #35 (117)?(!0, None);
  #36 (130)?(None, None);
  #37 RETURN(None, None);

}
function encode() {
  #0 RECV(None, None);
  #1 ASSIGN(None, '');
  #2 ASSIGN(None, 0);
  #3 JMP(->-81, None);
      #4 DO_FCALL_BY_NAME(None, 'dechex');
          #5 DO_FCALL_BY_NAME(None, 'ord');
              #6 FETCH_DIM_R(None, None);
              #7 (117)?(None, None);
          #8 (129)?(None, None);
          #9 (117)?(None, None);
      #10 (129)?(None, None);
  #11 ASSIGN(None, None);
  #12 (121)?(None, None);
  #13 IS_EQUAL(None, 1);
  #14 JMPZ(None, ->-94);
  #15 CONCAT('0', None);
  #16 ASSIGN_CONCAT(None, None);
  #17 JMP(->-96, None);
  #18 ASSIGN_CONCAT(None, None);
  #19 PRE_INC(None, None);
  #20 (121)?(None, None);
  #21 IS_SMALLER(None, None);
  #22 JMPNZ(None, ->134217612);
  #23 RETURN(None, None);

}

#0 ASSIGN(None, 'input_your_flag_here');
#1 DO_FCALL_BY_NAME(None, 'encrypt');
#2 SEND_VAL('this_is_a_very_secret_key', None);
#3 (117)?(None, None);
#4 (130)?(None, None);
#5 IS_IDENTICAL(None, '85b954fc8380a466276e4a48249ddd4a199fc34e5b061464e4295fc5020c88bfd8545519ab');
#6 JMPZ(None, ->-136);
#7 ECHO('Congratulation! You got it!', None);
#8 EXIT(None, None);
#9 ECHO('Wrong Answer', None);
#10 EXIT(None, None);



其实这段代码缺失了很多关键信息,在这里Ricter已经准确的瞎j8猜出了逻辑并还原了php代码(膜!),而且写出了逆向加密的代码(XOR可逆,直接把密文输入enc函数再算一遍即可得到明文),如下:

<?php

function encrypt() {
    $t = "";
    $s = "\x85\xb9T\xfc\x83\x80\xa4f'nJH$\x9d\xddJ\x19\x9f\xc3N[\x06\x14d\xe4)_\xc5\x02\x0c\x88\xbf\xd8TU\x19\xab";
    $k = 'this_is_a_very_secret_keythis_is_a_very_secret_keythis_is_a_very_secret_keythis_is_a_very_secret_keythis_is_a_very_secret_key';
    mt_srand(1337);
    for ($i=0; $i<37; $i ++) {
        $n = mt_rand(0, 255);
        $r = ord($s[$i]) ^ $n  ^ ord($k[$i]);
        $t .= chr($r);
    }
    return $t;
}

echo encrypt();

执行后可得到flag。

但这个脚本我俩执行完都是乱码,于是怀疑还原的不对,毕竟opcache的粗粒度指令丢失了很多信息。

事实是主办方线上题目的PHP版本是7.0,但生成这个opcache的版本是7.2(主办方后续发hint澄清),导致mt_rand函数在设置相同seed的情况下仍有不同结果,因此解密失败。

然而我们在这里继续尝试使用vld插件还原出完整的opcode,再精确还原出php代码。

精确指令还原

VLD插件与OPCODE不再赘述。

apt-get install php7.0-dev
wget http://pecl.php.net/get/vld-0.14.0.tgz
tar -xzvf vld-0.14.0.tgz 
cd vld-0.14.0/
cat README.rst 
which php-config
phpize
./configure --with-php-config=/usr/bin/php-config --enable-vld
make && make install

php --ini
vi /etc/php/7.0/cli/php.ini
service apache2 restart
php -dvld.active=1 -dvld.execute=0 phpinfo.php

现在需要在本地把opcache跑起来,然后通过vld插件拿到opcode。

本地环境安装vld之后,在php.ini开启opcache.enable_cli
然后本地创建/var/www/html/flag.php,生成opcache,用010editor解除system_idtimestamp值,写入我们待解的opcache,然后将其放到/tmp/cache对应目录下。

root@iZj6ccwgu73ligyn42bic9Z:/var/www/html# php -d vld.active=1 -d vld.execute=0 -dvld.save_dir=png -dvld.save_paths=1 -f /var/www/html/flag.php


Finding entry points
Branch analysis from position: 0
Jump found. (Code = 43) Position 1 = 7, Position 2 = 9
Branch analysis from position: 7
Jump found. (Code = 79) Position 1 = -2
Branch analysis from position: 9
Jump found. (Code = 79) Position 1 = -2
filename:       /var/www/html/flag.php
function name:  (null)
number of ops:  11
compiled vars:  !0 = $flag
line     #* E I O op                           fetch          ext  return  operands
-------------------------------------------------------------------------------------
  27     0  E >   ASSIGN                                                   !0, 'input_your_flag_here'
  29     1        INIT_FCALL                                               'encrypt'
         2        SEND_VAL                                                 'this_is_a_very_secret_key'
         3        SEND_VAR                                                 !0
         4        DO_UCALL                                         $2
         5        IS_IDENTICAL                                     ~1      $2, '85b954fc8380a466276e4a48249ddd4a199fc34e5b061464e4295fc5020c88bfd8545519ab'
         6      > JMPZ                                                     ~1, ->9
  30     7    >   ECHO                                                     'Congratulation%21+You+got+it%21'
  35     8      > EXIT
  32     9    >   ECHO                                                     'Wrong+Answer'
  35    10      > EXIT

branch: #  0; line:    27-   29; sop:     0; eop:     6; out1:   7; out2:   9
branch: #  7; line:    30-   35; sop:     7; eop:     8; out1:  -2
branch: #  9; line:    32-   35; sop:     9; eop:    10; out1:  -2
path #1: 0, 7,
path #2: 0, 9,


Function encrypt:
Finding entry points
Branch analysis from position: 0
Jump found. (Code = 42) Position 1 = 32
Branch analysis from position: 32
Jump found. (Code = 44) Position 1 = 34, Position 2 = 12
Branch analysis from position: 34
Jump found. (Code = 62) Position 1 = -2
Branch analysis from position: 12
Jump found. (Code = 44) Position 1 = 34, Position 2 = 12
Branch analysis from position: 34
Branch analysis from position: 12
filename:       /var/www/html/flag.php
function name:  encrypt
number of ops:  38
compiled vars:  !0 = $pwd, !1 = $data, !2 = $cipher, !3 = $pwd_length, !4 = $data_length, !5 = $i
line     #* E I O op                           fetch          ext  return  operands
-------------------------------------------------------------------------------------
  16     0  E >   RECV                                             !0
         1        RECV                                             !1
  17     2        INIT_FCALL                                               'mt_srand'
         3        SEND_VAL                                                 1337
         4        DO_ICALL
  18     5        ASSIGN                                                   !2, ''
  19     6        STRLEN                                           ~6      !0
         7        ASSIGN                                                   !3, ~6
  20     8        STRLEN                                           ~6      !1
         9        ASSIGN                                                   !4, ~6
  21    10        ASSIGN                                                   !5, 0
        11      > JMP                                                      ->32
  22    12    >   INIT_FCALL                                               'chr'
        13        INIT_FCALL                                               'ord'
        14        FETCH_DIM_R                                      $6      !1, !5
        15        SEND_VAR                                                 $6
        16        DO_ICALL                                         $6
        17        INIT_FCALL                                               'ord'
        18        MOD                                              ~8      !5, !3
        19        FETCH_DIM_R                                      $7      !0, ~8
        20        SEND_VAR                                                 $7
        21        DO_ICALL                                         $8
        22        BW_XOR                                           ~7      $6, $8
        23        INIT_FCALL                                               'mt_rand'
        24        SEND_VAL                                                 0
        25        SEND_VAL                                                 255
        26        DO_ICALL                                         $8
        27        BW_XOR                                           ~6      ~7, $8
        28        SEND_VAL                                                 ~6
        29        DO_ICALL                                         $6
        30        ASSIGN_CONCAT                                 0          !2, $6
  21    31        PRE_INC                                                  !5
        32    >   IS_SMALLER                                       ~6      !5, !4
        33      > JMPNZ                                                    ~6, ->12
  24    34    >   INIT_FCALL                                               'encode'
        35        SEND_VAR                                                 !2
        36        DO_UCALL                                         $6
        37      > RETURN                                                   $6

branch: #  0; line:    16-   21; sop:     0; eop:    11; out1:  32
branch: # 12; line:    22-   21; sop:    12; eop:    31; out1:  32
branch: # 32; line:    21-   21; sop:    32; eop:    33; out1:  34; out2:  12
branch: # 34; line:    24-   24; sop:    34; eop:    37; out1:  -2
path #1: 0, 32, 34,
path #2: 0, 32, 12, 32, 34,
End of function encrypt

Function encode:
Finding entry points
Branch analysis from position: 0
Jump found. (Code = 42) Position 1 = 20
Branch analysis from position: 20
Jump found. (Code = 44) Position 1 = 23, Position 2 = 4
Branch analysis from position: 23
Jump found. (Code = 62) Position 1 = -2
Branch analysis from position: 4
Jump found. (Code = 43) Position 1 = 15, Position 2 = 18
Branch analysis from position: 15
Jump found. (Code = 42) Position 1 = 19
Branch analysis from position: 19
Jump found. (Code = 44) Position 1 = 23, Position 2 = 4
Branch analysis from position: 23
Branch analysis from position: 4
Branch analysis from position: 18
Jump found. (Code = 44) Position 1 = 23, Position 2 = 4
Branch analysis from position: 23
Branch analysis from position: 4
filename:       /var/www/html/flag.php
function name:  encode
number of ops:  24
compiled vars:  !0 = $string, !1 = $hex, !2 = $i, !3 = $tmp
line     #* E I O op                           fetch          ext  return  operands
-------------------------------------------------------------------------------------
   3     0  E >   RECV                                             !0
   4     1        ASSIGN                                                   !1, ''
   5     2        ASSIGN                                                   !2, 0
         3      > JMP                                                      ->20
   6     4    >   INIT_FCALL                                               'dechex'
         5        INIT_FCALL                                               'ord'
         6        FETCH_DIM_R                                      $4      !0, !2
         7        SEND_VAR                                                 $4
         8        DO_ICALL                                         $4
         9        SEND_VAR                                                 $4
        10        DO_ICALL                                         $4
        11        ASSIGN                                                   !3, $4
   7    12        STRLEN                                           ~5      !3
        13        IS_EQUAL                                         ~4      ~5, 1
        14      > JMPZ                                                     ~4, ->18
   8    15    >   CONCAT                                           ~4      '0', !3
        16        ASSIGN_CONCAT                                 0          !1, ~4
        17      > JMP                                                      ->19
  10    18    >   ASSIGN_CONCAT                                 0          !1, !3
   5    19    >   PRE_INC                                                  !2
        20    >   STRLEN                                           ~5      !0
        21        IS_SMALLER                                       ~4      !2, ~5
        22      > JMPNZ                                                    ~4, ->4
  13    23    > > RETURN                                                   !1

branch: #  0; line:     3-    5; sop:     0; eop:     3; out1:  20
branch: #  4; line:     6-    7; sop:     4; eop:    14; out1:  15; out2:  18
branch: # 15; line:     8-    8; sop:    15; eop:    17; out1:  19
branch: # 18; line:    10-    5; sop:    18; eop:    18; out1:  19
branch: # 19; line:     5-    5; sop:    19; eop:    19; out1:  20
branch: # 20; line:     5-    5; sop:    20; eop:    22; out1:  23; out2:   4
branch: # 23; line:    13-   13; sop:    23; eop:    23; out1:  -2
path #1: 0, 20, 23,
path #2: 0, 20, 4, 15, 19, 20, 23,
path #3: 0, 20, 4, 18, 19, 20, 23,
End of function encode

还原出的php逻辑和之前猜的一样。

<?php

function encrypt($var_0, $var_1) {
    mt_srand(1337);
    $var_2 = '';
    $var_3 = strlen($var_0); // key_length
    $var_4 = strlen($var_1); // flag length

    for ($var_5=0; $var_5<$var_4; ++$var_5) {
        $var_2 .= chr(
            ord($var_1[$var_5]) ^ ord($var_0[$var_5 % $var_3]) ^ mt_rand(0, 255)
        );
    }
    return $var_2;
}

$s = "\x85\xb9T\xfc\x83\x80\xa4f'nJH$\x9d\xddJ\x19\x9f\xc3N[\x06\x14d\xe4)_\xc5\x02\x0c\x88\xbf\xd8TU\x19\xab";
echo encrypt("this_is_a_very_secret_key", "$s");

  • P.S. 用条件竞争出的兄弟真地秀!