0CTF 2018 EZDOOR(WEB) Writeup
[CTF]
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。
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。
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_id和timestamp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！